Characterization of corrosion resistance of chromium-free coatings for aeronautical application

Coating systems protect metal substrates from environmental corrosion attack. However, although having good performances, the presence of specific metal ions in their composition represents a potential source of environmental contamination. The necessity to conciliate corrosion protection with environmental impact is; thereby, primary. This work was designed to study the performance of low environmental impact conversion coatings (surface pre-treatment and high-solids epoxy primer), such as those free from hexavalent chromium (Cr VI), layered on different substrates of aluminium alloy [Al 7075 (T6) unclad and Al 2024 (T3) unclad] widely used in aerospace applications. Substrate surfaces were first treated by environmentally friendly Cr-free products. Successively, on pre-treated substrates, high-solids, chromate-free epoxy primer was applied. The capability to protect substrates from corrosion phenomena was evaluated following exposure of 4 groups of samples to in situ marine atmosphere at the Genoa Experimental Marine Station (G.E.M.S.) of CIV.R.-I,SMAR., located in the port of Genoa, for 8, 16, and 24 months and in accordance with UNI EN ISO 8565:1997 Standard. Accelerated degradation of conversion coating was also studied by electrochemical impedance spectroscopy. To characterize the coating systems, the interface metal substrate/conversion coating, sections from different samples were submitted to microscope techniques. The experimental results reported in this paper give useful information regarding the protective power against substrate corrosion of some Cr-free conversion coatings that have, as compared to traditional Cr VI containing products, a lower impact on workers' health and on the environment.