Al/SiC thermal spray coatings for corrosion protection of Mg–Al alloys in humid and saline environments

The corrosion behaviour of thermal spray Al/SiC coatings deposited on AZ31, AZ80 and AZ91D Mg–Al–Zn alloys was evaluated in neutral salt fog (ASTM B 117) and high relative humidity (98% RH, 50 °C) environments. The findings revealed porous as-sprayed coatings with paths providing access of the corrosive media to the magnesium substrates. This resulted in galvanic corrosion at the substrate/coating interfaces and formation of magnesium corrosion products, which eventually led to coating spalling in the case of salt fog tests after protracted times of exposure. The application of a cold-pressing post-treatment improved the corrosion performance of the coatings. Thus, in high humidity atmosphere, corrosion signs were only visible at the Al/SiC interfaces in the outermost surface of the coatings and, in salt fog environment, the galvanic corrosion of the substrates was delayed.