Cobalt-molybdenum-phosphorus alloys: Electroplating and corrosion properties

Abstract

The relation between the composition, morphology, and corrosion resistance of the electrolytic CoMoP coatings plated at various current densities from citrate baths containing ammonia or hydrazine and various amounts of sodium hypophosphite is studied. Phosphorus was included in the coatings as a result of the electrochemical reduction of hypophosphite ions. Ternary Co-Mo-P alloys containing up to 26 at % Mo and up to 3 at % P were electroplated. Coatings of the highest quality were obtained from a bath containing hydrazine admixture at a temperature of 50°C. Corrosion resistance of CoMoP coatings increases with an increase in the Mo content and a certain decrease in the phosphorus amount in the deposit. The best protective properties were demonstrated by coatings obtained at higher concentrations of sodium hypophosphite and high deposition current densities when CoMoP alloys containing 23 ± 2 at % Mo and up to 1.5 at % P were formed.