SUBSTRATE ROUGHNESS, DEPOSIT THICKNESS AND THE CORROSION OF ELECTROLESS NICKEL COATINGS

The affect of substrate roughness and coating thickness on the corrosion resistance of electroless nickel coatings on mild steel in a 0.99M Na2SO4 + 0.01 M H2SO4 + 0.05M NaCl solution has been investigated using electrochemical techniques. The coating was electrochemically more active than pure nickel. The anodic polarization, corrosion potential, and corrosion current depend on the substrate roughness and coating thickness. The substrate roughness decreased for finishes in the order, as-ground, 240 grit, 600 grit, and 1 μm diamond polish, but the corrosion current on relatively thin coatings decreased in the order 240 grit, as-ground, 600 grit, and 1 μm diamond finish. The corrosion potential and the corrosion current of coatings more than about 10μm thick were independent of the surface roughness and similar to those observed with pure nickel. The fraction porosity was estimated to be about 0.005 in a coating about 5 μm thick on a 600 SiC grit substrate. © 1990 Chapman and Hall Ltd.