Corrosion behaviour of as deposited and PMMA treated TiN coating systems deposited by cathodic arc technique

Vapour deposited coatings usually contain microscopic defects, which can provide corrosive media with easy access to the substrate. Consequently, formation of galvanic cells between the coating and substrate can initiate localised corrosion at the defective sites. In this study, TiN coatings were deposited on mild steel (MS) and stainless steel (SS) substrates using a cathodic arc technique. A post-deposition treatment with polymethyl methacrylate (PMMA) was applied to both TiN/MS and TiN/SS systems to block access of corrosion media to the substrates through the coating defects. The as deposited and PMMA treated coating systems were subjected to electrochemical tests in 3·5%NaCl solution. Potentiodynamic polarisation testing results indicated that the PMMA treatment led to a higher corrosion potential and a lower anodic current density for both TiN/MS and TiN/SS systems when compared with their as deposited counterparts. Extended exposure in 3·5%NaCl caused severe localised corrosion in the as deposited TiN/MS due to the presence of coating defects and poor corrosion resistance of the substrate. Electrochemical impedance spectroscopic measurements demonstrated that the PMMA post-deposition treatment significantly improved corrosion resistance of both TiN/MS and TiN/SS systems by effectively sealing the open voids or pores associated with the coating defects.