Electroplating of crack-free corrosion resistant Co-Mo alloy coatings

Abstract

Thick crack-free Co-Mo coatings containing 12%–18% molybdenum can be electroplated on metal substrates from an aqueous citrate complex bath containing sodium molybdate and a cobalt salt. The deposits have a columnar microstructure and a hardness of about 400 VHN. The internal tensile stress in the deposits is about 5 kg/mm², does not change significantly with molybdenum content, but shows a slight decrease when the cathode current density increases. These Co-Mo deposited alloys are well-crystallized supersaturated solid solutions with a close-packed hexagonal structure. The cathodic current efficiencies vary between 60% and 75%, depending on experimental conditions. The resistance to acids, especially concentrated hydrochloric, is excellent. Corrosion behaviour of deposits is improved by adequate heat treatment and reaches that of their cast counterparts.