Corrosion of Al has to be counteracted by first anodising the Al parts and applying further protective coatings. During anodising, aluminium reacts with the electrolyte and a layer of aluminium oxide is formed on the surface of the aluminium specimen. This coating is highly porous and is subject to attack from the environment and corrosive elements. Therefore, anodised aluminium 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 aluminium oxide anodised layer for corrosion protection a process involving boiling water containing chromate is still commonly used.
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 aluminium. Alternative sealing methods have also been proposed for example with Ni(II), Co(II), 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 aluminium. 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 (UV radiation, low pressure, large temperature range) experience by commercial aircraft during operation.
Therefore, of the previously identified approaches Cr(III)-containing or silicate-forming sealing solutions in REACH compliant processes are preferred options. An adaption of the electrical TSA cycle for improved corrosion resistance without negative impact on fatigue life of components was developed. Detailed investigations and characterisation of the obtained corrosion protected surfaces via ESEM, Raman + IR-spectroscopy and ESCA were performed.
WP1 progress towards objective
A test matrix had 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 only use Al alloy AA 2024 unclad as a final decision of alloys to be tested. The test matrix had 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 progress towards objective
The objective was to establish a standard 'reference' sample and this had 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 seemed 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 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) had 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 was 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 was 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 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 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 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 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 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 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.