An analysis of environmental factors affecting corrosion behavior of thermal spray cermet coatings

Primarily thermal-spray coatings have been developed to combat excessive degradation of components due to mechanical wear. However, these coatings are increasingly being required to function in aqueous environments where corrosive attack is possible. The durability of thermal spray coatings in terms of corrosion resistance is the focus of this paper. Two types of coatings have been studied; High-Velocity Oxy-Fuel (HVOF) and thermal spray-fused. The HVOF coating was WC-Co-Cr and the spray-fused coatings were WC-Co based and Ni-Cr-Si-B. Samples have been exposed to aqueous environments at a range of different temperatures and of varying salinity (500ppm and 35,000ppm Total Dissolved Solids) in order to simulate freshwater and seawater environments. The detailed material loss and degradation mechanisms have been investigated using electrochemical-monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron and atomic force microscopy and x-ray microanalysis. The study has demonstrated that there is a strong dependence on coating durability for all the coatings as a function of temperature but salinity has a lesser effect. However there are interesting differences in the extent and detailed mechanisms of such effects between the different coatings.