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What is electroless nickel plating?

 

Electroless coating (also known as autocatalytic coating) is defined as the deposition of a metallic coating by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited. The electroless process has several advantages over electroplating:

  • virtually unlimited throwing power
  • little or no excess deposit at high points
  • deposits of excellent physical and chemical properties
  • reduced hydrogen charging (hydrogen embrittlement)
  • ability to coat surfaces which would be difficult or impossible by electroplating

The principal disadvantage of the electroless process is high cost.

By far the most common form of electroless coating is nickel plating, which is actually a nickel-phosphorus alloy (2-10% P). Electroless nickel has the unusual quality of being amorphous and not crystalline in structure. These coatings possess high hardness, natural lubricity, and good wear and abrasion resistance.

Typical applications for electroless nickel include fasteners, pipe and valves. The coating thickness ranges from 2.5 to 150µm, but most components used for engineering purposes have a thickness of around 50 - 100µm. The greater thickness is usually required when rough surfaces are encountered.

Electroless nickel is sometimes an economical treatment to improve the performance of carbon steel in mildly corrosive environments (such as chlorides, trace acids, caustic solutions) and in situations where light wear may occur in service. It is, however, difficult to deposit electroless nickel on chromium-containing steels. Electroless nickel can be deposited on the internal diameter of tubular components and other difficult to access surfaces.

The most common pitfall associated with the use of electroless nickel plating is inadequate surface preparation. Maximum adhesion depends on eliminating surface contaminants and generating a completely active surface to initiate plating on all areas. Electroless nickel is a cathodic or non-sacrificial metal coating, unlike zinc or aluminium. Therefore, it must cover the entire surface of the component to provide corrosion resistance. Accelerated corrosion can occur at localised areas where the coating is defective or penetrated by the corrosion media, or where unfavourable galvanic contacts are set up.

There are variants of electroless nickel plating, the most common of which include:

  • composite electroless nickel, in which SiC (silicon carbide) particles are co-deposited with the nickel to enhance its strength and wear resistance
  • duplex electroless nickel where an undercoat containing 14% phosphorus is used with a top coat of 5% phosphorus.

See further information about Materials & Corrosion Management or please contact us.

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