Electroless Nickel Plating
Electroless Nickel Plating
Electroless Nickel Plating (ENP): Electroless Nickel Plating (ENP) is the deposit of a nickel-alloy coating by chemical reduction – without the electric current that’s used in electroplating processes. The majority of ENP for engineering purposes is a nickel phosphorus deposit containing 2 to 14% phosphorus. The higher the phosphorus content the greater the corrosion resistance, however the compromise on increased phosphorus content is a decrease in hardness. ENP is deposited by reducing nickel ions to metallic nickel with a chemical reducing agent such as sodium hydrophosphite. Thickness of 25-75m is common, high phosphorus ENP can edge above 75m although this does start to push the limits of ENP.
The benefits of electroless nickel plating
ENP offers excellent corrosion resistance to common corrodents such as salt water, carbon dioxide ,oxygen and hydrogen sulphide.
High phosphorus deposits of ENP (10-14% phos) is also amorphous, which means that there are no grain or phase boundaries to create initiation sites for corrosion.
The uniformity of ENP versus electrolytic deposits is also advantageous, creating a uniformly thick coating across the whole substrate; even in slots, holes and inside walls of tubing.
Without heat treatment corrosion resistant high phosphorus ENP still provides good hardness and wear resistance properties.
ENP offers a cost-effective solution for applications subject to conditions where wear and corrosion are factors; extending service life and providing a lower cost alternative to corrosion resistant alloys, particularly where carbon steel suffers localised corrosion; flange attack or weld corrosion.
What are the properties of the deposit and factors affecting them?
ENP offers excellent corrosion protection and uniformity of deposit along with good hardness. The phosphorus content of the deposit can be altered to best fit the application.
At Poudrafshan our technical team understand each customers’ application before recommending the optimum phosphorus level – the higher the phosphorus content the greater the corrosion resistance but lower the surface hardness.
Hardness of ENP can be increased by heat treatment but this produces a microcracked deposit with reduced corrosion resistance.
ENP deposits uniformly, even on complex shapes, unlike electroplating.
How does heat treatment affect ENP?
ENP is at its most corrosion resistant in its amorphous phase. Heat treatment causes particles of nickel phosphide to precipitate, destroying the amorphous character of the deposit. With high phos ENP deposits this occurs at temperatures between 330 and 360C.
How well does ENP adhere to substrates?
Adhesion is dependent on effective cleaning of the substrate. With good surface preparation adhesion to carbon steel is between 200-420 MPa. With stainless steels the bond strength is usually between 160-200 Mpa.
What factors affect plating quality?
There are several factors affecting plating quality and the majority should be effectively managed by your surface finishing partner.
– Surface preparation: Rough, badly machined surfaces with uneven metal surfaces, burrs or cold shuts cannot be effectively plated.
– Cleaning: Surfaces must be free from oils, dirt and soaps formed by saponification of oils by alkaline cleaners.
– Control of the plating baths: Temperature, pH, nickel ion concentration and hypophosphite concentration all need to be carefully managed to ensure high quality, consistent plating finish is achieved.
What are the fabrication / finishing requirements before plating?
Weld spatter should be removed, sharp edges radiused and weld roots ground smooth. When refurbishing / remanufacturing components it is important that corroded areas are ground to a flat surface.
COMMON APPLICATIONS OF ELECTROLESS NICKEL COATINGS:
- Aerospace Components:For wear resistance, corrosion protection, chemical resistivity and lubricity on valves, pistons, engine shafts, engine mounts, compressor blades and other flight-critical components.
- Electrical Equipment:Due to solderability and conductivity properties.
- Packaging & Handling Machinery: Due to wear resistance, cleanliness and attractive finish.
- Chemical Manufacturing and Transport Equipment:Due to chemical resistance.
- Molds & Dies:Corrosion protection minimizes erosion and abrasion. Low coefficient of friction improves release.
- Food Service Equipment:For superior corrosion and wear resistance, attractive finish and cleanliness.
- Plastics Manufacturing Equipment:For durability and release properties on injection molds and extrusion dies.
- Oil & Gas Components: For corrosion protection in harsh undersea and underground environments for parts such as valves, pumps, pipe fittings and others.
- Printing Industry Equipment: For abrasion resistance on conveyance and chemical resistance on printer cylinders.
- Automotive Components:For wear protection and corrosion resistance on pistons, cylinders, gears, differential pinon ball shafts, fuel injectors, ball studs, transmission thrust washers, knuckle pins, hose couplings, heat sinks and others.
- Salvage:For restoration of parts to their original dimensions or to bring mis-machined parts into tolerance.
- Stainless Steel Replacement:Electroless nickel can serve as a cost effective replacement for 300 and 400 series stainless steel in some applications