Corrosion behaviour of hybrid coatings

The corrosion resistance of hard coatings depends on pinhole defects. Many attempts to reduce them have been made but the corrosion problem could not be eliminated completely. An alternative way to achieve better corrosion resistance is to introduce a corrosion-resistant metallic interlayer between the substrate and the hard coating. Potentiodynamic methods have been used to investigate the corrosion protection value of various hybrid coatings systems. Combinations of electrochemically deposited (ECD) hard chromium and electroless nickel interlayers with titanium nitride and chromium nitride coatings produced by are-physical vapour deposition have been tested. Anodic polarization curves show that the corrosion behaviour is strongly interconnected with the microdefects present in the coatings. Because its structure shows many defects in the form of pinholes and droplets even in combination with interlayers the corrosion behaviour of TiN is unsatisfactory, Compared to TiN, CrN exhibits better corrosion resistance mainly in combination with ECD nickel as an interlayer.