Crystallization mechanism and corrosion property of electroless nickel phosphorus coating during intermediate temperature oxidation

Electroless Ni-P coating was deposited on steel substrate and the effect of intermediate temperature oxidation on crystallization mechanism and corrosion properties of the coating was investigated. Ni-P coatings were annealed at three different temperatures, viz. 200 degrees C, 400 degrees C and 600 degrees C for 2 h in air. Formation of nickel oxide (NiO) was observed in the coating upon annealing beyond the crystallization temperature (330 degrees C). Crystallization mechanism provided insight about the step by step formation of long range ordered Ni,Ni3P and NiO phases.Improvement in the corrosion resistance of Ni-P coating compared to bare steel was found to be similar to 21% on annealing at 400 degrees C in air which gradually increased to similar to 31% on annealing the coating at 600 degrees C in air. Increasedcorrosion resistance at 400 degrees C annealed, coating was attributed to the formation of crystalline Ni and Ni3P phases. Two simultaneously effects hive identified for the increased corrosion resistance of the coating annealed at 600 degrees C in air. (a) Formation of NiO layer which acts as a passivation layer and protects the underlying P enriched layer and (b) absence of an interdiffusion layer from substrate to coating. (C) 2015 Elsevier B.V. All rights reserved.