Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy

The protective properties of trivalent chromium process (TCP) coatings grown on a 2024-T3 aluminium alloy were studied on the basis of electrochemical measurements performed both in sulphate and chloride solutions and neutral salt spray tests. The influence of the alloy microstructure and surface state was studied: two batches, each one characterized by its own coarse intermetallic particle distribution, and two surface states, i.e. laminated and polished, were considered. Results showed that in 0.1 M Na2SO4, the protective properties of the TCP coatings decreased when the roughness of the initial surface increased. Furthermore, improved protective properties were observed for a TCP coating grown on a surface containing a lower amount of Al-Cu-Mg IMCs in the initial microstructure. The most plausible explanation is that a fast kinetics of coating growth, either associated to strong initial roughness or a great surface copper coverage, was detrimental for the protective properties of the coatings. In more aggressive solutions, i.e. 0.5 M NaCl solution or for neutral salt spray tests, the differences are not significant. The findings are highly relevant for industrial applications: the results showed that variations in batches, for a same type of alloy, or in initial surface state should not be detrimental for the corrosion resistance of the TCP coated samples. However, the conversion process had to be adapted for different types of alloys, characterized by their own microstructure