Final Report Summary - NANOCOAT (Nano-structured Aluminium Oxide Coatings)
Project context and objectives:
Nanomaterials? They measure from 1 to 100 nanometers, are smaller than a virus, but bear huge promises for industry, but also energy, health or multimedia areas.
For more than a decade, nanomaterials have entered our daily life, but here the goal was to know more about some industrial applications: the project results are expected to have a significant impact on the competitiveness and economic performance of the small and medium-sized enterprise (SME) participants by expanding existing markets and developing new applications. The project outputs will demonstrate clear market potential and will address specific technology market demands for improved functional anodised aluminium materials derived from nano-structured surface coatings.
Conventional processes for anodised aluminium oxide products are well established across the range of market applications. Standard, long-established manufacturing technologies are in use throughout Europe.
Anodising aluminium is a proven industrial manufacturing technique for improving corrosion resistance, increasing material strength and introducing cosmetic finishes such as metal colouring. Anodic aluminium Oxide (AAO) industrial products include:
- decorative or coloured Al surfaces: sealed colourless; electrochemical or chemical colouring,
- anodised base for organic coatings: liquid or powder painting processes,
- corrosion resistance: hard anodising to produce hard films on Al and its alloys with excellent abrasion and corrosion resistance,
- adhesive bonding: porous coatings as primers for surface bonding Al.
The introduction of markedly different technological approaches, which offer significant enhancements in properties and therefore provide differentiation from existing products, will create new market opportunities. The proposers consider that the innovative templated anodisation processes in this research will provide the necessary product differentiation.
The output results will be implemented by the surface finishing industry. The impact of the sector is very broad, because of the wide range of products that play a critical role in industries of great strategic importance to the European Union (EU). Examples include: civil and military aerospace and maritime, construction, industrial hydraulic applications, automotive, construction, chemicals, food etc.
Companies carrying out these large manufacturing activities in important industry sectors are all customers of the SME manufacturers that form the vast majority of the surface finishing / engineering sector.
Any important impacts on the European surface treatment industry are highly significant to Europe as the sector employs more than 500 000 persons for a total turnover of EUR 40 billion. This industry has over 30 000 SME members in Europe, with the majority employing less than 20 people. The vast majority of these companies are also categorised as SMEs in terms of turnover. The biggest group (53 %) has an annual sales volume of between EUR 1 and 5 million.
The industry is broadly distributed across all EU countries. In the vast majority of cases, the types of processes carried out are largely the same wherever they are applied. Typically, SMEs produce components that are sold to end user companies, often very large, who incorporate these components into final products.
The result is that there are no significant geographical barriers. Also, competition between SMEs tends to occur relatively locally, often within single countries, so there are few geographical inter-regional competition issues acting as barriers between SMEs.
This SME intensive industry sector is under increasing competitive pressure from the global market, particularly competition from regions with lower intrinsic costs and less demanding legislative controls. In the context of this highly competitive environment, there is a clear need for new technology to produce higher specification and added value products.
Hence, there is a strong need for SMEs in general to create their own intellectual property and develop new products with a clear economic impact leading to increased competitiveness.
Science and technology (S&T) objectives
The overall aim of the proposed research is to develop improved functionality and new coating applications for aluminium anodisation products in surface engineering.
The project will advance recent research innovations in research laboratories at research and technological development (RTD) participant Armines towards the preparation of ordered nanoporous, nanostructured materials using anodisation methods and applying the methods to develop commercially relevant novel materials.
The proposed research will aim to address the market potential for these new and improved materials.
The expertise and capabilities provided by the project participants will ensure that the following S&T objectives can be realistically achieved and verified within the project timescale:
- research the techniques for producing nano-structured anodised aluminium materials under industrially relevant conditions,
- develop process protocols including pre-treatments and post-treatments,
- scale-up processes for the treatment of larger areas and adapt processes to treat complex-shapes,
- design and construct an experimental prototype system,
- operate the prototype system over an extended period and address scale-up issues,
- use industrial grade aluminium alloys, in addition to pure aluminium,
- carry out full structural characterisation of materials,
- carry out functional characterisation to evaluate the performance of materials across a range of existing and potential industrial applications,
- consider production issues relating to environmental legislation and apply a simplified life cycle analysis (LCA) methodology,
- complete a full techno-economic assessment of the project outputs,
- develop a plan for exploitation and dissemination.
Project results:
The anodisation of the aluminium is well known as an industrial process used in many applications in which, subsequently at an electrochemical oxidation by anoding process, it is possible to increase the thickness of the alumina layer naturally present when aluminium is in contact with the oxygen (and water) present in atmosphere.
Objectives of the anodisation processes are essentially devoted to anti-corrosion, decorative, wear resistance purposes. The formation of oxide on the surface of the aluminium protects the metal: the oxide layer, even it is porous, presents a good resistance to the corrosion due to a barrier layer formed at the interface of aluminium metal and oxide layer. The properties of this oxide layer could be improved by controlling the aluminium anodisation process and by using specific parameters.
In particular, important research works have been done since about twenty years to control the layer formation to obtain a special structure which is interesting: the anodic aluminium oxide grown in hexagonal nano-structure, well ordered, if parameters of anodic oxidation are well controlled. This growing is linked also to the nature of the aluminium substrate: pure Al, Al alloys, Al with inclusions.
The goal of the first work package (by Armines) is to determine and define the parameters of the process for each nature of Al substrate to obtain a regular structure of the anodic aluminium oxide depending on aluminium composition and geometrical form.
In particular:
- to define and select operating parameters to generate porous layers with regular arrangements (nano well-ordered structure) over the widest range of surfaces and substrate,
- to define the operating parameters to transfer the results at an industrial level.
1. Anodisation techniques
1.1 Marking and double anodisation
Two methods can be used to initiate the arrangement of the structure: pre-texturing or double anodisation method. Both goals are the same: creating a network of nano-cavities on the surface of the substrate which are the 'germs' of the nano well-ordered structure.
- The pre-texturing method consists in engraving the surface of the substrate using a gauge to pattern the surface following the wanted structure inscribed in a 'negative' form compare to the structure to be obtained. The pattern corresponds to the cell size of the future ordered nanostructure.
- The double anodisation method consists of making a first anodisation to start the ordered structure as the scheme shows the principle. This first oxide layer created is completely dissolved in acid solution. The oxide layer is completely removed of the aluminium substrate and lives on the surface marks of the ordered structure. The second anodisation starts from theses marks and grows in a nano well-ordered form.
In the course of the NANOCOAT project, we used only the double anodisation method because it is the method which allows obtaining large nanostructured surface, the pretexturing method is somewhere limited in area and is only used to engrave surfaces equivalent to the surface of the mold.
In the case of double anodisation process, the first anodisation layer (oxide layer) is dissolved in an etching solution of 6 % H3PO4 and 1.8 % CrO3.
2. Laboratory equipments
The electrolyte is in a double wall glass tank with an approximately volume of 1.5 litre. As the anodisation is an exothermic reaction, the electrolyte has to be cooled to control the temperature of the solution. The coolant is on the external part (blue on the picture): an external cooling system allows regulating the temperature of the cooling liquid at a fixed value which is linked to the wanted temperature of the electrolyte. The nature of the electrolyte studied is variable and described in next paragraph.
This equipment is sufficient for small samples but for larger one, the volume of electrolyte has been increase to about 10 litres.
This configuration with a larger volume allows the possibilities to realise anodisation of larger samples even a higher power is necessary (same voltage but higher intensity), without temperature variations during the anodisation process (increasing temperature by Joules effect).
3. Process to obtain anodic aluminium oxide ordered
3.1 Samples
In a first time, we used only pure aluminium samples from Goodfellow (samples bought to Goodfellow Company), with a purity at 99.999 % (ref: AL0006920) in sheet of 1 mm thick.
To obtain a regular structure, these samples have been polished mechanically with SiC abrasive paper from P640 to P4000. Then the surface is polished with fibre plate and diamond solution from 3µm to 1µm.
3.2 Etching
Before anodising, the aluminium sample is etched by phosphoric acid at 10 % during 30 minutes to remove the native oxide layer.
The roughness of the surface has been monitored and standard results obtained by mechanical profilometry measurements are:
- starting aluminium: Ra = 240 nm - 300 nm Rz = 4.3 µm
- after polishing: Ra = 60 nm - 90 nm Rz = 1.5 µm.
3.3 Anodisation: Influence of the electrolyte
Following our experience and a literature survey, it appears that the nature of the electrolyte is one of the criteria which define the size of the pores.
Literature indicates that three different types of electrolytes have been investigated and could be used: sulphuric acid, oxalic acid and phosphoric acid.
Tests with each of recommended electrolyte have been conducted during the first part of the programme.
The structure is ordered and the size of the pores is around 25 nm in diameter. The interpores distance is about 60 nm. Thickness of the oxide layer after 2 hours is approximately 6 µm.
The structure is ordered and the size of the pores is about 35 nm in diameter and about 95 nm for interpores distance. Thickness of the oxide layer after 2 hours is approximately 16 µm.
In the case of phosphoric acid, the structure seems to be less ordered but the pore size is larger: around 120 nm in diameter for pores and around 150 nm for interpores distance. Thickness of the oxide layer after 2 hours is approximately 8 µm.
When phosphoric acid electrolyte is used, it has to be noted that the recommended value for voltage between anode and cathode is around 190 V. But due to our rectifier characteristics, it is impossible to increase the voltage to this high value and the voltage has been limited at about 160 V (maximum of our rectifier).
3.4 Anodisation: Influence of the double anodisation
After the first anodisation process, the nanoporous structure is created but the pores are not well ordered neither well define. The pores seem to be unevenly present on the surface of the structure and the size of the holes and cells are different.
3.5 Anodisation: Shape of the subtract
Usually the sample comes from a sheet of pure aluminium. Trying to study influence of the sample geometry, anodisation of a small cylinder (6 mm diameter, length 7 cm) of the same aluminium purity (99.999 % Al) from Goodfellow Company has been realised. This anodisation process on pure aluminium cylinder has been realised in an oxalic acid solution, using same parameters as for flat samples. The aluminium oxide layer grows with an ordered structure as for flat samples after a double anodisation process.
As presented in pictures 12, annex I (SEM examination at different magnification) formation of a well-ordered structure on surface of cylindrical samples (diameter 6 mm) is not influenced by the shape of the substrate.
3.6 Anodisation: Influence of the purity of the aluminium
Influence of the substrate purity has been studied on aluminium alloy (AG3-5754) compared to results obtained on pure aluminium.
Structure on AG3-5754 is more irregular than for pure aluminium sample even an ordered structure appears in the alumina obtained on these alloys: some perturbations are present in the network of the structure. The presence of foreign atoms in the alloys disrupt the formation of alumina column and so the formation of the nano well-ordered structure.
As for alloys, impurities (precipitate) included in the aluminium influence really the arrangement of the structure.
The influence of inclusions in the substrate has already been investigated during the PhD research works of Pascal Thomas in EMSE Laboratory (2008). Results shown that these impurities give perturbation in the formation of the well-ordered structure: due to the nature of impurities, formation of alumina structure is impeded and so some defaults appears in the ordered structure.
3.7 Anodisation: Aluminium alloys
One test has been made with an aluminium alloys (aluminium series 2XXX) which contains approximately 4 % of copper. In this case, the well-ordered structure is not present. However, an adjustment of parameters could improve the quality of the structure and the determination of well-ordered parameters will be discuss in following paragraphs.
3.8 Anodisation : Influence of the surface preparation
Following our experiments during the WP1, surface preparation is important to obtain well ordered structure.
Surface of samples has to be degreased (acetone + water + ethylic alcohol) and polished before first anodisation. The necessity of polishing is important as growing of pores occurred perpendicularly to the surface and so if the surface is too rough, pores cannot grow in an ordered manner.
Mechanical polishing and electrochemical polishing have been compared and results appear to be equivalent. This is due to the fact that during the process of double anodisation, the first step (first anodisation) removes the strain-hardened layer created during mechanical polishing. There is no real difference between the two types of preparation.
However, voluntarily, no solution using chloric, or perchloric acid and/or other chemical hazardous components has been tested and used during the programme as industrial applications would be hazardous
3.9 Anodisation: Influence of the temperature
Previous experiments (Cf PhD thesis of P. Thomas) have evidenced that the optimal temperature to obtain ordered structure is between -3°C and 20°C. When the temperature increases it occurs a degradation of the nanostructure between 30°C and 40°C (for oxalic acid) and at a temperature higher than 40°C the structure is clearly disordered.
During the NANOCOAT project, more experiments have been done to define the exact influence of the temperature, in particular using different solution and substrate composition.
3.10 Pores opening
The size of the pores could be enlarged by dissolving a part of the wall of cells. The dissolution of the aluminium oxide is made by etching the sample in phosphoric acid.
3.11 Coloration of the anodic oxide layer
The aluminium anodic layer could be coloured by filling porous structure by different solution or by electrolysis. Some tests have been done in cooperation with the NANOCOAT partners ProMet and C-Tech.
4. Anodisation parameters definition
During a meeting of the RTDs performers (Gardanne (France) in November 2011), it appears that some complement of information from industrial partner Promet are important due to the anodisation parameters, more particularly on the electrolyte and on the voltage control.
Promet makes industrial hard anodisations by using the already defined high concentred electrolyte composed by mixing sulphuric acid (2.2 M) and oxalic acid (0.2 M) at a temperature of -3 to 0°C.
More, the process itself is slightly different as for industrial reasons (too important intensity at the beginning), voltage of the anodisation process is increased from 0 to approximately 40 -50 V in 5 minutes then a stable period is running for 1 hours, anodisation is ending by an increase of the voltage to 60 V. At the laboratory scale, the voltage is increased very rapidly and maintained constant along the anodisation process.
For the final application it will be interesting to know exactly the differences on the final layer.
But as a results of this meeting, it was decided that the anodisation solution which will be used in the future will be the Promet solution: 220 g/L (2.2 M) of H2SO4 and 15 g/L (0.2 M) of COOH.
4.1 Specific electro formulation
As shown in the previous paragraph 4.3 the diameters of the pores depend on the nature of the electrolyte. The table here after summarise the composition of the different electrolyte used during the NANOCOAT project.
4.2 Operating protocol
Investigations to define the better condition of anodisation have been made and this protocol will be used for all samples produced in laboratory conditions.
- Samples (2 x 6 cm) are taken from pure aluminium plate (Goodfellow supplier).
- Surface preparation is realised by polishing the sample with different paper of SiC and diamond solutions to obtain mirror surface without scratch.
- Before anodisation, the sample is degreased with acetone and etched in a H3PO4 (10 %) solution at room temperature during 30 minutes.
- The voltage is progressively increased to the value wanted without limitation of current.
- The time of the first anodisation has been fixed at 3 hr to be sure that the structure is well ordered.
- After the first anodisation, the sample is rinsed and the oxide layer is dissolved in a solution of 6 % H3PO4 and 1.8 % CrO3 during 2 hours to remove completely the oxide.
- The sample is then rinsed again and dried: so it is ready for the second anodisation.
- The second anodisation is made using the same protocol for electrolytical parameters.
- The duration of the anodisation process is defined in accordance with the oxide thickness intended.
4.3 Industrial application
Tries for application of the Laboratory parameters to industrial hard anodisation have been made in the Promet premises. During common tests made in May 2012, double anodisation process has been tested in an industrial environment on pure aluminium sheets (A5 format).
1. The first sample (S1) has been hanged on a titanium jig (Promet system) for the first anodisation procedure:
- Surface preparation: Etching in soda solution, acid etching in proprietary (Promet solution - rinsing after each operation.
- Anodisation in a tank (1000 L) solution (- 3° C): 220 g/L H2SO4 - 15 g/L Oxalic acid at 20 V during about 40 min (ramp of current) - 2 to 3 A/dm².
- Coloration in mineral black solution (commercial)
-Result: Colouration of S1 seems to be 'pale' on about half of the surface due to an insufficient thickness of the alumina layer.
2. Definition of the operating procedure:
- Dissolution of the first anodisation for the first sample S1 second anodisation of first sample S1.
-In the same time and operation, first (and single one) anodisation for the second sample (S2) same conditions as previously except: duration 100 min voltage is increasing from 20 V to 40V progressively.
- Coloration of the two samples (single - S2 and double anodisation S1) in mineral black solution.
- Sealing in a proprietary solution (boiling aqueous solution with acetate salts).
Results and report made during these tests are presented in the D6.2 deliverable.
Following some difficulties in the realisation of the anodisation in Promet company (essentially due to the new formulation of the electrolyte: mixed of sulphuric and oxalic acid), tests in the same condition (using the industrial bath) have been made in Laboratory of EMSE / Armines.
These different results demonstrate that it is possible to obtain a well-ordered structure on pure aluminium with an industrial electrolyte. Tests to improve the process using sulfo-oxalic acid have been realised. Results rae presented in the following paragraph.
5. Improvement of the anodisation parameters
After definition of the electrolyte composition, the others parameters having an influence on the anodisation process have been investigated.
5.1 Improvement of the anodisation parameters using industrial solution (sulfo-oxalic acid) (pure aluminium)
As a general point-of-view, during the first anodisation, the internal evolution of the structure becomes homogeneous after a lap of time (not necessary well-ordered). So by examination of the structure obtained after a first anodisation it is possible to define if the 'stable' structure obtains with the parameters used during the anodisation procedure allows the formation of a nano well-ordered structure.
By examination of the remaining structure obtained on the aluminiuum substrate after dissolution of first alumina layer, it is possible to know the conditions for obtaining the best well-ordered structure. The following schema indicates the procedure the first anodisation.
If general conditions are correct and well defined, the structure obtained at the end of the first anodisation would be well-ordered. The second anodisation following the structure 'printed' by the first anodisation, will induce a well-ordered structure. This method allows determining the parameters of anodisation.
The protocol we used to characterise the anodisation parameters is:
- polish the aluminium,
- make a first anodisation,
- dissolve the aluminium anodised layer,
- examination of the surface by SEM.
5.2 Determination of the better potential on pure aluminium
Following first experiments, it appears that the most important parameter is the voltage between aluminium (working electrode / anode) and counter electrode (cathode). However, we have studied also the influence of temperature.
- Influence of voltage
Following these results, the best potential is assumed to be 35 to 45 V, but due to the joules effect and cost reason, the potential used will be 35 to 40 V. This method of 'ordered-potential' determination will be used in the future for industrial aluminium alloys.
Same experiments have been used for the determination of the best temperature. As a result, the best temperature has been defined to be -3°C to 0°C.
5.3 Influence of the temperature of the electrolyte
The samples made a 20°C shows a nano-structure but not a nano well-ordered structure compare with samples made a 0°C.
5.4 Determination of the better potential on aluminium alloys 6082
As previously indicated, observation is made on the surface after dissolution of the alumina layer.
A high potential is also requested to obtain an ordered structure. This conclusion has been verified by realisation of double anodisation on aluminium alloy (6082). The structure appears nano well-ordered, even if it is less ordered than on pure aluminium.
5.5 Determination of the better potential on aluminium alloys 2024
During these experiments on 2024, a nanostructured alumina layer has been obtained but it is not well-ordered as for pure aluminium or 6082 alloy.
5.6 Ordered structure on other aluminium alloys (6060 and 5005)
These tests have been made on parts from Sobinco company which is in contact with a partner of NANOCOAT project the Falex Tribology NV company. Information on this Sobinco company will be provided by Falex Tribology.
1. Tests in EMSE / Armines laboratory
The company Sobinco is interested by realisation of well-ordered nanostructure on aluminium alloy parts. The interest for this company is to increase the wear and corrosion resistance of parts used in building in particular for decorative application.
For testing the process, Sobinco company has sent to some partners (C-Tech and FALEX) some parts. The parts received are part of door: handle part (alloy 5005), door hinge (alloy 6060)
The objective is to obtain a nano well-ordered nanostructure on the industrial samples received. Sobinco has not been able to give us samples without anodisation process. So the first operation has been to dissolve the alumina layers on the samples before testing our anodisation process. This dissolution has been made by the method currently used, using a mixed of phosphoric and chromic (CrVI) acid.
The operating procedure for testing double anodisation process is the following:
- dissolution of the industrial (Sobinco) alumina layer in H3PO4 + CrO3: 3 hours at 55°C,
- first anodisation in H2SO4 (220 g/l) + (COOH)2 (11 g/l) at 40 V,
- dissolution of the layer in H3PO4 + CrO3 at 55°C,
- second anodisation in H2SO4 (220 g/l) + (COOH)2 (11 g/l) at 40 V,
- colouring: using C-Tech method.
During this study, by using 40 V as potential difference between parts and cathode, it appears that the rate of formation of the alumina layer is greatly improved. This rate of formation is about 7 to 10 µm/min instead of 0.5 to 1 µm/min in the best case for industrial process.
This rate of formation has been verified by Promet in his jobshop: confirmation of the improvement of the rate of formation using a higher voltage than classical process. This increase in rate of formation was anticipated but the high deposition rate value is interesting for industrial application.
Due to the high voltage inducing high current intensity particularly at the beginning of the anodisation process, the number of parts treated in one operation (on one jig) is reduced. But as the rate of formation of alumina layer is greatly improve (x7 or x10), it is certainly possible to treat more parts during the same time used for previous treatment at lower voltage.
Tests are in progress to complete and to certify these results.
As conclusion, by using a double anodisation, it is possible to create a well-ordered structure on industrial part (and alloys). The impurities of the alloys have a large influence on the arrangement. This difference in composition explains why the structure of the alloy 5005 is better ordered than the alloy 6060.
2. Tests in Promet lab and workshop on door parts from Sobinco
Following experiments and results obtained in EMSE / Armines laboratory, Promet has tested the process on door hinge (6060). Parts anodisation has been realised by manual and automatic system (driven by PC system) and characterisation of the results have been made by SEM examination after dissolution.
Results show that it is possible to realise well-ordered alumina structure in industrial partner who confirms also that the rate of formation is higher than expected.
6. Titanium anodisation
By using the same parameters defined for the anodisation of the aluminium, sample of titanium have been treated. An oxide layer has been formed on the surface of the titanium but without a real ordered structure. A porous structure is visible but it has not been possible to create a well-ordered structure.
At the end of NANOCOAT project, an interesting paper has been found following our bibliography survey 'Electrolyte influence on the anodic synthesis of TiO2 nanotube arrays' Ref: Journal of Non-crystalline Solids 354 (2008) p.5233-5235 Authors: V. Vega et al.
The electrolyte solutions which have been tested by the authors contain hydrofluoric acid. Due to the lack of time, experiments and tests have been not continued on titanium alloys.
7. Microhardness and scratch test
Evaluation of the microhardness of the alumina layers has been made by Vickers indentation method.
8. Conclusions
The double anodisation creates the ordered structure and the alumina well-ordered structure depends on a specific voltage.
During the NANOCOAT project, following industrial proposal, the tested solution selected and used has been H2SO4 (220 g/l) + (COOH)2 (11 g/l) (PROMET formulation), and it has been possible to obtain a well-ordered structure on pure aluminium and some aluminium alloys, except for alloy 2XXX series certainly due to the high concentration of copper in this kind of alloys. The oxidation potential of copper appears to be the reason of the lack of success for this kind of alloys. These series of aluminium are well known to have a special comportment in anodisation processes.
Following the demand of Falex Tribology NV company, some parts from a Belgium company specialised in the furniture of aluminium parts (domestics) (Sobinco) have been tested: it has been possible to realise well-ordered alumina structure on these kind of industrial alloys and industrial parts.
The method developed during the project has been tested by industrial partners (Promet) and results obtained at laboratory scale have been reproduced in industrial environment. Results are innovative as to our knowledge it is the first time that well-ordered alumina structure have been realised on aluminium alloys (5XXX and 6XXX series).
Black coloration (without opening pores) appears uniform and attractive after the double anodisation. Microhardness of the alumina layer remains constant after simple or double anodisation: formation of pores through the alumina layer do not modified the penetration of the indenter.
Results obtained by nanoscratch tests are non consistant and the method has to be re-considered as it gives too macroscopic results in front of the nanostructure
Work packages (WPs) 2 and 3 (C-Tech)
Key WPs that have been instrumental in testing the industrial viability of the new process were WP2 (Laboratory scale process design and development) and WP3 (Process scale-up), which were intended to be part of a three-step scale-up progression from the original research and technological development (RTD0 studies through to the industrial SME production lines.
RTD work that demonstrated the concept of nano-structured anodised aluminium oxide coatings and inspired the present work, focused on the ability to achieve the structures at the laboratory scale, using equipment not optimised for the industrial environment. C-Tech therefore set about constructing a system designed to emulate a realistic and flexible but reduced version of the industrial anodising lines used by the SME manufacturing partners Promet and Ashton & Moore, complete with all of the equipment necessary for pre- and post-processing parts to be anodised. The stage allowed the process to be studied in a realistic environment while not encroaching on the day to day operation of the full-scale anodising lines. Without the construction and operation of the pilot system, the study would not have been possible.
Initially this progression was intended to be done with an initial scale-up from the lab-scale of the anodising tank to a lab-scale version of the full anodising line, followed by a larger, optimised pilot scale system. At the outset, however, it was clear that in order to recreate the industrial process at the first scale-up stage, the balance of plant necessary to achieve practical targets (e.g. process heating and cooling, agitation) constituted a key part of the process, and that equipment size was not necessarily as important. It was therefore deemed more practical and efficient to amalgamate the laboratory and pilot systems, and concentrate efforts into constructing a high-quality transferrable system in two intermediary steps from lab-to-pilot-to-production.
In general, the S&T aims from WP2 were designed to develop a series of protocols for the process that would be key to the successful transfer of intellectual property (IP) and achieve high quality nanostructured anodised oxide coatings at the industrial scale. However, careful consideration of all process steps was necessary in order to assess the viability of operating the process at this industrial scale, and included:
- surface preparation, including cleaning and chemical etching for different applications,
- anodising, including all electrical and physical operating conditions necessary to achieve the desired results (e.g. current, voltage, temperature, electrolyte composition, agitation requirements,
- all ancillary and downstream processing stages, including rinsing, sealing and colouring for different applications
Accordingly, for WP3, the S&T aims were designed to achieve the necessary scale-up towards an industrial level, with a detailed analysis of the anodising process and all of the necessary ancillary equipment that are normally associated with the anodising step, including:
- pre- and post-treatment tanks for cleaning, etching, sealing and colouring,
- rinse tanks, individual to each step in the process to avoid cross-contamination,
- anodising tank, complete with ancillary cooling and agitation systems, and power electronics and control system.
In detail, the results in accordance with the S&T objectives have included the following:
- Develop process protocols including pre- and post-treatments
The starting points for the development of process protocols have been two-fold:
Standard anodising processes
Ashton & Moore and Promet supplied key information about their existing anodising processes, the process steps and their operating conditions, the materials and equipment involved, and product applications and their individual tribological requirements. Falex Tribology also provided important information about the tribological tests that the materials must undergo in order to quantify the performance and compare directly with conventional anodising.
C-Tech has actively researched anodising, both from an industrial perspective and academic studies. Current practices in all aspects of industrial anodising, including the different alloys involved, has been reviewed through standard literature:
- Sheasby and Pinner, 'The surface treatment and finishing of aluminium and its alloys', ASM International and Finishing Publications Ltd., 2001 (6th Ed.)
- Canning, The Canning Handbook Surface Finishing Technology, W. Canning plc, 1982 (23rd Ed.)
- Metals Handbook, Vol. 2, ASM International, 1990 (10th Ed.)
As a result of this background research, C-Tech drew up a preliminary flow sheet showing each individual step of a proposed prototype, in which the NANOCOAT process was to be scaled up and tested in a fully industrialised environment. Many aspects of the NANOCOAT process were found to be identical to those found in current industrial anodising plant, including the pre- and post- anodising treatment steps. However, these steps needed to be included in the prototype system to keep the process comparable. The preliminary flowsheet was reviewed by all partners before the construction process.
A final process flowsheet was produced at the end of the project, following lengthy operation of the prototype, including some retrofitting and optimisation of the equipment and the operating conditions. This iterative process involved providing anodised samples to KUL and Falex for characterisation, and comparison with conventionally anodised samples. A final set of conditions has been presented in the final flowsheet, in the deliverable report D3.2 - Operating procedures and final process flow sheet.
- Design and construct an experimental prototype system
The initial strategy for the scale-up process was to produce a small lab-scale system where initial samples would be anodised to compare the results with initial work carried out at Armines and to address possible scale-up issues for a larger prototype system. However, during the early investigations of how the laboratory and prototype systems would be constructed, it was clear that the minimum practical size of anodising tank for a meaningful laboratory study would be large enough to be considered a prototype. Smaller systems of just a few litres would be difficult to practically agitate using the planned system, and to control at a steady state, and there would be few differences from the system operated by Armines. Also, scaling up twice from a small prototype to a larger one, and then to industrial scale would serve little purpose, as the same scaling issues would need to be considered twice. For these reasons, a single lab / prototype system was built to develop the process before transferring to an SME onsite demonstration.
The prototype system, contains all necessary equipment to prepare and anodise small industrial scale parts up to around 20 x 20 x 10 cm, from the initial cleaning and etching stages through to colouring and sealing.
- Operate the prototype system over an extended period and address scale-up issues
The prototype system was operated over an extended period, looking at critical operating parameters such as anodising power, temperature and time, heat and mass transfer by agitation, removal of heat from the system, and practical issues of anodising twice with an intermediary stripping process.
The traditional method of agitation is by bubbling air through the tank. This has the effect of moving hot electrolyte away from the components, replacing it with cooler liquid from the bulk of the tank. It also helps to disperse gas and replace ions lost in the oxidation process. The NANOCOAT process however, uses voltages and currents much higher than normally used, producing more heat and affecting the ability to anodise and reducing product quality. The effect of increasing agitation by higher air flow through the liquid would have negative effects, by increasing evaporative losses and acid spray from the tank, and energy losses from additional throughput.
A more efficient method of agitation was tested using eduction, where fluid is pumped around an external circuit and back in through the eductors, which are strategically placed within the tank. Since the electrolyte is circulated through an external heat exchanger to cool, the eduction system could be incorporated into this circuit. The eductors improve efficiency, since only one fifth of the fluid directed towards the components needs to be pumped around the circuit. The remainder is drawn from the tank through the sides of the eductor and out with the pumped fluid.
Computational fluid dynamics (CFD) was used to model the circulation system to find the best configuration of flowrate, number and position of eductors.
As the optimisation process drove the move to more aggressive electrical conditions (40V; 860 mAcm-2), anodisation of larger parts became impossible due to the limitations of C-Tech's rectifier. This prompted the construction of an in-house custom-built system capable of operating at much higher currents. However, further issues arose when attempting to achieve 40V from the start, which resulted in a runaway current and again limited by the power supply.
A control system was added to allow accurate ramped or stepped control of the voltage. This allowed an initial period at slightly lowered voltage to promote the anodisation process, before climbing to the desired set-point of 40 V.
Typical conditions for the optimised anodisation follow:
Anodisation 1: Initial voltage - 25 V; ramp to 40 V over 60 s; hold at 40 V for 2 min
Anodisation 2: Initial voltage - 32 V; ramp to 40 V over 18 s; hold at 40 V for 2 min
This produced a coating of 34 µm (±2 µm).
- Use industrial grade aluminium alloys, in addition to pure aluminium
Initial operation of the prototype system involved anodising pure aluminium to emulate preliminary work performed at Armines and compare the resulting coatings. Subsequent operations involved anodising different alloy more appropriate to industrial applications. 2000 series (high copper content) proved particularly difficult to anodise, producing little or no coating, and so efforts were concentrated on 5000 and 6000 series, common industrial grades.
- Scale-up processes for the treatment of larger areas and adapt processes to treat complex shapes
A number of measures were taken to attract a supplier or end user of aluminium products, including advertising on a number of relevant 'Linkedin' message boards, and on the project web-site, to provide parts for industrial studies.
Sobinco, a Belgian aluminium door and window frame manufacturer, provided the project with various components, including a handles, a cover plates, and hinge parts, to the project, which have been anodised and characterised, both tribologically by Falex, and using standard salt spray testing by KUL. The parts were made from 5000 and 6000 series alloys and had requirements of wear resistance (hinge), corrosion resistance (external handles and covers), and colour retention (handles and covers).
Further scale-up has been achieved by Promet, who has carried out the process on their industrial anodising line.
- Consider production issues relating to environmental legislation and apply a simplified life cycle analysis methodology
A streamlined life cycle analysis was carried out, and is available as a deliverable report D5.3.
- Complete a full techno-economic assessment of the project outputs
Contribution to a full techno-economic assessment was done, comparing energy consumption of the NANOCOAT process with a conventional process.
In addition to the S&T results, a list of possible applications follows. Taking into account the additional energy requirements and capital expenditure for larger components, the process would be limited to niche or high performance applications where cost is not a consideration. Examples may include:
- optical applications,
- space applications,
- small aerospace components,
- specialised automotive components,
- high value consumer products.
WP 6 by KU Leuven: Functional and structural characterisation of anodised materials
1. Objectives of KUL WP
The main objective of the KUL WP was to characterise the coloured anodised aluminium oxide (AAO) samples delivered by C-Tech and ProMet. In particular, the following four sub-objectives were identified, namely:
1.1. To investigate the morphological surface structure of the AAO samples. A special attention was given to the detection of the presence of nanostructured, perfectly ordered pores.
1.2. To determine the layer thickness of the AAO samples obtained by making cross-sectional views.
1.3. To investigate the friction and wear behaviour at macro-loads of the AAO samples.
1.4. To investigate qualitatively the corrosion resistance of the AAO samples after a 1000 hours acetic acid salt spray (AASS) test.
Attempts have been made to correlate these characteristics to the anodising parameters employed.
2. Samples investigated and selected testing procedure
AAO samples were grown onto flat substrates made of four different Al grades, namely grade 1050 (C-Tech), grade 6082 (C-Tech), grade 5754 (C-Tech), and grade 2017 (Promet). These samples were investigated for their morphological surface structure, layer thickness, wear behaviour, and corrosion resistance.
In addition, AAO samples were grown onto flat substrates made of Al grade 5754 (C-Tech), and onto Sobinco door hinges and door covers, made of Al grade 6060 (C-Tech) and grade 5005 (C-Tech) respectively. These samples were investigated for their corrosion resistance. A reference door hinge and door cover anodised by Sobinco were included for comparison.
The AAO samples were produced under different anodising conditions in order to achieve a nanostructured surface morphology with a perfectly ordered pore distribution.
Prior to characterisation and testing, the as-delivered AAO samples were ultrasonically cleaned in acetone for 15 minutes, rinsed with ethanol, and dried in a vacuum chamber for 30 minutes at 3.4 mbar. In order to make the AAO samples conductive for SEM investigation, an Au layer was sputtered for 1 minute.
2.1 Morphological surface structure
The morphological surface structure was investigated using a SEM-EDX XL 30 FEG (FEI) at a beam voltage of 10 kV and a working distance of approximately 10 mm.
2.2 Layer thickness
Three different methods have been employed to obtain cross sectional views and hence determine the AAO layer thickness, namely:
First method: Mechanical deformation approach
It consists in clamping the flat coupons between jaws and on screwing inducing a progressive bending of the samples till 180° is reached or till breaking of the sample.
Second method: Metallographic approach
This method is based on classic metallographic procedure to obtain cross sectional views. It may be helpful in order to confirm the data obtained by the 1° method and to detect very thin top layers, or ductile ones.
It consists of the following steps:
- sample on its side embedded in resin,
- followed by mechanical grinding (22 - 14 -7 µm), mechanical polishing (Nap, 3-1 µm), and
- a final oxide diamond polishing (ODP, SiO, 0.1 µm).
The AAO layer thickness was then determined using optical microscopy.
Third method: Ion beam milling approach
This method is based on the ion milling in a SEM-FIB equipment leading to a cross section in a small area without any mechanical interaction as is the case in the first and second method.
2.3 Friction and wear behaviour
Reciprocating sliding wear tests at macro-loads were performed in Fretting II equipment,. The contact configuration was a ball-on-flat with an Al2 O3 ball as counterbody. After the sliding tests, the worn samples were ultrasonically cleaned in acetone for 15 minutes and rinsed in ethanol in order to remove the debris. The morphology and chemical composition of the wear track was investigated using SEM and EDX respectively.
2.4 Corrosion resistance
The AASS test chamber was prepared according to the ISO 3769-1976 standard. The test temperature inside the chamber was 34 - 35°C. The pH of the acetic acid solution was 2.9 - 3.0. Samples were placed inside the chamber at an inclination angle of 15°. The set-up of the salt spray chamber is shown Figure 4. Samples were rinsed with warm water and dried prior to visual inspection.
3. Collaborative work
- Flat coupons, Sobinco door hinges, and Sobinco door covers with different Al grades were anodised by C-Tech and ProMet. Details on those samples were provided in Section 2.
- Discussions among the industrial partners and RTD performers allowed to improve the anodising conditions in view of obtaining a nanostructured AAO surface morphology. Anodising parameters which had a significant effect of the final surface morphology and corrosion resistance include the imposed voltage, stripping step, pore opening step, sealing step, and dwell time.
- Progress reports regarding the outcome of the characterisation and test results were sent out by KUL.
4. Main achievements
4.1 Morphological surface structure, layer thickness, friction and wear behaviour
- An overview of the morphological surface structure, layer thickness, friction and wear behavior of AAO grown onto Al 1050, 6082, and 2018.
- An overview of the AAO surface morphologies achieved on Al grade 6082 after single step and double step anodising shows that a nanostructured morphology with a perfectly ordered pore distribution is obtained on Al grade 6082, under double step anodising without a pore opening nor a sealing step and in absence of a dye. Instead, a phosphate-like visual appearance is observed. Introduction of a sealing step under the same anodising conditions results in the evolution of a nanostructured, perfectly ordered pore distribution to a partially ordered pore distribution. In addition, a decrease in layer thickness and increase in running-in period is noticed.
- An overview of the structural and tribological features achieved on a nanostructured morphology with a perfectly ordered pore distribution. At low magnification, pitting phenomena and grain boundary effects induced by the anodising process are revealed. At high magnification, pore sizes ranging from 20 to 23 nm are observed. A coefficient of friction around 0.7 is recorded after 1000 sliding cycles, resulting in wear track depth of approximately 0.9 µm.
- An overview of the wear track depth versus the AAO thickness for Al 1050; 6082, and 2015. The broadest range of AAO layer thicknesses is achieved on Al 1050. Wear through is observed on Al 1050 and 6082. No direct correlation between AAO layer thickness and wear depth is found.
- An overview of the effect of anodising parameters on the morphological surface structure, layer thickness, friction and wear behaviour of AAO samples grown onto Al 1050 and 6082. For both Al grades, the anodising time, pore opening step, and sealing step during double step anodising have an effect on the structural and tribological features. In contrast, during single step anodising, the aforementioned parameters do not seem to induce a significant change.
- A comparison of the AAO surface morphology obtained after a ramp-up voltage process with and without dwell time. The imposition of a dwell time seems to result in a less pronounced pitting corrosion.
4.2 Corrosion resistance
Reference door hinges and covers treated by Sobinco have a superior corrosion resistance as compared to the door hinges and covers treated by C-Tech and ProMet. These reference samples only show a small amount of white rust spots and slight discoloration on both the front and at its edges.
A qualitative ranking of the AAO samples after 1000 hours AASS test has been made based on the amount of white rust spots. It is shown that the samples can be classified in three groups, namely lowest, intermediate, and highest amount of white rust spots.
Preliminary results indicate that the lowest amount of white rust spots is observed under the following conditions:
- single anodised stripped samples seem to exhibit a better corrosion resistance as compared to single anodised unstripped samples under the same anodising conditions for Al 1050,
- double anodising on stripped samples results in an improved corrosion resistance as compared to single anodising on stripped samples for Al 1050, 5754, and 6082,
- imposition of two minutes dwell time on single anodised unstripped samples result in an improved corrosion resistance as compared to the same samples without dwell time for Al 1050.
Imposition of dwell time seems to have different effects depending on the anodising conditions and Al grade used. Two minutes dwell time imposed on:
- double anodised stripped samples results in an inferior corrosion resistance, whereas on single anodised unstripped samples an improved corrosion resistance is observed for Al 1050,
- double anodised stripped samples does not seem to affect the corrosion resistance for Al 5754.
In general, AAO grown onto Al 1050 and 5754 seems to exhibit a better corrosion resistance than onto Al 6082.
5. Work in progress
AASS tests on door hinges and door covers anodised by ProMet and C-Tech have been in progress even after the project officially ended. These tests have terminated on January 14, 2013.
6. Conclusions
6.1 Morphological surface structure, layer thickness, friction and wear behaviour
- The majority of AAO samples produced under various anodising conditions exhibit a nanostructured morphology with only a partially or no ordered pore distribution. In addition, cracks and surface pits are often observed. The broadest range of AAO layer thicknesses is achieved on Al 1050.
- The coefficient of friction ranges from 0.6 to 0.9 after 500 sliding cycles on all Al grades. Lower wear resistance on Al 1050 is observed. No direct correlation between AAO layer thickness and wear depth is found.
- Only one nanostructured morphology with a perfectly ordered pore distribution (range of pore sizes : 20 - 23 nm) has been obtained on Al grade 6082. This particular morphology with a phosphate-like appearance is produced under double step anodising without a pore opening nor a sealing step and in absence of a dye. However, surface pitting phenomena and grain boundary effects induced by the anodising process are revealed. A coefficient of friction around 0.7 is recorded after 1000 sliding cycles, resulting in a wear track depth of approximately 0.9 µm.
- No general correlation of structural features to synthesis parameters has been found due to too wide variation of anodising parameters used. However, preliminary results show that:
- the introduction of a sealing step under the anodising conditions mentioned in the previous paragraph, results in the evolution of a nanostructured, perfectly ordered pore distribution to a partially ordered pore distribution, a decrease in layer thickness, and an increase in running-in period on Al 6082,
- the imposition of a dwell time during a ramp-up voltage process results in a less pronounced pitting corrosion on Al 1050,
- the anodising time, pore opening step, and sealing step during double step anodising have an effect on the structural and tribological features. In contrast, during single step anodising, the aforementioned parameters do not seem to induce a significant change for both Al 1050 and 6082.
6.2 Corrosion resistance
- Reference Sobinco door hinges and covers have a superior corrosion resistance as compared to the door hinges and covers anodised by C-Tech and Promet.
- A qualitative ranking of the AAO samples after 1000 hours AASS tests has been made based on the amount of white rust spots. No general correlation of corrosion resistance to synthesis parameters has been found due to too wide variation of anodising parameters used. However, preliminary results indicate that an improved corrosion resistance is achieved by:
- stripping of single anodised samples on Al 1050,
- double anodising of stripped samples on Al 1050, 5754, and 6082,
- imposition of dwell time on single anodised unstripped samples on Al 1050.
Imposition of dwell time seems to have different effects on the corrosion resistance depending on the Al grade and anodising conditions employed. On Al 1050, the dwell time improves or deteriorates the corrosion resistance depending on the anodising conditions. In contrast, on Al 5754 the dwell time does not seem to affect the corrosion resistance.
In general, AAO grown onto Al 1050 and 5754 seems to exhibit a better corrosion resistance than onto Al 6082.
Potential impact:
Expected impacts at the European and/or international level
The technology studied during the two years in the NANOCOAT project has clear market potential and will have a strong impact on the economic prospects the participants via two routes):
- The SME participants will use the technology directly in their own manufacturing operations and/or directly in the services they provide.
- The SME participants will market the technology through process licensing to other manufacturing organisations (via product type, market area, geographical region)
In the latter case, this will lead to improved industrial competitiveness and improved employment across the EU. The surface finishing sector is widely distributed and approximately EUR 30 billion (30 x 109) of finished value product is dependent upon its capabilities. In Germany, there are about 2100 businesses in the surface finishing industry with about 56,000 employees and a total market volume of EUR 4 billion (4 x 109). The total size of the market in France has been estimated as EUR 6 billion. The added value of the sector is much higher, because metal finishing accounts for only 5 % of the total value of the product. Therefore the estimated total added value in Germany, for example, is EUR 80 billion (4 % gross domestic product of Germany).
Anodised aluminium products are established in the following industrial application areas:
- Anodising has been used in building construction for more than 60 years. Anodising gives the safest and hardest architectural finish currently available. Anodised aluminium is easy to clean and requires little maintenance.
- Coating is the process of covering the aluminium surface with a suitable film (layer) made of organic compounds, typically powder or wet paint. Many aluminium components may be painted for various reasons, starting with decoration up to protection against specific environmental influences.
- Decorative finishes are available through metallic finishes and a wide variety of colours. Anodising offers a large increasing number of gloss and colour alternatives and minimises or eliminates colour variations. Unlike other finishes, anodising allows the aluminium to maintain its metallic appearance.
- Adhesive bonding was developed by the aerospace industry. It produces a porous oxide that is used mainly as a pretreatment for preparing adhesively bonding aluminium structures in the aerospace industry and to some extent in automotive. The anodised coating is used as an adhesive bonding primer coat on aircraft and aerospace alloy sheets. This is an excellent surface for the epoxy adhesive and also improves corrosion resistance.
The specific anodising markets to be addressed during the project are concerned with improved functionality for traditional anodisation processes and new coatings.
Improved functionality of existing coatings will be explored in the following large market areas: corrosion and abrasion resistance, adhesive bonding, anodised base for organic coatings / painting, coloured architectural finishes.
The statistics for the market size for existing industrial applications are available from the Association for European Surface Treatment on Aluminium (ESTAL):
- anodised Al: 279 million m2 or 828 thousand tonnes,
- anodised and Coated Al: 441 million m2 or 1.22 million tones.
The construction industry accounts for 59 % of anodised products and 78 % of anodised and coated products. Other applications are: automotive and transportation; aerospace; industrial products; electronic equipment; furniture; household appliances and decorative design; sports and leisure; packaging. ESTAL represents 452 plants with 15 000 employees in SME manufacturers.
Within the surface treatment industry as a whole, the conversion of aluminium represents about 15 % of the market including around 10 % for anodisation alone i.e. EUR 4 billion. The European aluminium surface treatment market grew by 25 % between 2000 and 2005.
New application markets will be assessed, which are based on the enhanced performance of nanostructured coatings. As conventional materials are reaching the limits or their performance and industrial components are subjected to increasing demands in terms of mechanical performance (speeds, loads, temperatures), many industrial manufacturers are facing difficulties taking the 'next technological step'. There are many difficulties to overcome to solve the limitations of current systems.
Surface treatments that can be easily and economically produced by electrochemical techniques (aluminium anodisation) will have an advantage over vapour deposited coatings for many industrial sectors. Price, size and robustness of the production process are key elements to reach an economically viable product. Some examples of new market applications are given:
- improved lubricity,
- anti fretting,
- tribocorrosion surfaces,
- a specific application is the improvement of emissivity of black anodisation layers obtained by absorption of Ni and/or cobalt sulphides for space applications.
Nanomaterials? They measure from 1 to 100 nanometers, are smaller than a virus, but bear huge promises for industry, but also energy, health or multimedia areas.
For more than a decade, nanomaterials have entered our daily life, but here the goal was to know more about some industrial applications: the project results are expected to have a significant impact on the competitiveness and economic performance of the small and medium-sized enterprise (SME) participants by expanding existing markets and developing new applications. The project outputs will demonstrate clear market potential and will address specific technology market demands for improved functional anodised aluminium materials derived from nano-structured surface coatings.
Conventional processes for anodised aluminium oxide products are well established across the range of market applications. Standard, long-established manufacturing technologies are in use throughout Europe.
Anodising aluminium is a proven industrial manufacturing technique for improving corrosion resistance, increasing material strength and introducing cosmetic finishes such as metal colouring. Anodic aluminium Oxide (AAO) industrial products include:
- decorative or coloured Al surfaces: sealed colourless; electrochemical or chemical colouring,
- anodised base for organic coatings: liquid or powder painting processes,
- corrosion resistance: hard anodising to produce hard films on Al and its alloys with excellent abrasion and corrosion resistance,
- adhesive bonding: porous coatings as primers for surface bonding Al.
The introduction of markedly different technological approaches, which offer significant enhancements in properties and therefore provide differentiation from existing products, will create new market opportunities. The proposers consider that the innovative templated anodisation processes in this research will provide the necessary product differentiation.
The output results will be implemented by the surface finishing industry. The impact of the sector is very broad, because of the wide range of products that play a critical role in industries of great strategic importance to the European Union (EU). Examples include: civil and military aerospace and maritime, construction, industrial hydraulic applications, automotive, construction, chemicals, food etc.
Companies carrying out these large manufacturing activities in important industry sectors are all customers of the SME manufacturers that form the vast majority of the surface finishing / engineering sector.
Any important impacts on the European surface treatment industry are highly significant to Europe as the sector employs more than 500 000 persons for a total turnover of EUR 40 billion. This industry has over 30 000 SME members in Europe, with the majority employing less than 20 people. The vast majority of these companies are also categorised as SMEs in terms of turnover. The biggest group (53 %) has an annual sales volume of between EUR 1 and 5 million.
The industry is broadly distributed across all EU countries. In the vast majority of cases, the types of processes carried out are largely the same wherever they are applied. Typically, SMEs produce components that are sold to end user companies, often very large, who incorporate these components into final products.
The result is that there are no significant geographical barriers. Also, competition between SMEs tends to occur relatively locally, often within single countries, so there are few geographical inter-regional competition issues acting as barriers between SMEs.
This SME intensive industry sector is under increasing competitive pressure from the global market, particularly competition from regions with lower intrinsic costs and less demanding legislative controls. In the context of this highly competitive environment, there is a clear need for new technology to produce higher specification and added value products.
Hence, there is a strong need for SMEs in general to create their own intellectual property and develop new products with a clear economic impact leading to increased competitiveness.
Science and technology (S&T) objectives
The overall aim of the proposed research is to develop improved functionality and new coating applications for aluminium anodisation products in surface engineering.
The project will advance recent research innovations in research laboratories at research and technological development (RTD) participant Armines towards the preparation of ordered nanoporous, nanostructured materials using anodisation methods and applying the methods to develop commercially relevant novel materials.
The proposed research will aim to address the market potential for these new and improved materials.
The expertise and capabilities provided by the project participants will ensure that the following S&T objectives can be realistically achieved and verified within the project timescale:
- research the techniques for producing nano-structured anodised aluminium materials under industrially relevant conditions,
- develop process protocols including pre-treatments and post-treatments,
- scale-up processes for the treatment of larger areas and adapt processes to treat complex-shapes,
- design and construct an experimental prototype system,
- operate the prototype system over an extended period and address scale-up issues,
- use industrial grade aluminium alloys, in addition to pure aluminium,
- carry out full structural characterisation of materials,
- carry out functional characterisation to evaluate the performance of materials across a range of existing and potential industrial applications,
- consider production issues relating to environmental legislation and apply a simplified life cycle analysis (LCA) methodology,
- complete a full techno-economic assessment of the project outputs,
- develop a plan for exploitation and dissemination.
Project results:
The anodisation of the aluminium is well known as an industrial process used in many applications in which, subsequently at an electrochemical oxidation by anoding process, it is possible to increase the thickness of the alumina layer naturally present when aluminium is in contact with the oxygen (and water) present in atmosphere.
Objectives of the anodisation processes are essentially devoted to anti-corrosion, decorative, wear resistance purposes. The formation of oxide on the surface of the aluminium protects the metal: the oxide layer, even it is porous, presents a good resistance to the corrosion due to a barrier layer formed at the interface of aluminium metal and oxide layer. The properties of this oxide layer could be improved by controlling the aluminium anodisation process and by using specific parameters.
In particular, important research works have been done since about twenty years to control the layer formation to obtain a special structure which is interesting: the anodic aluminium oxide grown in hexagonal nano-structure, well ordered, if parameters of anodic oxidation are well controlled. This growing is linked also to the nature of the aluminium substrate: pure Al, Al alloys, Al with inclusions.
The goal of the first work package (by Armines) is to determine and define the parameters of the process for each nature of Al substrate to obtain a regular structure of the anodic aluminium oxide depending on aluminium composition and geometrical form.
In particular:
- to define and select operating parameters to generate porous layers with regular arrangements (nano well-ordered structure) over the widest range of surfaces and substrate,
- to define the operating parameters to transfer the results at an industrial level.
1. Anodisation techniques
1.1 Marking and double anodisation
Two methods can be used to initiate the arrangement of the structure: pre-texturing or double anodisation method. Both goals are the same: creating a network of nano-cavities on the surface of the substrate which are the 'germs' of the nano well-ordered structure.
- The pre-texturing method consists in engraving the surface of the substrate using a gauge to pattern the surface following the wanted structure inscribed in a 'negative' form compare to the structure to be obtained. The pattern corresponds to the cell size of the future ordered nanostructure.
- The double anodisation method consists of making a first anodisation to start the ordered structure as the scheme shows the principle. This first oxide layer created is completely dissolved in acid solution. The oxide layer is completely removed of the aluminium substrate and lives on the surface marks of the ordered structure. The second anodisation starts from theses marks and grows in a nano well-ordered form.
In the course of the NANOCOAT project, we used only the double anodisation method because it is the method which allows obtaining large nanostructured surface, the pretexturing method is somewhere limited in area and is only used to engrave surfaces equivalent to the surface of the mold.
In the case of double anodisation process, the first anodisation layer (oxide layer) is dissolved in an etching solution of 6 % H3PO4 and 1.8 % CrO3.
2. Laboratory equipments
The electrolyte is in a double wall glass tank with an approximately volume of 1.5 litre. As the anodisation is an exothermic reaction, the electrolyte has to be cooled to control the temperature of the solution. The coolant is on the external part (blue on the picture): an external cooling system allows regulating the temperature of the cooling liquid at a fixed value which is linked to the wanted temperature of the electrolyte. The nature of the electrolyte studied is variable and described in next paragraph.
This equipment is sufficient for small samples but for larger one, the volume of electrolyte has been increase to about 10 litres.
This configuration with a larger volume allows the possibilities to realise anodisation of larger samples even a higher power is necessary (same voltage but higher intensity), without temperature variations during the anodisation process (increasing temperature by Joules effect).
3. Process to obtain anodic aluminium oxide ordered
3.1 Samples
In a first time, we used only pure aluminium samples from Goodfellow (samples bought to Goodfellow Company), with a purity at 99.999 % (ref: AL0006920) in sheet of 1 mm thick.
To obtain a regular structure, these samples have been polished mechanically with SiC abrasive paper from P640 to P4000. Then the surface is polished with fibre plate and diamond solution from 3µm to 1µm.
3.2 Etching
Before anodising, the aluminium sample is etched by phosphoric acid at 10 % during 30 minutes to remove the native oxide layer.
The roughness of the surface has been monitored and standard results obtained by mechanical profilometry measurements are:
- starting aluminium: Ra = 240 nm - 300 nm Rz = 4.3 µm
- after polishing: Ra = 60 nm - 90 nm Rz = 1.5 µm.
3.3 Anodisation: Influence of the electrolyte
Following our experience and a literature survey, it appears that the nature of the electrolyte is one of the criteria which define the size of the pores.
Literature indicates that three different types of electrolytes have been investigated and could be used: sulphuric acid, oxalic acid and phosphoric acid.
Tests with each of recommended electrolyte have been conducted during the first part of the programme.
The structure is ordered and the size of the pores is around 25 nm in diameter. The interpores distance is about 60 nm. Thickness of the oxide layer after 2 hours is approximately 6 µm.
The structure is ordered and the size of the pores is about 35 nm in diameter and about 95 nm for interpores distance. Thickness of the oxide layer after 2 hours is approximately 16 µm.
In the case of phosphoric acid, the structure seems to be less ordered but the pore size is larger: around 120 nm in diameter for pores and around 150 nm for interpores distance. Thickness of the oxide layer after 2 hours is approximately 8 µm.
When phosphoric acid electrolyte is used, it has to be noted that the recommended value for voltage between anode and cathode is around 190 V. But due to our rectifier characteristics, it is impossible to increase the voltage to this high value and the voltage has been limited at about 160 V (maximum of our rectifier).
3.4 Anodisation: Influence of the double anodisation
After the first anodisation process, the nanoporous structure is created but the pores are not well ordered neither well define. The pores seem to be unevenly present on the surface of the structure and the size of the holes and cells are different.
3.5 Anodisation: Shape of the subtract
Usually the sample comes from a sheet of pure aluminium. Trying to study influence of the sample geometry, anodisation of a small cylinder (6 mm diameter, length 7 cm) of the same aluminium purity (99.999 % Al) from Goodfellow Company has been realised. This anodisation process on pure aluminium cylinder has been realised in an oxalic acid solution, using same parameters as for flat samples. The aluminium oxide layer grows with an ordered structure as for flat samples after a double anodisation process.
As presented in pictures 12, annex I (SEM examination at different magnification) formation of a well-ordered structure on surface of cylindrical samples (diameter 6 mm) is not influenced by the shape of the substrate.
3.6 Anodisation: Influence of the purity of the aluminium
Influence of the substrate purity has been studied on aluminium alloy (AG3-5754) compared to results obtained on pure aluminium.
Structure on AG3-5754 is more irregular than for pure aluminium sample even an ordered structure appears in the alumina obtained on these alloys: some perturbations are present in the network of the structure. The presence of foreign atoms in the alloys disrupt the formation of alumina column and so the formation of the nano well-ordered structure.
As for alloys, impurities (precipitate) included in the aluminium influence really the arrangement of the structure.
The influence of inclusions in the substrate has already been investigated during the PhD research works of Pascal Thomas in EMSE Laboratory (2008). Results shown that these impurities give perturbation in the formation of the well-ordered structure: due to the nature of impurities, formation of alumina structure is impeded and so some defaults appears in the ordered structure.
3.7 Anodisation: Aluminium alloys
One test has been made with an aluminium alloys (aluminium series 2XXX) which contains approximately 4 % of copper. In this case, the well-ordered structure is not present. However, an adjustment of parameters could improve the quality of the structure and the determination of well-ordered parameters will be discuss in following paragraphs.
3.8 Anodisation : Influence of the surface preparation
Following our experiments during the WP1, surface preparation is important to obtain well ordered structure.
Surface of samples has to be degreased (acetone + water + ethylic alcohol) and polished before first anodisation. The necessity of polishing is important as growing of pores occurred perpendicularly to the surface and so if the surface is too rough, pores cannot grow in an ordered manner.
Mechanical polishing and electrochemical polishing have been compared and results appear to be equivalent. This is due to the fact that during the process of double anodisation, the first step (first anodisation) removes the strain-hardened layer created during mechanical polishing. There is no real difference between the two types of preparation.
However, voluntarily, no solution using chloric, or perchloric acid and/or other chemical hazardous components has been tested and used during the programme as industrial applications would be hazardous
3.9 Anodisation: Influence of the temperature
Previous experiments (Cf PhD thesis of P. Thomas) have evidenced that the optimal temperature to obtain ordered structure is between -3°C and 20°C. When the temperature increases it occurs a degradation of the nanostructure between 30°C and 40°C (for oxalic acid) and at a temperature higher than 40°C the structure is clearly disordered.
During the NANOCOAT project, more experiments have been done to define the exact influence of the temperature, in particular using different solution and substrate composition.
3.10 Pores opening
The size of the pores could be enlarged by dissolving a part of the wall of cells. The dissolution of the aluminium oxide is made by etching the sample in phosphoric acid.
3.11 Coloration of the anodic oxide layer
The aluminium anodic layer could be coloured by filling porous structure by different solution or by electrolysis. Some tests have been done in cooperation with the NANOCOAT partners ProMet and C-Tech.
4. Anodisation parameters definition
During a meeting of the RTDs performers (Gardanne (France) in November 2011), it appears that some complement of information from industrial partner Promet are important due to the anodisation parameters, more particularly on the electrolyte and on the voltage control.
Promet makes industrial hard anodisations by using the already defined high concentred electrolyte composed by mixing sulphuric acid (2.2 M) and oxalic acid (0.2 M) at a temperature of -3 to 0°C.
More, the process itself is slightly different as for industrial reasons (too important intensity at the beginning), voltage of the anodisation process is increased from 0 to approximately 40 -50 V in 5 minutes then a stable period is running for 1 hours, anodisation is ending by an increase of the voltage to 60 V. At the laboratory scale, the voltage is increased very rapidly and maintained constant along the anodisation process.
For the final application it will be interesting to know exactly the differences on the final layer.
But as a results of this meeting, it was decided that the anodisation solution which will be used in the future will be the Promet solution: 220 g/L (2.2 M) of H2SO4 and 15 g/L (0.2 M) of COOH.
4.1 Specific electro formulation
As shown in the previous paragraph 4.3 the diameters of the pores depend on the nature of the electrolyte. The table here after summarise the composition of the different electrolyte used during the NANOCOAT project.
4.2 Operating protocol
Investigations to define the better condition of anodisation have been made and this protocol will be used for all samples produced in laboratory conditions.
- Samples (2 x 6 cm) are taken from pure aluminium plate (Goodfellow supplier).
- Surface preparation is realised by polishing the sample with different paper of SiC and diamond solutions to obtain mirror surface without scratch.
- Before anodisation, the sample is degreased with acetone and etched in a H3PO4 (10 %) solution at room temperature during 30 minutes.
- The voltage is progressively increased to the value wanted without limitation of current.
- The time of the first anodisation has been fixed at 3 hr to be sure that the structure is well ordered.
- After the first anodisation, the sample is rinsed and the oxide layer is dissolved in a solution of 6 % H3PO4 and 1.8 % CrO3 during 2 hours to remove completely the oxide.
- The sample is then rinsed again and dried: so it is ready for the second anodisation.
- The second anodisation is made using the same protocol for electrolytical parameters.
- The duration of the anodisation process is defined in accordance with the oxide thickness intended.
4.3 Industrial application
Tries for application of the Laboratory parameters to industrial hard anodisation have been made in the Promet premises. During common tests made in May 2012, double anodisation process has been tested in an industrial environment on pure aluminium sheets (A5 format).
1. The first sample (S1) has been hanged on a titanium jig (Promet system) for the first anodisation procedure:
- Surface preparation: Etching in soda solution, acid etching in proprietary (Promet solution - rinsing after each operation.
- Anodisation in a tank (1000 L) solution (- 3° C): 220 g/L H2SO4 - 15 g/L Oxalic acid at 20 V during about 40 min (ramp of current) - 2 to 3 A/dm².
- Coloration in mineral black solution (commercial)
-Result: Colouration of S1 seems to be 'pale' on about half of the surface due to an insufficient thickness of the alumina layer.
2. Definition of the operating procedure:
- Dissolution of the first anodisation for the first sample S1 second anodisation of first sample S1.
-In the same time and operation, first (and single one) anodisation for the second sample (S2) same conditions as previously except: duration 100 min voltage is increasing from 20 V to 40V progressively.
- Coloration of the two samples (single - S2 and double anodisation S1) in mineral black solution.
- Sealing in a proprietary solution (boiling aqueous solution with acetate salts).
Results and report made during these tests are presented in the D6.2 deliverable.
Following some difficulties in the realisation of the anodisation in Promet company (essentially due to the new formulation of the electrolyte: mixed of sulphuric and oxalic acid), tests in the same condition (using the industrial bath) have been made in Laboratory of EMSE / Armines.
These different results demonstrate that it is possible to obtain a well-ordered structure on pure aluminium with an industrial electrolyte. Tests to improve the process using sulfo-oxalic acid have been realised. Results rae presented in the following paragraph.
5. Improvement of the anodisation parameters
After definition of the electrolyte composition, the others parameters having an influence on the anodisation process have been investigated.
5.1 Improvement of the anodisation parameters using industrial solution (sulfo-oxalic acid) (pure aluminium)
As a general point-of-view, during the first anodisation, the internal evolution of the structure becomes homogeneous after a lap of time (not necessary well-ordered). So by examination of the structure obtained after a first anodisation it is possible to define if the 'stable' structure obtains with the parameters used during the anodisation procedure allows the formation of a nano well-ordered structure.
By examination of the remaining structure obtained on the aluminiuum substrate after dissolution of first alumina layer, it is possible to know the conditions for obtaining the best well-ordered structure. The following schema indicates the procedure the first anodisation.
If general conditions are correct and well defined, the structure obtained at the end of the first anodisation would be well-ordered. The second anodisation following the structure 'printed' by the first anodisation, will induce a well-ordered structure. This method allows determining the parameters of anodisation.
The protocol we used to characterise the anodisation parameters is:
- polish the aluminium,
- make a first anodisation,
- dissolve the aluminium anodised layer,
- examination of the surface by SEM.
5.2 Determination of the better potential on pure aluminium
Following first experiments, it appears that the most important parameter is the voltage between aluminium (working electrode / anode) and counter electrode (cathode). However, we have studied also the influence of temperature.
- Influence of voltage
Following these results, the best potential is assumed to be 35 to 45 V, but due to the joules effect and cost reason, the potential used will be 35 to 40 V. This method of 'ordered-potential' determination will be used in the future for industrial aluminium alloys.
Same experiments have been used for the determination of the best temperature. As a result, the best temperature has been defined to be -3°C to 0°C.
5.3 Influence of the temperature of the electrolyte
The samples made a 20°C shows a nano-structure but not a nano well-ordered structure compare with samples made a 0°C.
5.4 Determination of the better potential on aluminium alloys 6082
As previously indicated, observation is made on the surface after dissolution of the alumina layer.
A high potential is also requested to obtain an ordered structure. This conclusion has been verified by realisation of double anodisation on aluminium alloy (6082). The structure appears nano well-ordered, even if it is less ordered than on pure aluminium.
5.5 Determination of the better potential on aluminium alloys 2024
During these experiments on 2024, a nanostructured alumina layer has been obtained but it is not well-ordered as for pure aluminium or 6082 alloy.
5.6 Ordered structure on other aluminium alloys (6060 and 5005)
These tests have been made on parts from Sobinco company which is in contact with a partner of NANOCOAT project the Falex Tribology NV company. Information on this Sobinco company will be provided by Falex Tribology.
1. Tests in EMSE / Armines laboratory
The company Sobinco is interested by realisation of well-ordered nanostructure on aluminium alloy parts. The interest for this company is to increase the wear and corrosion resistance of parts used in building in particular for decorative application.
For testing the process, Sobinco company has sent to some partners (C-Tech and FALEX) some parts. The parts received are part of door: handle part (alloy 5005), door hinge (alloy 6060)
The objective is to obtain a nano well-ordered nanostructure on the industrial samples received. Sobinco has not been able to give us samples without anodisation process. So the first operation has been to dissolve the alumina layers on the samples before testing our anodisation process. This dissolution has been made by the method currently used, using a mixed of phosphoric and chromic (CrVI) acid.
The operating procedure for testing double anodisation process is the following:
- dissolution of the industrial (Sobinco) alumina layer in H3PO4 + CrO3: 3 hours at 55°C,
- first anodisation in H2SO4 (220 g/l) + (COOH)2 (11 g/l) at 40 V,
- dissolution of the layer in H3PO4 + CrO3 at 55°C,
- second anodisation in H2SO4 (220 g/l) + (COOH)2 (11 g/l) at 40 V,
- colouring: using C-Tech method.
During this study, by using 40 V as potential difference between parts and cathode, it appears that the rate of formation of the alumina layer is greatly improved. This rate of formation is about 7 to 10 µm/min instead of 0.5 to 1 µm/min in the best case for industrial process.
This rate of formation has been verified by Promet in his jobshop: confirmation of the improvement of the rate of formation using a higher voltage than classical process. This increase in rate of formation was anticipated but the high deposition rate value is interesting for industrial application.
Due to the high voltage inducing high current intensity particularly at the beginning of the anodisation process, the number of parts treated in one operation (on one jig) is reduced. But as the rate of formation of alumina layer is greatly improve (x7 or x10), it is certainly possible to treat more parts during the same time used for previous treatment at lower voltage.
Tests are in progress to complete and to certify these results.
As conclusion, by using a double anodisation, it is possible to create a well-ordered structure on industrial part (and alloys). The impurities of the alloys have a large influence on the arrangement. This difference in composition explains why the structure of the alloy 5005 is better ordered than the alloy 6060.
2. Tests in Promet lab and workshop on door parts from Sobinco
Following experiments and results obtained in EMSE / Armines laboratory, Promet has tested the process on door hinge (6060). Parts anodisation has been realised by manual and automatic system (driven by PC system) and characterisation of the results have been made by SEM examination after dissolution.
Results show that it is possible to realise well-ordered alumina structure in industrial partner who confirms also that the rate of formation is higher than expected.
6. Titanium anodisation
By using the same parameters defined for the anodisation of the aluminium, sample of titanium have been treated. An oxide layer has been formed on the surface of the titanium but without a real ordered structure. A porous structure is visible but it has not been possible to create a well-ordered structure.
At the end of NANOCOAT project, an interesting paper has been found following our bibliography survey 'Electrolyte influence on the anodic synthesis of TiO2 nanotube arrays' Ref: Journal of Non-crystalline Solids 354 (2008) p.5233-5235 Authors: V. Vega et al.
The electrolyte solutions which have been tested by the authors contain hydrofluoric acid. Due to the lack of time, experiments and tests have been not continued on titanium alloys.
7. Microhardness and scratch test
Evaluation of the microhardness of the alumina layers has been made by Vickers indentation method.
8. Conclusions
The double anodisation creates the ordered structure and the alumina well-ordered structure depends on a specific voltage.
During the NANOCOAT project, following industrial proposal, the tested solution selected and used has been H2SO4 (220 g/l) + (COOH)2 (11 g/l) (PROMET formulation), and it has been possible to obtain a well-ordered structure on pure aluminium and some aluminium alloys, except for alloy 2XXX series certainly due to the high concentration of copper in this kind of alloys. The oxidation potential of copper appears to be the reason of the lack of success for this kind of alloys. These series of aluminium are well known to have a special comportment in anodisation processes.
Following the demand of Falex Tribology NV company, some parts from a Belgium company specialised in the furniture of aluminium parts (domestics) (Sobinco) have been tested: it has been possible to realise well-ordered alumina structure on these kind of industrial alloys and industrial parts.
The method developed during the project has been tested by industrial partners (Promet) and results obtained at laboratory scale have been reproduced in industrial environment. Results are innovative as to our knowledge it is the first time that well-ordered alumina structure have been realised on aluminium alloys (5XXX and 6XXX series).
Black coloration (without opening pores) appears uniform and attractive after the double anodisation. Microhardness of the alumina layer remains constant after simple or double anodisation: formation of pores through the alumina layer do not modified the penetration of the indenter.
Results obtained by nanoscratch tests are non consistant and the method has to be re-considered as it gives too macroscopic results in front of the nanostructure
Work packages (WPs) 2 and 3 (C-Tech)
Key WPs that have been instrumental in testing the industrial viability of the new process were WP2 (Laboratory scale process design and development) and WP3 (Process scale-up), which were intended to be part of a three-step scale-up progression from the original research and technological development (RTD0 studies through to the industrial SME production lines.
RTD work that demonstrated the concept of nano-structured anodised aluminium oxide coatings and inspired the present work, focused on the ability to achieve the structures at the laboratory scale, using equipment not optimised for the industrial environment. C-Tech therefore set about constructing a system designed to emulate a realistic and flexible but reduced version of the industrial anodising lines used by the SME manufacturing partners Promet and Ashton & Moore, complete with all of the equipment necessary for pre- and post-processing parts to be anodised. The stage allowed the process to be studied in a realistic environment while not encroaching on the day to day operation of the full-scale anodising lines. Without the construction and operation of the pilot system, the study would not have been possible.
Initially this progression was intended to be done with an initial scale-up from the lab-scale of the anodising tank to a lab-scale version of the full anodising line, followed by a larger, optimised pilot scale system. At the outset, however, it was clear that in order to recreate the industrial process at the first scale-up stage, the balance of plant necessary to achieve practical targets (e.g. process heating and cooling, agitation) constituted a key part of the process, and that equipment size was not necessarily as important. It was therefore deemed more practical and efficient to amalgamate the laboratory and pilot systems, and concentrate efforts into constructing a high-quality transferrable system in two intermediary steps from lab-to-pilot-to-production.
In general, the S&T aims from WP2 were designed to develop a series of protocols for the process that would be key to the successful transfer of intellectual property (IP) and achieve high quality nanostructured anodised oxide coatings at the industrial scale. However, careful consideration of all process steps was necessary in order to assess the viability of operating the process at this industrial scale, and included:
- surface preparation, including cleaning and chemical etching for different applications,
- anodising, including all electrical and physical operating conditions necessary to achieve the desired results (e.g. current, voltage, temperature, electrolyte composition, agitation requirements,
- all ancillary and downstream processing stages, including rinsing, sealing and colouring for different applications
Accordingly, for WP3, the S&T aims were designed to achieve the necessary scale-up towards an industrial level, with a detailed analysis of the anodising process and all of the necessary ancillary equipment that are normally associated with the anodising step, including:
- pre- and post-treatment tanks for cleaning, etching, sealing and colouring,
- rinse tanks, individual to each step in the process to avoid cross-contamination,
- anodising tank, complete with ancillary cooling and agitation systems, and power electronics and control system.
In detail, the results in accordance with the S&T objectives have included the following:
- Develop process protocols including pre- and post-treatments
The starting points for the development of process protocols have been two-fold:
Standard anodising processes
Ashton & Moore and Promet supplied key information about their existing anodising processes, the process steps and their operating conditions, the materials and equipment involved, and product applications and their individual tribological requirements. Falex Tribology also provided important information about the tribological tests that the materials must undergo in order to quantify the performance and compare directly with conventional anodising.
C-Tech has actively researched anodising, both from an industrial perspective and academic studies. Current practices in all aspects of industrial anodising, including the different alloys involved, has been reviewed through standard literature:
- Sheasby and Pinner, 'The surface treatment and finishing of aluminium and its alloys', ASM International and Finishing Publications Ltd., 2001 (6th Ed.)
- Canning, The Canning Handbook Surface Finishing Technology, W. Canning plc, 1982 (23rd Ed.)
- Metals Handbook, Vol. 2, ASM International, 1990 (10th Ed.)
As a result of this background research, C-Tech drew up a preliminary flow sheet showing each individual step of a proposed prototype, in which the NANOCOAT process was to be scaled up and tested in a fully industrialised environment. Many aspects of the NANOCOAT process were found to be identical to those found in current industrial anodising plant, including the pre- and post- anodising treatment steps. However, these steps needed to be included in the prototype system to keep the process comparable. The preliminary flowsheet was reviewed by all partners before the construction process.
A final process flowsheet was produced at the end of the project, following lengthy operation of the prototype, including some retrofitting and optimisation of the equipment and the operating conditions. This iterative process involved providing anodised samples to KUL and Falex for characterisation, and comparison with conventionally anodised samples. A final set of conditions has been presented in the final flowsheet, in the deliverable report D3.2 - Operating procedures and final process flow sheet.
- Design and construct an experimental prototype system
The initial strategy for the scale-up process was to produce a small lab-scale system where initial samples would be anodised to compare the results with initial work carried out at Armines and to address possible scale-up issues for a larger prototype system. However, during the early investigations of how the laboratory and prototype systems would be constructed, it was clear that the minimum practical size of anodising tank for a meaningful laboratory study would be large enough to be considered a prototype. Smaller systems of just a few litres would be difficult to practically agitate using the planned system, and to control at a steady state, and there would be few differences from the system operated by Armines. Also, scaling up twice from a small prototype to a larger one, and then to industrial scale would serve little purpose, as the same scaling issues would need to be considered twice. For these reasons, a single lab / prototype system was built to develop the process before transferring to an SME onsite demonstration.
The prototype system, contains all necessary equipment to prepare and anodise small industrial scale parts up to around 20 x 20 x 10 cm, from the initial cleaning and etching stages through to colouring and sealing.
- Operate the prototype system over an extended period and address scale-up issues
The prototype system was operated over an extended period, looking at critical operating parameters such as anodising power, temperature and time, heat and mass transfer by agitation, removal of heat from the system, and practical issues of anodising twice with an intermediary stripping process.
The traditional method of agitation is by bubbling air through the tank. This has the effect of moving hot electrolyte away from the components, replacing it with cooler liquid from the bulk of the tank. It also helps to disperse gas and replace ions lost in the oxidation process. The NANOCOAT process however, uses voltages and currents much higher than normally used, producing more heat and affecting the ability to anodise and reducing product quality. The effect of increasing agitation by higher air flow through the liquid would have negative effects, by increasing evaporative losses and acid spray from the tank, and energy losses from additional throughput.
A more efficient method of agitation was tested using eduction, where fluid is pumped around an external circuit and back in through the eductors, which are strategically placed within the tank. Since the electrolyte is circulated through an external heat exchanger to cool, the eduction system could be incorporated into this circuit. The eductors improve efficiency, since only one fifth of the fluid directed towards the components needs to be pumped around the circuit. The remainder is drawn from the tank through the sides of the eductor and out with the pumped fluid.
Computational fluid dynamics (CFD) was used to model the circulation system to find the best configuration of flowrate, number and position of eductors.
As the optimisation process drove the move to more aggressive electrical conditions (40V; 860 mAcm-2), anodisation of larger parts became impossible due to the limitations of C-Tech's rectifier. This prompted the construction of an in-house custom-built system capable of operating at much higher currents. However, further issues arose when attempting to achieve 40V from the start, which resulted in a runaway current and again limited by the power supply.
A control system was added to allow accurate ramped or stepped control of the voltage. This allowed an initial period at slightly lowered voltage to promote the anodisation process, before climbing to the desired set-point of 40 V.
Typical conditions for the optimised anodisation follow:
Anodisation 1: Initial voltage - 25 V; ramp to 40 V over 60 s; hold at 40 V for 2 min
Anodisation 2: Initial voltage - 32 V; ramp to 40 V over 18 s; hold at 40 V for 2 min
This produced a coating of 34 µm (±2 µm).
- Use industrial grade aluminium alloys, in addition to pure aluminium
Initial operation of the prototype system involved anodising pure aluminium to emulate preliminary work performed at Armines and compare the resulting coatings. Subsequent operations involved anodising different alloy more appropriate to industrial applications. 2000 series (high copper content) proved particularly difficult to anodise, producing little or no coating, and so efforts were concentrated on 5000 and 6000 series, common industrial grades.
- Scale-up processes for the treatment of larger areas and adapt processes to treat complex shapes
A number of measures were taken to attract a supplier or end user of aluminium products, including advertising on a number of relevant 'Linkedin' message boards, and on the project web-site, to provide parts for industrial studies.
Sobinco, a Belgian aluminium door and window frame manufacturer, provided the project with various components, including a handles, a cover plates, and hinge parts, to the project, which have been anodised and characterised, both tribologically by Falex, and using standard salt spray testing by KUL. The parts were made from 5000 and 6000 series alloys and had requirements of wear resistance (hinge), corrosion resistance (external handles and covers), and colour retention (handles and covers).
Further scale-up has been achieved by Promet, who has carried out the process on their industrial anodising line.
- Consider production issues relating to environmental legislation and apply a simplified life cycle analysis methodology
A streamlined life cycle analysis was carried out, and is available as a deliverable report D5.3.
- Complete a full techno-economic assessment of the project outputs
Contribution to a full techno-economic assessment was done, comparing energy consumption of the NANOCOAT process with a conventional process.
In addition to the S&T results, a list of possible applications follows. Taking into account the additional energy requirements and capital expenditure for larger components, the process would be limited to niche or high performance applications where cost is not a consideration. Examples may include:
- optical applications,
- space applications,
- small aerospace components,
- specialised automotive components,
- high value consumer products.
WP 6 by KU Leuven: Functional and structural characterisation of anodised materials
1. Objectives of KUL WP
The main objective of the KUL WP was to characterise the coloured anodised aluminium oxide (AAO) samples delivered by C-Tech and ProMet. In particular, the following four sub-objectives were identified, namely:
1.1. To investigate the morphological surface structure of the AAO samples. A special attention was given to the detection of the presence of nanostructured, perfectly ordered pores.
1.2. To determine the layer thickness of the AAO samples obtained by making cross-sectional views.
1.3. To investigate the friction and wear behaviour at macro-loads of the AAO samples.
1.4. To investigate qualitatively the corrosion resistance of the AAO samples after a 1000 hours acetic acid salt spray (AASS) test.
Attempts have been made to correlate these characteristics to the anodising parameters employed.
2. Samples investigated and selected testing procedure
AAO samples were grown onto flat substrates made of four different Al grades, namely grade 1050 (C-Tech), grade 6082 (C-Tech), grade 5754 (C-Tech), and grade 2017 (Promet). These samples were investigated for their morphological surface structure, layer thickness, wear behaviour, and corrosion resistance.
In addition, AAO samples were grown onto flat substrates made of Al grade 5754 (C-Tech), and onto Sobinco door hinges and door covers, made of Al grade 6060 (C-Tech) and grade 5005 (C-Tech) respectively. These samples were investigated for their corrosion resistance. A reference door hinge and door cover anodised by Sobinco were included for comparison.
The AAO samples were produced under different anodising conditions in order to achieve a nanostructured surface morphology with a perfectly ordered pore distribution.
Prior to characterisation and testing, the as-delivered AAO samples were ultrasonically cleaned in acetone for 15 minutes, rinsed with ethanol, and dried in a vacuum chamber for 30 minutes at 3.4 mbar. In order to make the AAO samples conductive for SEM investigation, an Au layer was sputtered for 1 minute.
2.1 Morphological surface structure
The morphological surface structure was investigated using a SEM-EDX XL 30 FEG (FEI) at a beam voltage of 10 kV and a working distance of approximately 10 mm.
2.2 Layer thickness
Three different methods have been employed to obtain cross sectional views and hence determine the AAO layer thickness, namely:
First method: Mechanical deformation approach
It consists in clamping the flat coupons between jaws and on screwing inducing a progressive bending of the samples till 180° is reached or till breaking of the sample.
Second method: Metallographic approach
This method is based on classic metallographic procedure to obtain cross sectional views. It may be helpful in order to confirm the data obtained by the 1° method and to detect very thin top layers, or ductile ones.
It consists of the following steps:
- sample on its side embedded in resin,
- followed by mechanical grinding (22 - 14 -7 µm), mechanical polishing (Nap, 3-1 µm), and
- a final oxide diamond polishing (ODP, SiO, 0.1 µm).
The AAO layer thickness was then determined using optical microscopy.
Third method: Ion beam milling approach
This method is based on the ion milling in a SEM-FIB equipment leading to a cross section in a small area without any mechanical interaction as is the case in the first and second method.
2.3 Friction and wear behaviour
Reciprocating sliding wear tests at macro-loads were performed in Fretting II equipment,. The contact configuration was a ball-on-flat with an Al2 O3 ball as counterbody. After the sliding tests, the worn samples were ultrasonically cleaned in acetone for 15 minutes and rinsed in ethanol in order to remove the debris. The morphology and chemical composition of the wear track was investigated using SEM and EDX respectively.
2.4 Corrosion resistance
The AASS test chamber was prepared according to the ISO 3769-1976 standard. The test temperature inside the chamber was 34 - 35°C. The pH of the acetic acid solution was 2.9 - 3.0. Samples were placed inside the chamber at an inclination angle of 15°. The set-up of the salt spray chamber is shown Figure 4. Samples were rinsed with warm water and dried prior to visual inspection.
3. Collaborative work
- Flat coupons, Sobinco door hinges, and Sobinco door covers with different Al grades were anodised by C-Tech and ProMet. Details on those samples were provided in Section 2.
- Discussions among the industrial partners and RTD performers allowed to improve the anodising conditions in view of obtaining a nanostructured AAO surface morphology. Anodising parameters which had a significant effect of the final surface morphology and corrosion resistance include the imposed voltage, stripping step, pore opening step, sealing step, and dwell time.
- Progress reports regarding the outcome of the characterisation and test results were sent out by KUL.
4. Main achievements
4.1 Morphological surface structure, layer thickness, friction and wear behaviour
- An overview of the morphological surface structure, layer thickness, friction and wear behavior of AAO grown onto Al 1050, 6082, and 2018.
- An overview of the AAO surface morphologies achieved on Al grade 6082 after single step and double step anodising shows that a nanostructured morphology with a perfectly ordered pore distribution is obtained on Al grade 6082, under double step anodising without a pore opening nor a sealing step and in absence of a dye. Instead, a phosphate-like visual appearance is observed. Introduction of a sealing step under the same anodising conditions results in the evolution of a nanostructured, perfectly ordered pore distribution to a partially ordered pore distribution. In addition, a decrease in layer thickness and increase in running-in period is noticed.
- An overview of the structural and tribological features achieved on a nanostructured morphology with a perfectly ordered pore distribution. At low magnification, pitting phenomena and grain boundary effects induced by the anodising process are revealed. At high magnification, pore sizes ranging from 20 to 23 nm are observed. A coefficient of friction around 0.7 is recorded after 1000 sliding cycles, resulting in wear track depth of approximately 0.9 µm.
- An overview of the wear track depth versus the AAO thickness for Al 1050; 6082, and 2015. The broadest range of AAO layer thicknesses is achieved on Al 1050. Wear through is observed on Al 1050 and 6082. No direct correlation between AAO layer thickness and wear depth is found.
- An overview of the effect of anodising parameters on the morphological surface structure, layer thickness, friction and wear behaviour of AAO samples grown onto Al 1050 and 6082. For both Al grades, the anodising time, pore opening step, and sealing step during double step anodising have an effect on the structural and tribological features. In contrast, during single step anodising, the aforementioned parameters do not seem to induce a significant change.
- A comparison of the AAO surface morphology obtained after a ramp-up voltage process with and without dwell time. The imposition of a dwell time seems to result in a less pronounced pitting corrosion.
4.2 Corrosion resistance
Reference door hinges and covers treated by Sobinco have a superior corrosion resistance as compared to the door hinges and covers treated by C-Tech and ProMet. These reference samples only show a small amount of white rust spots and slight discoloration on both the front and at its edges.
A qualitative ranking of the AAO samples after 1000 hours AASS test has been made based on the amount of white rust spots. It is shown that the samples can be classified in three groups, namely lowest, intermediate, and highest amount of white rust spots.
Preliminary results indicate that the lowest amount of white rust spots is observed under the following conditions:
- single anodised stripped samples seem to exhibit a better corrosion resistance as compared to single anodised unstripped samples under the same anodising conditions for Al 1050,
- double anodising on stripped samples results in an improved corrosion resistance as compared to single anodising on stripped samples for Al 1050, 5754, and 6082,
- imposition of two minutes dwell time on single anodised unstripped samples result in an improved corrosion resistance as compared to the same samples without dwell time for Al 1050.
Imposition of dwell time seems to have different effects depending on the anodising conditions and Al grade used. Two minutes dwell time imposed on:
- double anodised stripped samples results in an inferior corrosion resistance, whereas on single anodised unstripped samples an improved corrosion resistance is observed for Al 1050,
- double anodised stripped samples does not seem to affect the corrosion resistance for Al 5754.
In general, AAO grown onto Al 1050 and 5754 seems to exhibit a better corrosion resistance than onto Al 6082.
5. Work in progress
AASS tests on door hinges and door covers anodised by ProMet and C-Tech have been in progress even after the project officially ended. These tests have terminated on January 14, 2013.
6. Conclusions
6.1 Morphological surface structure, layer thickness, friction and wear behaviour
- The majority of AAO samples produced under various anodising conditions exhibit a nanostructured morphology with only a partially or no ordered pore distribution. In addition, cracks and surface pits are often observed. The broadest range of AAO layer thicknesses is achieved on Al 1050.
- The coefficient of friction ranges from 0.6 to 0.9 after 500 sliding cycles on all Al grades. Lower wear resistance on Al 1050 is observed. No direct correlation between AAO layer thickness and wear depth is found.
- Only one nanostructured morphology with a perfectly ordered pore distribution (range of pore sizes : 20 - 23 nm) has been obtained on Al grade 6082. This particular morphology with a phosphate-like appearance is produced under double step anodising without a pore opening nor a sealing step and in absence of a dye. However, surface pitting phenomena and grain boundary effects induced by the anodising process are revealed. A coefficient of friction around 0.7 is recorded after 1000 sliding cycles, resulting in a wear track depth of approximately 0.9 µm.
- No general correlation of structural features to synthesis parameters has been found due to too wide variation of anodising parameters used. However, preliminary results show that:
- the introduction of a sealing step under the anodising conditions mentioned in the previous paragraph, results in the evolution of a nanostructured, perfectly ordered pore distribution to a partially ordered pore distribution, a decrease in layer thickness, and an increase in running-in period on Al 6082,
- the imposition of a dwell time during a ramp-up voltage process results in a less pronounced pitting corrosion on Al 1050,
- the anodising time, pore opening step, and sealing step during double step anodising have an effect on the structural and tribological features. In contrast, during single step anodising, the aforementioned parameters do not seem to induce a significant change for both Al 1050 and 6082.
6.2 Corrosion resistance
- Reference Sobinco door hinges and covers have a superior corrosion resistance as compared to the door hinges and covers anodised by C-Tech and Promet.
- A qualitative ranking of the AAO samples after 1000 hours AASS tests has been made based on the amount of white rust spots. No general correlation of corrosion resistance to synthesis parameters has been found due to too wide variation of anodising parameters used. However, preliminary results indicate that an improved corrosion resistance is achieved by:
- stripping of single anodised samples on Al 1050,
- double anodising of stripped samples on Al 1050, 5754, and 6082,
- imposition of dwell time on single anodised unstripped samples on Al 1050.
Imposition of dwell time seems to have different effects on the corrosion resistance depending on the Al grade and anodising conditions employed. On Al 1050, the dwell time improves or deteriorates the corrosion resistance depending on the anodising conditions. In contrast, on Al 5754 the dwell time does not seem to affect the corrosion resistance.
In general, AAO grown onto Al 1050 and 5754 seems to exhibit a better corrosion resistance than onto Al 6082.
Potential impact:
Expected impacts at the European and/or international level
The technology studied during the two years in the NANOCOAT project has clear market potential and will have a strong impact on the economic prospects the participants via two routes):
- The SME participants will use the technology directly in their own manufacturing operations and/or directly in the services they provide.
- The SME participants will market the technology through process licensing to other manufacturing organisations (via product type, market area, geographical region)
In the latter case, this will lead to improved industrial competitiveness and improved employment across the EU. The surface finishing sector is widely distributed and approximately EUR 30 billion (30 x 109) of finished value product is dependent upon its capabilities. In Germany, there are about 2100 businesses in the surface finishing industry with about 56,000 employees and a total market volume of EUR 4 billion (4 x 109). The total size of the market in France has been estimated as EUR 6 billion. The added value of the sector is much higher, because metal finishing accounts for only 5 % of the total value of the product. Therefore the estimated total added value in Germany, for example, is EUR 80 billion (4 % gross domestic product of Germany).
Anodised aluminium products are established in the following industrial application areas:
- Anodising has been used in building construction for more than 60 years. Anodising gives the safest and hardest architectural finish currently available. Anodised aluminium is easy to clean and requires little maintenance.
- Coating is the process of covering the aluminium surface with a suitable film (layer) made of organic compounds, typically powder or wet paint. Many aluminium components may be painted for various reasons, starting with decoration up to protection against specific environmental influences.
- Decorative finishes are available through metallic finishes and a wide variety of colours. Anodising offers a large increasing number of gloss and colour alternatives and minimises or eliminates colour variations. Unlike other finishes, anodising allows the aluminium to maintain its metallic appearance.
- Adhesive bonding was developed by the aerospace industry. It produces a porous oxide that is used mainly as a pretreatment for preparing adhesively bonding aluminium structures in the aerospace industry and to some extent in automotive. The anodised coating is used as an adhesive bonding primer coat on aircraft and aerospace alloy sheets. This is an excellent surface for the epoxy adhesive and also improves corrosion resistance.
The specific anodising markets to be addressed during the project are concerned with improved functionality for traditional anodisation processes and new coatings.
Improved functionality of existing coatings will be explored in the following large market areas: corrosion and abrasion resistance, adhesive bonding, anodised base for organic coatings / painting, coloured architectural finishes.
The statistics for the market size for existing industrial applications are available from the Association for European Surface Treatment on Aluminium (ESTAL):
- anodised Al: 279 million m2 or 828 thousand tonnes,
- anodised and Coated Al: 441 million m2 or 1.22 million tones.
The construction industry accounts for 59 % of anodised products and 78 % of anodised and coated products. Other applications are: automotive and transportation; aerospace; industrial products; electronic equipment; furniture; household appliances and decorative design; sports and leisure; packaging. ESTAL represents 452 plants with 15 000 employees in SME manufacturers.
Within the surface treatment industry as a whole, the conversion of aluminium represents about 15 % of the market including around 10 % for anodisation alone i.e. EUR 4 billion. The European aluminium surface treatment market grew by 25 % between 2000 and 2005.
New application markets will be assessed, which are based on the enhanced performance of nanostructured coatings. As conventional materials are reaching the limits or their performance and industrial components are subjected to increasing demands in terms of mechanical performance (speeds, loads, temperatures), many industrial manufacturers are facing difficulties taking the 'next technological step'. There are many difficulties to overcome to solve the limitations of current systems.
Surface treatments that can be easily and economically produced by electrochemical techniques (aluminium anodisation) will have an advantage over vapour deposited coatings for many industrial sectors. Price, size and robustness of the production process are key elements to reach an economically viable product. Some examples of new market applications are given:
- improved lubricity,
- anti fretting,
- tribocorrosion surfaces,
- a specific application is the improvement of emissivity of black anodisation layers obtained by absorption of Ni and/or cobalt sulphides for space applications.
