Influence of particle size on the microstructure, hardness and corrosion resistance of electroless Ni-P-Al2O3 composite coatings

High phosphorus electroless nickel bath has been used to prepare composite coatings containing alumina powders (50 nm, 0.3 mu m and 1.0 mu m). Deposits were characterized for its structure, morphology and hardness. Incorporation of particle has a marginal influence on the composition. More amount of particle incorporation and uniform distribution was found in composite coatings obtained with 1.0-mu m (C3) alumina particles compared to 50-nm (C 1) and 0.3-mu m (C2) alumina particles. XRD results showed a broad peak of nickel and low intensity alumina peaks present in C3 composite coating in as-plated condition. A marginal improvement in hardness was noticed in as-plated composite coatings. A 15% increase in microhardness was observed in the heat-treated (400 degrees C for 1 h) composite coatings. Potentiodynamic polarization measurements made on these deposits in 3.5% sodium chloride solution showed that uniform corrosion occurred in C1 and C2 composite coatings whereas localized corrosion was observed in C3 composite coating. (c) 2005 Elsevier B.V. All rights reserved.