Electroless Nickel Plating
Electroless nickel plating is a process that is used to produce a fine layer of hard nickel on substances such as ceramic, steel, plastics, glass, and aluminum, etc. Electricity is not used in the process and the plating is done by reducing the nickel ions catalytically in a heated aqueous solution bath. This is known as autocatalytic chemical reduction. Electroless coatings have improved corrosion resistance. Typical products such as printing rolls, flow meters, airframes and aircraft parts, chemical vessels and valves, coaxial electrical connectors, computer equipment, hydraulic parts, injection molds, internal combustion engine parts, medical equipment, textile machinery, and rubber molds, etc. are electroless nickel-plated. The nickel layer is hard, wear and corrosion resistant and the deposition is uniform throughout.
The process of electroless nickel plating can produce a layer from 0.005 millimeters to 0.025 millimeters and the rate at which material is deposited is up to 20 microns per minute. The parts to be plated need to be polished and ground before plating because of the thinness of the coating, and because the coating will conform to any burrs and bumps on the surface of the parts.
In electroless plating, the item that is going to be coated is placed into the liquid bath which contains a nickel salt like nickel sulphate and an assortment of complexing agents, buffering agents and stabilizers/inhibitors. These items are used to control the speed of the process. One of the reducing agents that are used is sodium hypophosphite. The amount of its concentration in the reducing bath will alter the level of phosphorus in the coating layer of nickel. This adds to the hardness and corrosion resistance of the item along with lowering its surface friction characteristics.
Because of this, electroless nickel coating is ideal for drills and other types of cutting tools, as well as bearing surfaces and components for electronic devices. A high level of phosphorus content will usually provide a good corrosion resistant coating, but it is relatively soft. If you use a low level of phosphorus content in the coating the outcome will be reversed.
The main types of electroless nickel plating are nickel-phosphorus, nickel-boron, and poly alloys. The nickel-phosphorus process is used to produce a hard, corrosion resistant layer of phosphorus of three percent to seven percent and is used in industrial applications. Nickel-boron produces a layer of boron of up to 10 percent. Poly alloys plating produces plating containing varying percentages of cobalt, molybdenum, iron, boron, tungsten, rhenium, nickel and phosphorus.