Final Report Summary - SAA-SEAL (Corrosion protection of Aluminium unpainted parts: development of an appropriated Cr free sealing process on thin SAA layer (≤5 µm))
Executive Summary:
Anodizing the Al components and applying further protective coatings can counteract corrosion of Al very effectively. During anodizing, Al reacts with the electrolyte and a layer of aluminium oxide is formed, which is highly porous and is subject to corrosive attack. Therefore, anodized Al is normally further processed with a sealing as a final step. Sealed SAA (sulphuric acid anodizing) industrial processes providing thicker layers (~10 μm) are already on the market, but the missing step is to develop a well-suited process for thin layers (≤ 5 μm) that meets the corrosion resistance requirements. Hot water sealing is one of the widely used methods. However in order to close (seal) the pores in the anodized layer for corrosion protection a process involving boiling water containing chromate is still commonly used. Cr(VI)-based sealing solutions (CCC) have been employed for several decades, and remain one of the most effective and commonly-used methods to improve corrosion resistance of anodized Al despite the very toxic properties of Cr(VI). There is an urgent need to replace the toxic Cr(VI) with less harmful alternatives. Alternative sealing methods have been proposed, e.g. with Ni(II), Co(II), Ni(II) + Co(II), rare earth salts, alkali metal fluorides, alkanolamine phosphonates, Cr(III), fatty acids, silicates, etc. It should be noted that Ni(II), Co(II) and fluorides are not without health implications, whereas most organic molecules would be expected to have limited lifetimes under the extreme conditions (UV radiation, low pressure, large temperature range) experienced by commercial aircraft during operation.
In the project anodized AA 2024 samples (sheet and machined) were conversion treated with a Sealing composed of a combination of two silanes or by using of different additives in the sealing bath based on Mn/Mo/V/Ce/Zr. The investigated variants provided appropriate corrosion protection after 750 h SST at least comparable to commercially available variants. An adapted version of the electrical SAA cycle for improved corrosion resistance has been developed and tested in the project as well with good and promising success. The best results in SST (750 h-1176 h) with maximum 1 corrosion pit/dm² were obtained with a sealing at low temperature and few treatment time. The starting material influences (sheet and machined sample) as well as the pre-treatment influences (cleaning, etching etc.) were analysed and the results supported the better understanding of the promising sealing results obtained. Finally the sealing parameters have been optimized with respect to improved corrosion resistance and minimized energy consumption during processing.
Project Context and Objectives:
An important objective for the industrial usability is to reduce or eliminate potentially environmentally hazardous effluents from aluminium finishing processes. Particular interest is focused on development of Cr(VI)-, cobalt- and nickel-free aqueous sealant compositions for sealing thin aluminium oxide coatings, with similar or even better results as the ones obtained by an SAA process (Sulphuric Acid Anodizing). Therefore, the main objectives of the SAA-Seal project are:
1. The sealing process on thin (≤ 5 µm) anodized layers obtained by a SAA process has to be carried out by environmental friendly REACH compliant processes and products avoiding Chromium(VI), with optimized process parameters in order to minimize energy consumption.
2. The sealing process on thin layer SAA has to show sufficient corrosion protection in order to meet the corrosion resistance requirements in a 750 hours salt spray test.
3. The effect of the substrate composition (2024, 2618, AS7G06, Au5NKZR) and the effect of the production process (e.g.: cast, laminated, forged) on the corrosion resistance of the sealed SAA layers has to be investigated.
4. Finally, a successful technology transfer towards a supplier selected by the topic manager has to be performed.
In accordance with the topic manager (results of a teleconference on 6.03.2013) the objective 3 was modified and it has been decided that in WP5 (D5.1 MS6) CEST will focus its work on AA 2024 sheet and machined samples only and will not investigate other substrates. Instead the optimization of the sealing process with 5 different additives on thin SAA was investigated. Further investigations were aimed to obtain an improved understanding of the differences between machined and sheet samples.
Project Results:
Significant result 1: A good corrosion protection (less than 5 pits/dm² after 750 h SST) using different sealing treatments could be obtained by sealings with Mn/Mo additives and by a following conversion treatment based on Cr(III).
The best corrosion protection (SST 750 h and 1176 h) was obtained by a sealing solution with additives of Mn/Mo and Zr at low temperature and a few minutes treatment time with maximal 1 pit/dm² for sheet and machines samples.
Significant result 2: Reasons for the different behaviour of sheet and machined samples have been elucidated and the cause of cracking in coatings was explored.
Differences between sheet and machined samples consist in different number and size of particles (containing alloying elements) near to the surface. Particles high in Cu near to the surface of machined samples are bigger than the ones found on sheet samples and will have increased influence on the pore orientation formation during anodizing. A more columnar orientation was found in the case of sheet samples.
A more uniform surface morphology is observed with a supplementary pretreatment step such as alkaline etching. There is some evidence by SEM investigations that by this pretreatment step the Cu particles are almost completely removed and the resulting mass loss is much higher in comparison with standard SAA pretreatment. The reasons for cracking were investigated more systematically and in greater detail in order to clarify the influences on the occurrence of cracking by layer morphology, thickness, and vacuum exposure in the SEM (10-6 mbar). The cracks appear only in samples of SAA layer thicknesses > 4 µm and are induced by the exposure of the samples to vacuum during SEM inspection. No such cracks were found by LOM observation.
Potential Impact:
The employment of ECO friendly pre-treatment processes on sheet and machined samples was thoroughly studied, analysed and will be applied in the aircraft production processes. One major focus of the technological competition in aeronautics is the increased use of novel superior light-weight materials processed in ECO friendly manufacturing. The project SAA and Clean Sky contributes to technological and scientific leadership in the area of aeronautics light-weight design, and therewith strengthens the competitiveness of the European research and industry. The processes developed in the scope of SAA have high performance potentials for anodizing steps and chrome free sealing using chemicals fulfilling the REACH requirements. The dissemination and exploitation of project results was implemented in WP 9 (Reporting and Management), under the supervision of the project manager (PM). The PM was responsible for the release of documentations and reports and its updates during the course of the project. The PM has coordinated the exploitation activities and synergies within CEST and particularly among similar topics and topics depending on surface-treatment of lightweight metals. The topic manager has been systematically informed on the proposed work progress in order to
Support the final implementation of project results. Detailed planning and execution of these activities has been a part of the exploitation task. Project results have been disseminated at two levels: detailed and general. At the detailed level informal information and technical reports (midterm and final) and deliverables for each work package have been produced and have been distributed to the topic manager of Clean Sky ECO. Those reports are describing the technical details and the conclusions and outcome of the research. At the general and public level project results have been presented at appropriate international Conferences (Eurocorr 2013), and will be published in international journals. A book chapter in “Advanced Material Series: Advanced Materials for Agriculture, Food, and Environmental Safety” with the title: “Improvement of the Corrosion Resistance of Aluminium Alloys Applying different Types of Silanes” will be published on 30.05.2014 Edited by Ashutosh Tiwari and Mikael Syväjärvi (Scrivener Publishing LLC). A Patent application (Chrome free sealing) is in progress. Before such general activities have been taken place the topic manager of Clean Sky ECO was informed and the consent has been sought. Proprietary elements and previous Know how of the topic manager have been respected. The dissemination activities ensured that the increased technological capability of European partners in taking full advantage of the superior characteristics of the novel REACH compatible pre-treatment/SAA thin anodisation and chrome free sealing processes become evident to the world-wide aerospace community. Any dissemination/exploitation was made in accordance with the grant agreement where the policy for securing Intellectual Property Rights and for licensing is defined.
List of Websites:
not applicable
Anodizing the Al components and applying further protective coatings can counteract corrosion of Al very effectively. During anodizing, Al reacts with the electrolyte and a layer of aluminium oxide is formed, which is highly porous and is subject to corrosive attack. Therefore, anodized Al is normally further processed with a sealing as a final step. Sealed SAA (sulphuric acid anodizing) industrial processes providing thicker layers (~10 μm) are already on the market, but the missing step is to develop a well-suited process for thin layers (≤ 5 μm) that meets the corrosion resistance requirements. Hot water sealing is one of the widely used methods. However in order to close (seal) the pores in the anodized layer for corrosion protection a process involving boiling water containing chromate is still commonly used. Cr(VI)-based sealing solutions (CCC) have been employed for several decades, and remain one of the most effective and commonly-used methods to improve corrosion resistance of anodized Al despite the very toxic properties of Cr(VI). There is an urgent need to replace the toxic Cr(VI) with less harmful alternatives. Alternative sealing methods have been proposed, e.g. with Ni(II), Co(II), Ni(II) + Co(II), rare earth salts, alkali metal fluorides, alkanolamine phosphonates, Cr(III), fatty acids, silicates, etc. It should be noted that Ni(II), Co(II) and fluorides are not without health implications, whereas most organic molecules would be expected to have limited lifetimes under the extreme conditions (UV radiation, low pressure, large temperature range) experienced by commercial aircraft during operation.
In the project anodized AA 2024 samples (sheet and machined) were conversion treated with a Sealing composed of a combination of two silanes or by using of different additives in the sealing bath based on Mn/Mo/V/Ce/Zr. The investigated variants provided appropriate corrosion protection after 750 h SST at least comparable to commercially available variants. An adapted version of the electrical SAA cycle for improved corrosion resistance has been developed and tested in the project as well with good and promising success. The best results in SST (750 h-1176 h) with maximum 1 corrosion pit/dm² were obtained with a sealing at low temperature and few treatment time. The starting material influences (sheet and machined sample) as well as the pre-treatment influences (cleaning, etching etc.) were analysed and the results supported the better understanding of the promising sealing results obtained. Finally the sealing parameters have been optimized with respect to improved corrosion resistance and minimized energy consumption during processing.
Project Context and Objectives:
An important objective for the industrial usability is to reduce or eliminate potentially environmentally hazardous effluents from aluminium finishing processes. Particular interest is focused on development of Cr(VI)-, cobalt- and nickel-free aqueous sealant compositions for sealing thin aluminium oxide coatings, with similar or even better results as the ones obtained by an SAA process (Sulphuric Acid Anodizing). Therefore, the main objectives of the SAA-Seal project are:
1. The sealing process on thin (≤ 5 µm) anodized layers obtained by a SAA process has to be carried out by environmental friendly REACH compliant processes and products avoiding Chromium(VI), with optimized process parameters in order to minimize energy consumption.
2. The sealing process on thin layer SAA has to show sufficient corrosion protection in order to meet the corrosion resistance requirements in a 750 hours salt spray test.
3. The effect of the substrate composition (2024, 2618, AS7G06, Au5NKZR) and the effect of the production process (e.g.: cast, laminated, forged) on the corrosion resistance of the sealed SAA layers has to be investigated.
4. Finally, a successful technology transfer towards a supplier selected by the topic manager has to be performed.
In accordance with the topic manager (results of a teleconference on 6.03.2013) the objective 3 was modified and it has been decided that in WP5 (D5.1 MS6) CEST will focus its work on AA 2024 sheet and machined samples only and will not investigate other substrates. Instead the optimization of the sealing process with 5 different additives on thin SAA was investigated. Further investigations were aimed to obtain an improved understanding of the differences between machined and sheet samples.
Project Results:
Significant result 1: A good corrosion protection (less than 5 pits/dm² after 750 h SST) using different sealing treatments could be obtained by sealings with Mn/Mo additives and by a following conversion treatment based on Cr(III).
The best corrosion protection (SST 750 h and 1176 h) was obtained by a sealing solution with additives of Mn/Mo and Zr at low temperature and a few minutes treatment time with maximal 1 pit/dm² for sheet and machines samples.
Significant result 2: Reasons for the different behaviour of sheet and machined samples have been elucidated and the cause of cracking in coatings was explored.
Differences between sheet and machined samples consist in different number and size of particles (containing alloying elements) near to the surface. Particles high in Cu near to the surface of machined samples are bigger than the ones found on sheet samples and will have increased influence on the pore orientation formation during anodizing. A more columnar orientation was found in the case of sheet samples.
A more uniform surface morphology is observed with a supplementary pretreatment step such as alkaline etching. There is some evidence by SEM investigations that by this pretreatment step the Cu particles are almost completely removed and the resulting mass loss is much higher in comparison with standard SAA pretreatment. The reasons for cracking were investigated more systematically and in greater detail in order to clarify the influences on the occurrence of cracking by layer morphology, thickness, and vacuum exposure in the SEM (10-6 mbar). The cracks appear only in samples of SAA layer thicknesses > 4 µm and are induced by the exposure of the samples to vacuum during SEM inspection. No such cracks were found by LOM observation.
Potential Impact:
The employment of ECO friendly pre-treatment processes on sheet and machined samples was thoroughly studied, analysed and will be applied in the aircraft production processes. One major focus of the technological competition in aeronautics is the increased use of novel superior light-weight materials processed in ECO friendly manufacturing. The project SAA and Clean Sky contributes to technological and scientific leadership in the area of aeronautics light-weight design, and therewith strengthens the competitiveness of the European research and industry. The processes developed in the scope of SAA have high performance potentials for anodizing steps and chrome free sealing using chemicals fulfilling the REACH requirements. The dissemination and exploitation of project results was implemented in WP 9 (Reporting and Management), under the supervision of the project manager (PM). The PM was responsible for the release of documentations and reports and its updates during the course of the project. The PM has coordinated the exploitation activities and synergies within CEST and particularly among similar topics and topics depending on surface-treatment of lightweight metals. The topic manager has been systematically informed on the proposed work progress in order to
Support the final implementation of project results. Detailed planning and execution of these activities has been a part of the exploitation task. Project results have been disseminated at two levels: detailed and general. At the detailed level informal information and technical reports (midterm and final) and deliverables for each work package have been produced and have been distributed to the topic manager of Clean Sky ECO. Those reports are describing the technical details and the conclusions and outcome of the research. At the general and public level project results have been presented at appropriate international Conferences (Eurocorr 2013), and will be published in international journals. A book chapter in “Advanced Material Series: Advanced Materials for Agriculture, Food, and Environmental Safety” with the title: “Improvement of the Corrosion Resistance of Aluminium Alloys Applying different Types of Silanes” will be published on 30.05.2014 Edited by Ashutosh Tiwari and Mikael Syväjärvi (Scrivener Publishing LLC). A Patent application (Chrome free sealing) is in progress. Before such general activities have been taken place the topic manager of Clean Sky ECO was informed and the consent has been sought. Proprietary elements and previous Know how of the topic manager have been respected. The dissemination activities ensured that the increased technological capability of European partners in taking full advantage of the superior characteristics of the novel REACH compatible pre-treatment/SAA thin anodisation and chrome free sealing processes become evident to the world-wide aerospace community. Any dissemination/exploitation was made in accordance with the grant agreement where the policy for securing Intellectual Property Rights and for licensing is defined.
List of Websites:
not applicable
