Electroless nickel-phosphorus plating
Electroless nickel-phosphorus plating, also referred to as E-nickel, is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt.
Source: Wikipedia — Electroless nickel-phosphorus plating (CC BY-SA 4.0)