Electrodeposited Ni–Co–B Alloy Coatings: Preparation and Properties

Ternary Ni–Co–B alloys with boron content up to 27 at% were electrodeposited from sulfate-chloride electrolyte containing sodium decahydro-closo-decaborate as a boron source, and the effect of the boron content on the structure, morphology, corrosion, mechanical and magnetic properties of the alloy coatings was studied. With increasing the boron content the structure of the alloys is changed from polycrystalline to amorphous-nanocrystalline and then to amorphous, and the surface becomes more smooth and uniform. During prolonged corrosion testing in 3.5% NaCl solution the amorphous alloys with high boron content demonstrate a gradual decrease in the corrosion rate due to the formation of a rather compact film of the corrosion products consisting of nickel-cobalt hydroxides. In addition, the amorphous Ni–Co–B coatings are less susceptible to the pitting corrosion in comparison with polycrystalline and amorphous-crystalline ones. The boron doping of the Ni–Co alloys improves significantly their mechanical properties. Amorphous Ni–Co–B coatings containing 20 at% B and 10 at% Co demonstrate the highest microhardness and wear resistance. At high content of boron and cobalt, the Ni–Co–B alloys are magnetically soft ferromagnetic materials with high magnetic permeability and low coercivity.