Production and characterization of electrolytic nickel–niobium composite coatings

Ni–Nb composite coatings were obtained on carbon steel by galvanostatic electrolysis of Watts nickel bath, containing suspended niobium powders. The effect of cathodic current density, stirring rate of the electrolyte, and size of the Nb particles on the volume fraction of co-deposited Nb particles and deposit morphology was investigated. The microhardness and electrochemical behavior of the composites were also evaluated and compared with pure nickel coatings. The bath containing the largest Nb powders (50 μm mean size) led to low Nb content coatings and inhomogeneous distribution of particles. On the contrary, using 20-μm mean size particles, coatings with 8.5–19% Nb content and good particles distribution were obtained. In this case, the 400 rpm stirring rate led to the higher Nb content for all current densities investigated and the smoother coatings were obtained for both 400 and 550 rpm stirring rates. The microhardness of the composite coatings was higher than that of electrolytic Ni obtained under the same electrolysis conditions due to grain refining. Incorporation of Nb particles in Ni coatings improved the corrosion resistance of the deposits in NaCl and H2SO4 solutions.