Electrochemical nickel-phosphorus alloy formation

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. Nickelphosphorus 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 fee 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.