Most metal plating takes place via the application of electricity. There is a class of plating called electroless that, as the name implies, requires no electricity. The mechanism, oxidation-reduction, is the same, but it is accomplished via a chemical reaction instead of DC power. Metal ions with a positive charge are reduced to their elemental state with electrons from chemical reactions rather than electrons in the form of current. While a few metals can be deposited with this method, the most common is nickel.
Electroless nickel plating is accomplished via a heated bath (around 200F) of nickel hypophosphate. The nickel catalyzes on an activated metal surface and "grows" on the surface at a controlled rate.
The deposit is actually an alloy of nickel and phosphorous, and varying the amount of phosphorous can yield different desirable characteristics. Higher amounts of phosphorous, >10%, give the best corrosion protection. 5-10% phosphorous yields the most aesthetically pleasing finish. <5%, considered "low phos," gives the highest surface hardness. Various alloys also yield differing magnetic properties, a characteristic that the computer hard disc industry harnesses to optimize its products.
Because electroless nickel uses chemistry rather than electricity, the deposit is remarkably uniform across all geometries. This allows for internal features to be plated because there is no requirement that a surface "see" an anode (or cathode in the case of anodizing). It also facilitates predictable tolerances since there is not the variation that occurs from electricity's affinity for points and sharp edges in standard electroplating.
Where electroless nickel is used for corrosion protection, it protects by encapsulation, rather than the more familiar galvanic protection afforded by metals like zinc and cadmium. Those metals corrode preferentially to steel and sacrificially protect it to extend its service life. Nickel forms a natural oxide layer and is inert to many corrosive elements. Thus the thicker the nickel layer, the better the protection. The possibility of the coating being compromised decreases as the nickel thickness increases.
If electroless nickel is a candidate for your engineering application, or for more information on the finish, visit www.chemprocessing.com.
