Corrosion resistance in oxygen of electrolytic nickel and cobalt phosphorus coatings

Corrosion of Ni-P and Co-P alloys in oxygen was investigated in the temperature range 350-800°C on as-electrodeposited films and after annealing treatments. For as-electrodeposited films, both alloys oxidize by diffusion and the scales consist of an inner region rich in phosphorus and an outer region of oxides (NiO or Co3O4). Good corrosion resistance achieved after extended heating above 600°C in vacuum, very well marked for Ni-P alloys, is explained in terms of crystallization and grain coarsening. This indicates that at these high annealing temperatures the crystals of phosphides formed a continuous layer with embedded nickel crystals rendering the alloy extremely resistant to oxygen penetration.