Electroless nickel plating is a process used to plate a thin nickel-phosphorus or nickel-boron over steel, iron, copper, brass, copper alloys, and zincated aluminium or even on plastic. Electroless nickel plating is a process without the use of a DC power source and electrodes. Electroless nickel plating is an alternative to replace relatively complex traditional electro nickel plating. The chemical reaction of the plating process mainly depends on the presence of a reducing agent sodium hypophosphite solution (NaPO2H2·H2O) which reduces metal ions to form a deposit on the object. Because the resulting nickel deposit also functions as a catalyst to the forming of nickel phosphorous alloy deposit, electroless nickel plating is also named as autocatalytic process. There are three different types of deposits using electroless nickel plating.
- Nickel-phosphorus alloy deposit
A large portion of electroless nickel plating is using nickel-phosphorus alloy. The deposit is able to provide a good tribological characteristic with low frictional coefficient and is galling resistance. The deposit also exhibits high hardness and its hardness can be further increased using post heat treatment process.
- Nickel-boron alloy deposit
The deposit provides good electrical conductivity, low contact resistance, wear resistance and high solderability. As a result, they are widely applied in electronics and aerospace industries.
- Composite electroless nickel deposit
It can co-deposit soft particles (e.g. PTFE or hard particles) or hard particle (e.g. silicon carbide). Electroless nickel & PTFE deposit provides a low frictional coefficient layer and electroless nickel & hard particles can enhance wear resistance.
Properties of electroless nickel
Composite and structure
The main differences between electroless nickel plating and electro nickel plating are their purity and structure. The purity of nickel deposit is usually more than 99% in electro nickel plating process. However, when sodium hypophosphite is adopted as a reducing agent in electroless nickel plating process, the deposit typically contains 92% nickel and 8% phosphorus. Nickel-phosphorous deposition can be classified as low phosphorus 2-5%, medium phosphorus 6-9% and high phosphorus 10-13%. Percentage of phosphorus has a great influence on properties of deposit, for example, high phosphorus and low phosphorus deposits exhibit distinctly different properties of hardness and corrosion resistance.
The structure of electroless nickel deposit can be an amorphous structure or fine crystalline. When the phosphorus percentage increases above 10.5%, the deposit is considered as truly amorphous structure. Due to its amorphous structure, the possibility of inter-granular corrosion is greatly eliminated. Amorphous structure will offer a more effective barrier to corrosion.