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Chromate free and energy efficient sealing of TSA anodic films for corrosion protection

Final Report Summary - TARTASEAL (Chromate free and energy efficient sealing of TSA anodic films for corrosion protection)

Executive Summary:

Corrosion of aluminium (Al) has to be counteracted by first anodising the Al parts and applying further protective coatings. During anodising, Al reacts with the electrolyte and a layer of Al oxide is formed on the surface of the Al specimen. This coating is highly porous and is subject to attack from the environment and corrosive elements. Therefore, anodised Al is normally further processed with a sealing as a final step after anodising. A hot water sealing process is one of the widely used methods. However in order to close (seal) the pores in the Al oxide anodised layer for corrosion protection a process involving boiling water containing chromate is still commonly used. Chromium (Cr(VI)) based sealing solutions have been employed for several decades, but remain one of the most effective and commonly-used methods to improve corrosion resistance of anodised Al. Alternative sealing methods have also been proposed for example with Ni(II), cobalt (Co(II)), nickel (Ni(II)) and Co(II), rare earth salts, alkali metal fluorides, alkanolamine salts of phosphonic acids, Cr(III), fatty acids, silicates, etc. Already about 45 of the 92 naturally occurring elements have been considered as replacements for Cr(VI) in conversion coatings on Al. In general these approaches have not been as successful as the Cr(VI) sealing. Also 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, e.g. ultraviolet (UV) radiation, low pressure and large temperature range, experience by commercial aircraft during operation. Therefore, of the previously identified approaches Cr(III)-containing or silicate-forming sealing solutions in registration, evaluation, authorisation and restriction of chemicals (REACH) compliant processes are preferred options. An adaption of the electrical thermal spray aluminium (TSA) cycle for improved corrosion resistance without negative impact on fatigue life of components will be developed. Detailed investigations and characterisation of the obtained corrosion protected surfaces via environmental scanning electron microscope (ESEM), Raman and infrared (IR) spectroscopy and electron spectroscopy for chemical analysis (ESCA) will be performed.

Project Context and Objectives:

In the project six tasks will be considered. A main focus will be put on the investigation and understanding of cold sealed anodic films via chromium conversion coatings (CCC) on silanes systems with strongly reduced energy consumption. As mitigation, but also to provide a process with a maximum of corrosion protection the sixth task shall combine different separate measures. So it shall be considered, the combination of e.g. optimised hot water sealing followed by a conversion coating for extra protection and e.g. adapted TSA plus hot water seal or CCC. Aim is to meet the performance of the chromic acid anodised (CAA) film in combination with dichromate hot water sealing, which is usually 500 h salt spray test (SST) and more.

The assessment of the sealing options will be justified with the required standard salt spray test, but also with additional investigations like impedance spectroscopy (EIS), to get detailed information about the resistance of the generated films. Further aim is to correlate SST values and EIS data, so that EIS can be used as an additional very fast method for quality control of the sealed films in case of production problems. Scanning electron microscope (SEM) shall be used to investigate the surface modification after the sealing process and as a pre-assessment for selection if sealed coatings were suitable for paint application as alternative option for the post local sealing procedure.

For application of the different processes that lab galvanic line of the purchaser will be available for the sample preparation studies, including the paint application. The quality of achieved data shall be sufficient, so that the proposed process can be specified. Finally it is intended, to provide a commonly accessible and available EN with the relevant TSA process parameters, including the sealing parameters.

The following six tasks will be considered in the work packages (WPs). The tasks of the working plan are chosen in such a way to ensure that in any case the requirements for corrosion protection were met:

1. standard hot water sealing shall be used as reference for alternative options.
2. hot water sealing process supported by REACH compliant sealing additives like additions of rare earth elements and/or carboxylic acid and/or silane.
3. investigation of alternative sealing via chromate free conversions coatings, e.g. Cr (III), silanes etc.
4. adaptation of the electrical TSA cycle for improved corrosion resistance for unpainted parts without negative impacts on the fatigue cycle.
5. local sealing of partial painted parts based on task three procedures (Silane or CCC).
6. combination of individual measures with the aim to provide a maximum of achievable corrosion protection.

Project Results:

WP1 objective

It is the aim of this WPto design a suitable test matrix based on previous experience and a brief exploration of commercially-available sealing solutions and those reported in the literature. To assess and validate the processes to be investigated the testing will be done on a selection of aeronautic alloys like AA2024 clad, 7XXX series clad and unclad and the 6XXX series alloys. It is recognised that as a reference AA2024 unclad will be used. The final decision on the alloys to be investigated will be determined in the test program with the topic manager. The experimental matrix will then be employed in WP2 to WP8.

WP1 progress towards objective

A test matrix has been established and discussed in the Kick off meeting at Airbus Bremen on 6 December 2011 and it was decided by the topic manager to use only Al alloy AA 2024 unclad as a final decision of alloys to be tested. The test matrix has been established for being employed in WP2 to WP8. As commercial solutions, Alodine 5923 P from Henkel and Chromit Al 650 from SurTec were selected.

WP2 objective

To seal Al using the standard hot water sealing method as a reference for improved methods. Al will be anodised according to the TSA process and the oxide layer sealed in hot water to prepare standard 'reference' samples. The sealing time and temperature will be varied within reasonable limits to find the optimal conditions. This will provide a baseline from which to judge the 'improved' sealing procedures.

WP2 progress towards objective:

The objective was to establish a standard 'reference' sample and this has been fully achieved. The anodised AA2024 samples were sealed by hot water sealing (HWS) at varied temperatures and times, 95°C, 30 min and 60 min and at 120°C for 30 min. It was observed that the results obtained by ESEM and FIB for the sample sealed with HWS at 95 °C for 30 min and sample sealed with HWS at 95°C for 60 min are not very different, indicating that by increasing the hot water treatment time the performance will not be improved excessively. In the case of the sample sealed with HWS at 120 °C for 30 min only a very slight reduction in terms of crack formation seems to be apparent. From an economical point of view it was concluded and proposed to keep as a standard reference and baseline for HWS the treatment conditions of 95°C and 30 min.

WP3 objective

It is the objective to seal Al using REACH compliant sealing additives by hot water sealing. Commercially-available, REACH compliant sealing additives and related chemicals will be sourced. Al will then be anodised according to the TSA process and the oxide layer sealed in hot water with said additives. The suggested additives are based on additions of rare earth elements and/or carboxylic acid and/or silane.

WP3 progress towards objective:

In accordance with the objectives as additives for REACH compliant hot water sealing, carboxylic acids (hexanoic acid, octanoic acid) and water based silanes mixtures (bis-amino silane /vinyltriacetoxysilane) have been proposed and used. Carboxylic acids used as additives in hot water sealing at different temperatures and time formed layers of soap on the surface, which conferred hydrophobic properties. The protective layer formed improves the corrosion protection. A mixture of two silanes were used due to the fact that a bis-amino silane film alone offered an inferior corrosion protection performance on AA 2024; this film is positively charged (protonation of amino group) and attracts anions, i.e. chlorine (Cl-) and others from the environment, destroying the metal substrates and also MeOSi bonds. A mixture of silanes, bis-(trimethoxysilylpropyl) amine and vinyltriacetoxysilane (5:1 v/v) enhances the corrosion resistance of AA 2024. A small volume of bis-amino silane is enough to confer hydrophilic properties and to facilitate the formation of a homogenous film on TSA anodised AA 2024 sample. The pronounced hydrophobicity of a silane mixture film is the basis for a good protective layer performance in corrosive environments. In accordance with the topic manager rare earth elements as additives were not considered as sealing additives, because in previous project with Airbus, rare earth elements have not achieved the expectations in sealing performance.

WP4 objective

It is the objective to seal Al according via alternative chromate-free conversion coatings. Al samples will be anodised by the TSA process. A range of alternative, chromate-free conversion coatings will be applied according to the test program defined in WP1. Ideally these processes should operate at room temperature, with low-cost, permitted chemicals and therefore represent a safe and energy-efficient alternative to existing sealing processes. Alternatives will for example include Cr (III), silanes, or Al salts with alkoxy silanes or rare earth elements containing coatings (e.g. CeO2.2 H2O).

WP4 progress towards objective:

According to the kickoff meeting results it was decided in agreement with the topic manager to concentrate the research on: silanes (bis-(3-(triethoxysilyl) ethane, bis-[3-(triethoxysilyl) propyl] tetrasulfide) and oxyanions (MnO4-/VO43- and MnO4-/WO42-), ChromitAl 650 and Alodine 5923P. In accordance with the topic manager the initially foreseen rare earth elements and Al salts with alkoxy silanes will not be considered. Further in agreement with Airbus (project meeting on 26 June 26 2012) it was established to use a commercially available relatively low cost Cr (III) containing conversion coatings. ChromitAl 650 and Alodine 5923P were therefore included in the testing program but results have been reported in deliverable 5.1 (Annex 5) due to the fact that the chemicals have been received too late for testing. The influence of concentration (2% and 5%), time of conversion (2, 5 and 10 min) and time of hydrolysis (5 and 7 days) were studied for corrosion protection of silanes used for conversion of TSA anodised AA 2024. In the case of conversion treatment with Mn/V oxyanions there are some few advantages from an economical point of view, such as time (shorter time) and temperature (RT) in comparison with ChromitAl 650 treatment. The results obtained for 840 h of salt spray exposure for sample conversion coated with Mn/V for 5 min at RT compare favourably with the results obtained for a sample conversion coated with Chromit Al 650.

WP5 objective

It is the objective to optimise the electrical TSA cycle and to investigate the possible effects on the material properties of the base material (microstructure and fatigue properties). The TSA treatment may be performed with different electrical profiles (e.g. continuous, pulsed, pulsed with offset and so on). It is well known that the potential profile can have a significant effect on the resulting oxide layer. In this WPthe effect of the potential/ current profile on the anodic layer properties will be investigated. Special attention will be paid to the requirement that the optimisation of the TSA cycle should not deteriorate the fatigue properties of the corrosion protected components. This will be investigated by SEM of surface samples and micro sections as a pre-assessment and a final endurance limit measurement (Wöhler curve) of corrosion protected samples with the optimised TSA cycle in comparison to the standard TSA cycle. For the determination of the fatigue properties of material subjected to the standard and the optimised TSA cycle a subcontract will be issued to a laboratory experienced in fatigue property measurements and equipped with appropriate and sufficient testing machinery.

WP5 progress towards objective

In agreement with Airbus (project meeting on 26 June 2012) it was decided that the fatigue tests will not be performed and instead of these, the studies will be focused much more on TSA cycle optimisation. Four anodisation methods with different current type such direct current (DC), pulsed current (PC) and anodisations with different voltage ramps were investigated. The details of voltage ramps and different anodisation conditions are to be found in deliverable 5.1 (Annex 5). The layer properties were influenced by the different anodisation methods. The most promising anodising method developed is the anodisation by method four (all the parameters are listed in detail in Annex 5) due to the fact that the corrosion protection was the highest of all the methods investigated.

WP6 objective

To investigate the applicability of methods developed in WP4 to the local sealing of partially-painted parts. For application of the different processes the lab galvanic line of the purchaser will be available for the sample preparation studies, including the paint application. At CEST the most promising methods identified in WP4 will be examined for applicability to the partially-painted parts obtained from the purchaser. In particular the sealing at the three-phase interface between Al, paint and conversion coating must be examined microscopically (WP8).

WP6 progress towards objective

According with the WP6 objectives, the samples were painted at CEST and afterwards treated with the most promising methods identified in WP4, which were bis-[3-(triethoxysilyl) propyl]tetrasulfide = PSS (5%) and oxyanions (MnO4-/VO43) = Mn/V conversion coatings. A good compatibility between anodic layer (with and without HWS), conversion coating- and primer was observed by SEM/FIB characterisation. A good compatibility between top-coat and conversion coating was proven as well. PSS (5 %) and Mn/V are good candidates for Cr(III) conversion coating replacement because the SST and SEM/FIB results indicate that the three face interface between Al, paint and conversion coating is efficiently protected.

WP7 objective

To determine if synergistic effects may be generated by the combination of adapted TSA, alternative conversion coatings and/or hot water sealing additives. The most promising approaches developed in WP3, WP4 and WP5 will be combined to ascertain what benefits may be obtained from an improved combined approach. In this way it may be possible to obtain a process that combines the advantages of the individual techniques.

WP7 progress towards objective

The most promising results obtained in WP3, WP4 and WP5 were combined and used. The conversion coating with 5 % PSS silane and Mn/V treated with standard HWS significantly increases the corrosion resistance in comparison to anodised samples treated with conversion coating without HWS. The optimised TSA cycle of method 4 combined with a Mn/V conversion coating shows better results in comparison to samples having a standard anodisation and a Mn/V conversion coating.

WP8 objective

Characterisation of the anodic layers obtained in WP3 to WP7. The assessment of the sealing options will be justified with the required standard salt spray test, but also with additional investigations like EIS, to get detailed information about the resistance of the generated films. Further aim is to correlate SST values and EIS data, so that EIS can be used as an additional very fast method for quality control of the sealed films in case of production problems. SEM will be used to investigate the surface modification after the sealing process and as a pre-assessment for selection if sealed coatings were suitable for paint application as alternative option for the post local sealing procedure. Aim is to meet the performance of the CAA (chromic acid anodisation) film in combination with dichromate hot water sealing, which is usually 500 h salt spray test (SST) and more.

WP8 progress towards objective

Anodic films generated by TSA were characterised morphologically (ESEM/FIB) and electrochemically (EIS) and the results were correlated with SST. The results in SST are in good correlation with results by EIS. The barrier layer resistance values were higher for Mn/V oxyanions than for by 5 % Bis-[3-(triethoxysilyl) propyl] tetrasulfide (PSS) conversion coatings based on EIS studies. Both conversion layer variants were superior than other conventional conversion coatings based on Cr(III). Mn/V performed considerably better than ChromitAl-650, Alodine 5923P in 504 h salt spray exposure. This superior performance remained constant up to 840 h SST when samples were tested having a combination of HWS and conversion coating. It was concluded that the conversion with Mn/V is therefore a promising candidate for replacement of Cr(III) conversion coatings.

Potential Impact:

The employment of ecologically (ECO) friendly cleaning processes 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 TSA 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 TARTASEAL have high performance potentials for anodising steps using chemicals fulfilling the REACH requirements.

The dissemination and exploitation of project results was implemented in WP9, 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 WP 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 and will be published in international journals. 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 general activities ensured that the increased technological capability of European partners in taking full advantage of the superior characteristics of the novel REACH compatible cleaning and anodisation 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 finally determined.