CORROSION STUDIES ON NICKEL-SILICON NITRIDE COMPOSITE COATINGS

The electrochemical behaviour of sediment electro-deposited nickel-silicon nitride composite coatings on mild steel has been studied. These composite coatings provide better corrosion resistance in both 3.5% NaCl and 0.5M H2SO4 solution than nickel coatings. The higher the percentage of silicon nitride in the composite, the better is the corrosion resistance. Microstructural studies on these composite coatings have revealed the retention of silicon nitride particles in the coating after corrosion tests, which was indicative of very little or no galvanic corrosion. A reduced tendency to pitting of the composite coatings as compared to nickel coatings was also observed. X-ray analysis of composite coatings exposed to 0.5M H2SO4 revealed the presence of silicon dioxide in them, leading to enhanced passivation of the composite coatings with respect to pure nickel. (C) 1991 The Institute of Metals and the Wolfson Institute for Surface Engineering. The authors are in the Corrosion Laboratory, Department of Metallurgical Engineering, Indian Institute of Technology, Madras, India.