Alternative chemical conversion pre-treatment for aeronautical aluminium alloy: characterisation and anticorrosion performance

In recent years, intensive studies have been conducted in finding viable alternatives to chromium (VI) containing conversion coatings applied in the aerospace industry to protect aluminium alloys from corrosion. The objective of this study was to evaluate the anticorrosion performance of a possible substitute pre-treatment for the chromium (VI) based coatings. The product involved in this study was a commercially available conversion coating, based on trivalent chromium, PreCoat A32, which was applied onto aluminium alloy 2024-T81. Through the techniques of the potentiodynamic polarization measurements and electrochemical impedance spectroscopy analysis, it revealed the electrochemical behaviour of the coating. The humidity tests, thermal cycling and salt-spray chamber tests were performed to simulate more severe environments and conditions. The results obtained for salt spray, humidity and thermal cycling tests, were consistent with the electrochemical data. Atmospheric corrosion tests were performed in Kourou, French Guiana, to study the behaviour in a representative launch site environment. The resistance to atmospheric corrosion is confirmed to be comparable between PreCoat A32 and the Alodine 1200S films. The morphological observation was performed by scanning electron microscopy (SEM) and revealed different morphologies of the coatings, as a result of the different compositions. The semi-quantitative analysis of the chemical elements Energy-Dispersive X-ray Spectroscopy (EDS) revealed the presence of Cr in Alodine and Zr in PreCoat A32 as expected. The results show that the conversion coatings formed on the aluminium alloys exhibited good corrosion protection; making this pre-treatment, a promising alternative for Chromium (VI) based chemical conversion coatings.