Electroless nickel plating

Electroless nickel plating (EN) is an auto-catalytic reaction that deposits an even layer of nickel-phosphorus or nickel-boron alloy on the surface of a solid material, or substrate, like metal or plastic. The process involves dipping the substrate in a bath of plating solution, where a reducing agent, like hydrated sodium hypophosphite (NaPO2H2 · H2O), reacts with the material's ions to deposit the nickel alloy. The metallurgical properties of the alloy depend on the percentage of phosphorus, which can range from 2–5% (low phosphorus) to 11–14% (high phosphorus). Unlike electroplating, it is not necessary to pass an electric current through the plating solution to form a deposit. Electroless plating prevents corrosion and wear, and can be used to manufacture composite coatings by suspending powder in the bath. EN plating creates an even layer regardless of the geometry of the surface – in contrast to electroplating which suffers from flux-density issues as an electromagnetic field will vary due to the surface profile and result in uneven depositions. Depending on the catalyst, EN plating can be applied to non-conductive surfaces.The EN plating of metallic nickel from aqueous solution in the presence of hypophosphite was a chemical accident by Charles Adolphe Wurtz in 1844. In 1911, Francois Auguste Roux reported that metal was inevitably precipitated in its powder form; however, this was not used in practical applications. In its early stage, progress in the field remained slow until World War II. In 1946, Abner Brenner and Grace E. Riddell developed a process for plating the inner walls of tubes with nickel-tungsten alloy, derived from the citrate-based bath using an insoluble anode, which brought out the unusual reducing properties of hypophosphite. The U.S. Patent Office says that the patent it issued in 1950 differed from the earlier patent in that the Roux reaction was spontaneous and complete, while the Brenner and Riddell process was a controlled catalytic process so that deposition occurred only on catalytic surfaces immersed in the bath. Brenner later wrote that his patent was an accidental discovery similar that of Wurtz and Roux, and filed a patent to protect the rights of the United States government. A declassified US Army technical report in 1963 credits the discovery to Wurtz and Roux more than Brenner and Riddell. EN plating was attributed to the chemical reduction of nickel ions.Before plating, the surface of the material is cleaned by applying a series of chemicals. Unwanted solids left on the surface cause poor plating. After applying each pretreatment chemical, the surface is rinsed two to three times with water to completely remove the chemicals. Removing oils is known as de-greasing, and removing scaling is known as acid cleaning.EN plating does not require electrical power, sophisticated jigs, or racks. The chemical replenishment is costly but can be monitored automatically, and complex filtration is not required. The lifespan of the chemicals, however, is limited. EN plating deposits an even coating of a desired thickness and volume, even in recesses and blind holes. The plating can have a matte, semi-bright, or bright finish. Because EN plating is porous, the quality of the material structure is inferior compared to electrolytic processes.Low-phosphorus treatment is applied for deposits with hardness up to 60 on the Rockwell C scale. It offers a uniform thickness on inner and outer surfaces of complex shapes, which often eliminates the need for grinding after plating. It is also excellent for corrosion resistance in alkaline environments.The most common form of electroless nickel plating produces a nickel-phosphorus alloy coating. The phosphorus content can range from 2% to 13%. Such plating is used when wear resistance, hardness and corrosion protection are required. Applications include oilfield valves, rotors, drive shafts, paper handling equipment, fuel rails, optical surfaces for diamond turning, door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment. It is also commonly used as a coating in electronics printed circuit board manufacturing, typically with an overlay of gold to prevent corrosion. This process is known as electroless nickel immersion gold.