Corrosion behaviour and microstructure of two thermal spray coatings

Many of the state of the art thermal spray coatings (e.g. plasma, HVOF) have been developed with wear resistance as a primary aim. However, these coatings are increasingly being required to function in aqueous environments and, to date, their corrosion behaviour is not well defined. This paper comprises a description of a study of the corrosion behaviour of two types of thermal sprayed cermet coatings applied to a superduplex stainless steel (UNS S32760) substrate. Both coatings (86WC-10Co-4Cr and 50WC-50Ni-Cr-B-Si) were applied using a proprietary HVOF process. Each generic coating was studied in three forms: unsealed, sealed using a polymeric sealant, and with an interlayer of Inconel 625 applied between coating and substrate. Surfaces were tested in both the as sprayed condition and after abrasion and polishing to I µm. The coatings were exposed to seawater and a low total dissolved solids (TDS) water at 18 and 50°C. The corrosion behaviour and detailed corrosion mechanisms were investigated using electrochemical monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron microscopy, and X-ray microanalysis. The two coatings studied are shown to be susceptible to corrosion after relatively short periods in both the low TDS water and seawater. Distinct differences in the extent and mechanisms of attack are demonstrated. In addition, the findings from electrochemical accelerated tests are shown to correlate well with the results of detailed examination of specimens after longer term exposure to the aqueous environments.