Characterization and investigation of the corrosion resistance of chromium conversion coatings on heat-treated aluminum alloys used in the aerospace industry

The hexavalent chromium coating(HCC), known to be harmful to health and the environment, was replaced by a trivalent conversion process (TCP) coating. In this framework, different concentrations (25 wt%, 35 wt% and 50 wt%) of TCP coatings on 2024-T3 aluminum alloys used in aerospace applications were investigated. The corrosion performance of the coatings was evaluated using electrochemical techniques and a salt spray test and surface morphologies investigated. The results show that the TCP coating had a porous surface with higher roughness compared to HCC. In addition, the TCP coating was slightly more hydrophobic compared to the HCC coating. The open circuit potential (Eocp) of HCC was 40 mV more positive than that of TCP coatings. A passive region was observed for all samples, and Epit of 25 wt% TCP coating started at 250 mV more anodic than the untreated sample. Analysis of the corroded surface after polarization measurement showed that pits had formed in the coated samples. In addition, the coated samples showed capacitive behavior compared to the uncoated samples, implying improved charge transfer resistance. In addition, the electrical circuit model showed that the resistance (Rp) of the HCC coating was better than that of the TCP coating. However, analysis of the salt spray samples showed a lower density of pits in HCC compared to TCP-coated samples after 168 hours, probably due to the higher amount of Cr involved in the oxide layer. In general, it can be concluded that TCP coating, especially at low concentration, increases the corrosion resistance of 2024-T3 but is not higher than HCC due to the porous surface morphology that allows ionic activity and the weak oxide-forming elements.