A method of forming a nickel layer on a metal substrate forming the terminal contact area of the cathode casing for a zinc-air cell. A cathode casing in desired shape and size for a zinc-air cell is formed of a substrate metal such as cold rolled steel or stainless steel. A nickel layer is plated onto the cathode casing substrate metal over the terminal contact area and is subsequently heat treated resulting in reduction in the surface hardness. The method of heat treatment involves heating the substrate metal with nickel plated thereon in an inert atmosphere at temperatures preferably between about 860.degree. F. and 135.degree. F. The method reduces the nickel surface hardness to a level below 8 GPa. This improves the electrical contact with the hearing aid or other device being powered.