Combining excellent appearance with good corrosion resistance, nickel has long been used as a plating material. Prior to 1946, nickel coatings were applied to most substrates by using electricity (electroplating). Methods have since been developed that allow electroless nickel plating of most metals and also many nonmetallic substrates.
Most electroless nickel coatings contain 5 to 15% phosphorus. The high-nickel, low-phosphorous alloy deposited by chemical reduction without electric current is mil-for-mil more corrosion resistant than electroplated nickel. On properly prepared substrates, electroless nickel coatings are virtually pore-free.
Also, unlike electrolytic nickel, the thickness of electroless nickel deposits are distributed evenly over the entire surface even of complex parts. Blind holes, threads, small diameter holes, recesses or internal areas receive the same amount of plating as sharp corners, edges or flat surfaces. Therefore, total thickness can often be reduced, close tolerances can be maintained, and although electroless nickel can be machined, subsequent machining to size is frequently eliminated.
One of the chief uses of electroless nickel has been preventing corrosion of metals. Thus electroless nickel has found ready acceptance in fields such as missiles and rocketry, oil refineries, chemical plant equipment, tank interiors, and many other mechanical and electrical applications.
Due to its hardness, wear resistance, and uniform deposit thickness, electroless nickel coatings have also found wide usage in the field of pumps, compressors, hydraulic pistons and other moving parts. In fact, electroless nickel should be considered wherever there is a need for a hard, smooth, corrosion-resistant, and uniformly distributed metallic coating. Since it has a natural lubricity and, therefore high resistance to abrasion, it has often proved superior to hard chrome plating.
The electroless nickel plating process uses a nickel-phosphorus alloy to coat a substrate without an electric current, dissimilar to hard chrome plating. Instead, this process uses a mix of catalysts to perform the same tasks.
While electroless nickel plating can be used for a wide variety of industries, it is most commonly used for avaiation, plastics, engineering, and agricultural machinery industries. This plating is used to protect applications from wear, abrasion, corrosion, and ultimately add a hardness to the application.
Electroless nickel plating can be anywhere from 450 - 750 VHN, depending on how much phosphorus the coating contains. Low phosphorus plating, which has about 2-5% content, gives the substrate the hardest coating. Medium is less hard (at 6-9% phosphorus content) but plates the fastest. Medium phosphorus plating is the most commonly used coating. The last phosphorus level, high phosphorus plating (10-13% content), is the least hard but is the best for corrosion protection.