Electroless Nickel as a functional coating
It is the functionality and versatility of the EN coating that makes it so desirable as a finish. Properties such as high corrosion and wear resistance, natural lubricity, uniform thickness regardless of component geometry and high surface hardness makes Electroless Nickel the obvious choice.
Electroless Nickel describes the plating of nickel deposits onto a suitable catalytic substrate by chemical reduction. These deposits generally contain up to 12% phosphorus by weight and unlike electroplated coatings, electroless nickel (EN) is applied without the use of external DC current via rectifiers and anodes. Instead, the coating is deposited onto the part’s surface by reducing nickel ions to metallic nickel with a reducing agent. The process is autocatalytic and will build thickness continuously. The resulting nickel-phosphorus alloy possesses many unique and desirable characteristics making it a superior engineering material.
By avoiding the problems associated with electrolytically applied coatings (ie high and low current densities with corresponding high and low plating thicknesses on the part or no plate in recesses), EN deposits in contrast, are of uniform thickness and hard. The chemical deposition allows plating of uniform thickness on any section of a geometrically complex part exposed to fresh plating solution. Grooves, slots, blind holes, threads and even the inside of tubing will have the same thickness of plate. Hardness as plated is typically 480-600 VHN (48-55 Rockwell C) but can be substantially increased with heat treatment.
Additional properties of EN deposits include excellent wear resistance, low coefficient of friction, solderability and superior corrosion resistance (upto 1000 hours NSS). Adhesion is extremely high strength and the EN coating, by virtue of the inherent lubricity, is also anti-galling. EN deposits have the appearance of semibright to bright stainless steel, and as such, the coating is sometimes used for decorative applications.
Quality high phosphorus EN deposits belong to a group of compounds called “metallic glasses”. These are hard, amorphous compounds without grain boundaries or any crystal structure. The surface of the EN deposit is inert and very passive making it very resistant to oxidation and corrosion.
Electroless nickel was discovered by Brenner and Riddell in 1946 and the process was used soon after and licensed by General American under the name of ‘Kanigen’. The special properties of the EN deposit became widely known in industry and as the technology of the process evolved and developed, so did the applications. Engineers and designers have been specifying electroless nickel ever since for the following industries: Toolmaking, Firearms, Textiles, Food, Medical & Pharmaceutical, Printing, Marine, Aircraft, Electrical, Timber and Pulp & Paper, Hydraulics, Mining, Chemical and Oil & Petroleum. Specific examples would be Toolmaking – plastic moulding and extrusion dies for corrosion resistance to plastics fumes, wear resistance, part release and non-scaling water channels; Timber – corrosion and abrasion resistance, wear resistance (especially on saw blades when an EN/Silicon Carbide composite coating is used) and Hydraulics (and valves) – corrosion and abrasion resistance, lubricity and buildup of worn surfaces.
Other types of electroless nickel coatings are available and these are referred to as ‘Composite Coatings’. These particular coatings are formed when particles are encapsulated or co-deposited within the EN coating as it plates. These coatings can be tailored to specific applications. EN/PTFE(Teflon) deposits are extremely lubricious providing dry film lubrication and non-stick properties. EN/ hexagonal Boron Nitride deposits are harder than EN/PTFE deposits but offer similar benefits and with a wider operating temperature range. EN/Silicon Carbide deposits are very hard and offer extreme wear resistance.
Electroless nickel use in industry and manufacturing continues to grow and potential applications are limited only by the imagination.
For further information, contact: Phil Rogers, Tel 09 4433948, Email: [email protected]