Electrochemical Behavior, Microstructure, and Surface Chemistry of Thermal-Sprayed Stainless-Steel Coatings

Thermally sprayed stainless-steel coatings were produced with a wide range of deposition parameters. The electrochemical behavior of polished coatings was monitored for 3 weeks in 3.5 wt.% NaCl aqueous solution and compared to that of reference materials including a wrought stainless steel plate and a bulk ingot produced by arc melting of the spraying powder feedstock. Transitions in the polarization behavior are discussed based on the observed changes in coating microstructures as well as on the shifts in X-ray photoelectron spectra (XPS). Results show that the deposition parameters have a strong effect on the coating microstructures but the small differences in the polarization behavior of coatings mostly disappear after 1 week of testing. Microstructure evidence shows preferential corrosion at splats experiencing melting prior to deposition. Pitting and corrosion products between splat boundaries are also reported. XPS analysis shows that the coating surfaces are enriched in chromium oxides and hydroxides. Comparison between the coating and bulk stainless steels suggests that coating inherent defects play a major role on their impaired corrosion resistance.