Deposition and characterization of nickel–niobium composite electrocoatings

Abstract

Ni–Nb composite electrocoatings were obtained on carbon steel from Watts bath, containing suspended 20 μm size niobium powders. The effect of cathodic current density, electrolyte stirring rate and concentration of Nb particles in the bath on the deposit morphology and texture, volume fraction of co-deposited Nb particles and microhardness was investigated. The Ni–Nb composite layers presented a rough morphology with randomly oriented Ni grains, whereas pure Ni coatings obtained under the same experimental conditions were smooth and showed highly preferred orientation in the [110] or [100] direction. Stirring rate of the electrolyte and concentration of Nb particles in the bath are the main parameters affecting the incorporation of Nb particles. The Nb incorporated volume fraction was 11–14%, 17–19%, 27–32% and 34–37% for the 20 g L−1 Nb/550 rpm, 20 g L−1 Nb/400 rpm, 40 g L−1 Nb/400 rpm and 40 g L−1 Nb/550 rpm conditions, respectively. The microhardness of the Ni–Nb composite coatings obtained at 20 and 40 mA cm−2 was higher than that of pure Ni layers, due to grain refining. Incorporation of Nb particles in Ni coatings improved the corrosion resistance of the deposits in NaCl and H2SO4 solutions.