Electrochemical nickel–phosphorus alloy formation

Abstract

What is believed to be the first comprehensive review on electrochemical nickel-phosphorus alloy formation is presented. Nickel-phosphorus deposits are important surface finishes for industrial materials owing to their electrocatalytic activity for hydrogen evolution, special paramagnetic characteristics, excellent microhardness and corrosion resistance. Nickel- phosphorus is a relatively complex system and can form a number of stable compounds. Two mechanisms for the incorporation of phosphorus during electrodeposition of Ni-P have been proposed: a direct and an indirect mechanism. In the direct mechanism, the oxyacid in the bath, i.e. phosphorous acid (H₃PO₃), is reduced solely to the elemental state. In the indirect mechanism, phosphine (PH₃) is an intermediate stage in the formation of the Ni-P alloy. Electrodeposition of nickel takes place in a fcc structure with co-deposition of phosphorus in octahedral interstitial sites. The pH of the plating solutions is an important consideration for the electrodeposition of phosphorus alloys. It is well known that above a critical phosphorus content, the Ni-P alloy becomes amorphous; amorphous alloys tend to be harder and have higher contact resistance than their crystalline counterparts. Amorphous alloys prepared by electrodeposition are generally brittle or of low ductility.