Electrochemical methods for characterisation of thermal spray corrosion resistant stainless steel coatings

The use of thermal spray stainless steel coatings for protection of low alloyed steels against different types of corrosion is limited due to high porosity levels and oxide inclusions. In this paper electrochemical methods like corrosion potential monitoring and cyclic voltammetry are reported to monitor the corrosion resistance of thermal spray coatings. The studied stainless steel AISI 316 coatings are deposited by are spraying, plasma spraying or high velocity oxygen fuel (HVOF) spraying. The electrochemical tests are performed in a 0.5 M H2SO4 solution. It is found that none of the tested coatings had an electrochemical response equal to that of stainless steel alloy AISI 316. The results indicate that the HVOF coating has the highest corrosion resistance and the corrosion resistance of are spray coatings can be improved by spraying in an inert argon atmosphere. The electrochemical response of the studied are spray coatings is independent of substrate type (including the case of no substrate), indicating that the underlying alloy does not contribute to the corrosion process and only the coating itself is attacked. The HVOF coatings age rapidly in a 0.5 M H2SO4 solution. It is proposed that this is due to pore opening and pore widening.