When exposed to varied environmental conditions and corrosive uses, raw metals other than precious metals can quickly oxidise and corrode. Any component’s correct design must begin with a surface engineering study to verify that it will perform reliably for the product’s intended service life. Corrosion can cause functional difficulties and deficiencies even in very low-risk service applications.
Electroless nickel plating can protect against corrosive attacks from various sources, including galvanic corrosion, chemical attack, and erosion. Electroless nickel plating is a type of electroless nickel plating used on different metals, including steel, copper, brass, and aluminium alloys. Heavy equipment, oil and gas, power transmission and distribution, automotive, marine, and railway are just a few industries that use nickel plating to improve corrosion protection performance.
Characteristics of Electroless Nickel Plating
Electroless nickel plating differs from electrolytic nickel plating in that it does not rely on an external electron source for deposition. As a result, the Electroless nickel deposit has far better uniformity than electrolytic deposits because this auto-catalytic method does not require passing an electric current through parts to generate the plated deposit. Electroless nickel plating’s enhanced uniformity is a crucial deposit attribute that improves corrosion resistance on all features of a part, particularly those that are usually difficult to coat, including thru-holes and counterbores.
Another distinguishing feature of Electroless nickel plating is that the nickel co-deposits phosphorus in different proportions depending on the bath type, ranging from 4 to 13 per cent. The amount of Phosphorus in a deposit directly impacts its hardness, corrosion resistance, and ductility. Furthermore, post-plate heat treatment can be utilised to change the structure of electroless nickel plating, resulting in hardness of up to 70 Rc.
Options for Advanced Electroless Nickel Plating
Advanced Plating Technologies has developed multi-layer or duplex systems that include a copper and electrolytic nickel underplate before the electroless nickel deposit to improve the corrosion performance of electroless nickel plating. APT also has proprietary sealing technologies like our H4 and H5 molecular sealers, improving ENP’s salt spray performance by up to 2x.
Corrosion Protection Options for Electroless Nickel Plating
Traditional electroless nickel-phosphorus plating is divided into three categories: low Phosphorus (5% P), medium phosphorus (6-10% P), and high Phosphorus (>10% P) (11-13 per cent P). The phosphorus amount and deposit thickness can be modified based on design considerations and the desired corrosion performance. Advanced Plating Technologies provides both medium and high phosphorus electroless nickel plating. The following is a breakdown of each Electroless nickel plating type:
Phosphorus deficiency (11-13%): Electroless nickel with high Phosphorus (high-phos) offers the best corrosion resistance, is non-magnetic, has the lowest melting point (about 880°C) and is the most malleable of the Electroless nickel plating deposits. The deposit has an amorphous structure with a hardness of 48-55 Rc when plated, which can be enhanced to 66-70 Rc by post-plate heat treatment.
High Phosphorus electroless nickel has the lowest as-plated internal tension due to its amorphous nature, making it the best electroless nickel for heavy build deposits with thicknesses of up to 0.005 inches per side. The deposit has a semi-bright look that varies depending on the plated part’s raw surface polish. Compared to medium Phosphorus electroless nickel, the high phosphorus electroless nickel plating rate is slower, increasing cost.
Medium Phosphorus (6-10 per cent P): Electroless nickel with a medium phosphorus content (mid-phos) is known as the “workhorse” electroless nickel. It has a high melting point (1000 C), superior corrosion resistance, and solderability. The hardness of mid-phos electroless nickel-plated in a mixed amorphous/microcrystalline state is 58-62 Rc as plated and can be enhanced to 66-70 Rc with post-plate heat treatment options.
The appearance of medium phosphorous electroless nickel plating ranges from semi-bright to bright, depending on the raw surface finish of the item. The faster plating rate of mid-phosphorus EN plating, compared to high-phosphorus EN plating, is a common advantage.
Low Phosphorus (5 per cent or less P): Due to the micro-crystalline as-plated structure, low-phosphorus electroless nickel plating (low-phos) has the highest as-plated hardness (up to 60 Rc) and is the least ductile. Low-phos is the least corrosion resistant of the three electroless nickel kinds, although it is the most conductive and solderable when unplated. Because the commercial demand for medium or high phosphorus electroless nickel plating is often substantially higher, fewer metal finishing job shops provide low phosphorus electroless nickel.
Electroless Nickel Plating Heat Treatment
When it comes to electroless nickel plating, one of the most frequently asked questions is how heat treatment affects the deposit qualities. The following is a quick rundown of heat treatments:
Preplate Bakes: To increase adhesion and limit the possibility of hydrogen embrittlement, several specifications recommend preplate bakes and shot peening of higher stress ferrous alloys. The characteristics of future ENP are unaffected by prelate bakes.
Low-temperature post-plate bakes (500F) are commonly employed to improve adhesion, particularly in aluminium alloys, and ease hydrogen embrittlement in ferrous alloys. With these lower temperature bakes, there is no discernible change in deposit characteristics.
High-temperature post-plate bakes (500F-750F) are commonly utilised in both medium and high-phosphorus electroless nickel plating to convert amorphous to crystalline structures. The hardness of the deposit increases as the structure changes, while the corrosion resistance diminishes.