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Concept
Electroless nickel-phosphorus plating
Summary
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. It is the most common version of electroless nickel plating (EN plating) and is often referred by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.
Unlike electroplating, processes in general do not require passing an electric current through the bath and the substrate; the reduction of the metal cations in solution to metallic is achieved by purely chemical means, through an autocatalytic reaction. This creates an even layer of metal regardless of the geometry of the surface – in contrast to electroplating which suffers from uneven current density due to the effect of substrate shape on the electric resistance of the bath and therefore on the current distribution within it. Moreover, can be applied to non-conductive surfaces.
It has many industrial applications, from merely decorative to the prevention of corrosion and wear. It can be used to apply composite coatings, by suspending suitable powders in the bath.
The reduction of nickel salts to nickel metal by hypophosphite was accidentally discovered by Charles Adolphe Wurtz in 1844. In 1911, François Auguste Roux of L'Aluminium Français patented the process (using both hypophosphite and orthophosphite) for general metal plating.
However, Roux's invention does not seem to have received much commercial use. In 1946 the process was accidentally rediscovered by Abner Brenner and Grace E. Riddell of the National Bureau of Standards. They tried adding various reducing agents to an electroplating bath in order to prevent undesirable oxidation reactions at the anode. When they added sodium hypophosphite, they observed that the amount of nickel that was deposited at the cathode exceeded the theoretical limit of Faraday's law.
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Plating
Plating is a finishing process in which a metal is deposited on a surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.