Electroless nickel describes the plating of nickel deposits, which may contain phosphorus and boron, onto catalytic metallic or catalyzed non-metallic substrates by chemical reduction. Unlike electrolytically plated nickel coatings, electroless nickel coatings produce very uniform, hard and lubricious coatings, without an externally applied electric current, and are normally identified according to their phosphorus content.

By varying the percentage of phosphorus or boron in the coating, deposits can be produced to exhibit non-magnetic and highly corrosion resistant characteristics or hard deposits with excellent wear resistance.

Electroless nickel plating is a process for chemically applying nickel-alloy deposits onto metallic substrates using an auto catalytic immersion process without the use of electrical current. It differs from electroplating, which depends on an external source of direct (electrical) current to produce a deposit on the substrate material. Since electrical current cannot be distributed evenly throughout the component, it is very difficult to obtain uniform coatings with electrolytically applied deposits. Electroless nickel deposits, therefore, are not subject to the uniformity problems associated with electroplated coatings.

Electroless nickel is the preferred choice among functional coatings for irregularly shaped, highly detailed part geometries because of its completely uniform deposit thickness and close dimensional tolerance capabilities. Electroless nickel deposits can also contain up to 25% PTFE, silicon carbide, diamonds or other alloying materials, which result in deposit properties for superior to those of conventional electro-plated deposits.

Electroless Nickel Melting Point

Electroless nickel is an eutectic alloy with a wide melting range. Unlike a pure compound, it does not have a true melting point. The melting range for electroless nickel coatings varies depending upon the phosphorus content of the deposit. In fact, the melting range becomes wider as the phosphorus content is reduced.

Electroless Nickel Density

The density of electroless nickel coatings is inversely proportional to their phosphorus content. Densities vary from about 8.5 gm/cm3 for very low phosphorus deposits to 7.75 gm/cm3 for deposits containing about 10.5% phosphorus.

Electroless Nickel Electrical Resistivity

The electrical properties of these coatings also vary with composition. For high phosphorus deposits, coatings are significantly less conductive than conventional conductors such as copper. For low phosphorus deposits, electrical resistivity is about 20 µO-cm.

Because of the relatively thin layers used, however, for most applications, the resistance of electroless nickel is not significant.
Heat treatments precipitate phosphorus from the alloy and can increase the conductivity of electroless nickel by two to four times. The formulation of the plating solution can also effect conductivity. Electroless nickel boron yields the lowest resistivity of any commercial electroless nickel types. Phosphorus content also has a strong effect on the thermal expansion of electroless nickel.

Deposit Structure

Hypophosphite reduced electroless nickel is one of the very few metallic glasses used as an engineering material. Depending upon the bath formulation, which can have a dramatic effect on deposit structure through changes in plating rate, deposit content and deposit stress levels, the structure of these coatings varies according to their phosphorus composition.

Deposit Uniformity

One especially beneficial property of electroless nickel is its uniform coating thickness, which can effect the ultimate performance of the coating, and can also eliminate additional finishing to be required after plating. With electroplated coatings, thickness can vary significantly depending upon the part’s configuration and its proximity to the anodes.

With electroless nickel, the coating thickness is the same on any section of the part exposed to fresh plating solution, and can be controlled to suit the application. Grooves, slots, blind holes, and even the inside of tubing will have the same amount of coating as the outside part.

The mechanical properties of electroless nickel deposits are similar to those of other amorphous deposits. They have high strength, limited ductility and a high modulus of elasticity .

Tensile Strength

The ultimate tensile strength of most coatings is equal to many hardened steels and allows the coating to withstand a considerable amount of abuse without damage. The effect of phosphorus content upon the strength and strain at fracture of electroless nickel deposits exceeds 700 MPa (100kpsi).

Ductility

The ductility of electroless nickel coatings also varies with composition. The ductility of coatings is less than that of most engineering materials, but is adequate for most coating applications. Thin films of the deposit can be bent completely around themselves without fracture, and the coating has been used successfully for springs and bellows. Electroless nickel, however, should not be applied to articles which subsequently will be bent or drawn. Severe deformation will crack the deposit, reducing corrosion and abrasion resistance. With lower phosphorus deposits, or with deposits containing metallic or sulfur impurities, ductility is greatly reduced and may approach zero.

Deposit Appearance

Deposit appearance varies considerably depending upon bath formulation and substrate topography. Baths can be formulated to produce deposits that vary from matte to extremely bright.

Since electroless nickel deposits have virtually no leveling capabilities, these coatings mirror the finish of the surface to which they are applied. As a result, even a very bright deposit may appear dramatically less bright on a casting or blasted surface if compared to a similar deposit on a polished surface.
If corrosion protection, good deposit elongation, and low stress of high thicknesses with minimum pitting are desired, brightened deposits are normally not recommended.

Adhesion

The adhesion of electroless nickel coatings to most metals is excellent. The initial replacement reaction, which occurs with catalytic metals, together with the associated ability of the baths to remove submicroscopic soils, allows the deposit to establish metallic as well as mechanical bonds with the substrate.

With non-catalytic or passive metals, such as stainless steel, an initial replacement reaction does not occur and adhesion is reduced. Therefore, these metals must have a thin nano layer of electrolytic nickel to initiate the plating of electroless nickel.With metals such as aluminum, it is common practice to bake parts after plating for one to four hours at 130° to 200° (270° to 400°F) to increase the adhesion of the coating. These treatments relieve hydrogen from the part and the deposit and provide a very minor amount of codiffusion between the coating and substrate.

Hardness and Wear Resistance

One of the most important properties for many applications is hardness. As deposited, the micro-hardness of electroless nickel coatings is about 500 to 700 HK100. That is approximately equal to 45 to 58 HRC and equivalent to many hardened alloy steels. Heat treatment causes these alloys to precipitation harden and can produce hardness values as high as 1100 HK100, equal to most commercial hard chromium coatings.

For some applications, high temperature treatments cannot be tolerated because of part warpage or because of their effect on the substrate. For these, it is sometimes possible to use longer times at lower temperatures to obtain the desired hardness.

Treatment at 340°C (650°F) for four to six hours and at 290°C (550°F) for 10 to 12 hours are commonly used for electroless nickel deposits. Those treatments can produce hardness values of 950 to 1000 HK100. Treatments at 260°C (500°F) are also occasionally used, although the resulting hardness is lower. At temperatures of 230°C (450°F) and below, only a minimal increase in hardness is obtained. Accordingly, such treatments are only rarely used, except for hydrogen embrittlement relief or adhesion improvement.

Electroless nickel coatings also have excellent hot hardness. Up to about 400°C (750°F) the hardness of heat-treated electroless nickel is equal to or better than that of hard chromium coatings. As-deposited coatings also retain their hardness to this temperature, although at a lower level.

Wear and Abrasion Resistance

Electroless nickel coatings have excellent resistance to wear and abrasion, both in the as-deposited and hardened conditions. Laboratory tests have shown fully hardened coatings to have wear resistance equal to hard chromium under both dry and lubricated conditions. The excellent wear resistance of electroless nickel often allows it to replace high alloy materials and hard chromium.

Tests with electroless nickel coated V-blocks in a Falex Wear Tester have confirmed a similar relation between heat treatment and wear, and have shown the coating to be more resistant than hard chrome under lubricated wear conditions.

Tests show that after heat treatment, high phosphorus deposits provide the best resistance to adhesive wear.

Frictional Properties

The frictional characteristics of electroless nickel coatings are excellent. Their phosphorus content provides a natural lubricity, which helps to minimize heat buildup and reduces scoring and galling which can be very useful for applications such as plastic molding.

The corrosion resistance of an electroless nickel coating is a function of its composition. Most deposits are naturally passive and very resistant to attack in most environments. Their degree of passivity (and corrosion resistance), however, is greatly affected by their phosphorus content.

Often, the tramp constituents present in an electroless nickel bath are even more important to its corrosion resistance than its phosphorus content. Most coatings are applied from baths stabilized with lead, tin, cadmium, or sulfur. Codeposition of these elements in more than trace amounts causes a severe reduction in the coating’s passivity and corrosion resistance.

One of the most important variables affecting the corrosion of electroless nickel is its heat treatment. Reducing the phosphorus content of the remaining amorphous material reduces the coating’s corrosion resistance. The particles also create small active/passive corrosion cells, further contributing to the deposit’s destruction. Another effect of heat treating is that the deposit shrinks as it hardens, which can crack the coating and expose the substrate to attack.

Electroless nickel is normally applied for five different applications: coatings used for corrosion resistance, wear resistance, lubricity, solderability, or buildup of worn or over-machined surfaces. To varying degrees, these properties are utilized by all segments of industry, either separately or in combination.

Petroleum and Chemical Industries

The petroleum and chemical process industries are the largest users of electroless nickel for corrosion protection. High phosphorus coatings are commonly used to resist corrosion and erosion in aggressive brines, acids, and gasses. Common applications include valves, chokes, blowout preventers, mud pumps, sucker rod and submergible pumps, pipe, heat exchangers and separators, packers, safety valves, production tubing, and all types of downhole tools.

Medical, Dental and Pharmaceutical

Electroless nickel is also used for medical, dental and pharmaceutical equipment because of its superior corrosion and wear resistance. This equipment is often subject to severe abuse, but must remain completely reliable. Electroless nickel provides almost complete resistance to these environments and frequently allows steel or aluminum to be used instead of more expensive stainless steel.

Typical medical applications are scissors, suture needles, clamps, forceps and hubs for disposable hypodermic needles. In the pharmaceutical industry, extruders, sizing screens, pill sorters and filing equipment are common applications.

Printing and Textile Industries

The use of electroless nickel for the cylinders and rolls used in the printing and textile industries has grown greatly during the past several years. The ability of the coating to deposit uniformly allows the cylinder to be machined to size, balanced, and plated without subsequent finishing or grinding. The life of the equipment is also greatly extended by the lubricity and wear resistance of electroless nickel. Other common textile applications include thread guides, fiber feeds, fabric knives, heddles, bobbins, shuttles, rapiers, ratchets, knitting needles and picks.

Aerospace

In the aerospace industry, electroless nickel is used to protect the surface of light metals, such as aluminum, from corrosion and wear. It also enhances the appearance of these metals with a polished, stainless steel look. The coating is used on a variety of aircraft parts, including engine components, structural air frame and landing gear pieces, refueling systems, compressor blades, and servo valves. Its uniform thickness, and its ability to coat the inside of holes and recesses, makes electroless nickel an ideal coating for welded tanks and complex hydraulic valve and manifold systems.

Packaging and Handling

With packaging machinery and food handling equipment, electroless nickel is also used because of its excellent wear and corrosion resistance. The coatings provide an attractive finish and help to ensure the cleanliness of the part. Electroless nickel is used to handle such diverse products as sodium hydroxide, food grade acids and fish oils. Its uniform deposit is especially useful for hydraulic cylinders, worm feeds and extruders, shafts, chain belts and other close fitting parts. Common food handling applications include pneumatic canning machinery, hamburger molds and grills, bun warmers, baking pans, fryers and chocolate molds.

Mining

Equipment for mining operations is a growing application for electroless nickel coatings. Mining environments are both very corrosive and abrasive. Mine waters are typically acidic and can cause high rate of attack of unprotected steel. In addition, the dust produced during mining can result in severe erosion. Electroless nickel coatings have been found to withstand these conditions with little attack. Common applications are hydraulic components, framing, cylinder heads for jetting pumps, pipeline connections and tubing, and mine engine components.

Wood, Pulp and Paper

Wood handling, pulp and paper equipment operate under conditions of severe corrosion and abrasion. The salt and organic acids present in woods can cause high rates of attack on common materials. Electroless nickel coatings provide good protection against these conditions and are presently being used for knife holder cover plates and for abrading plates for wood cutting and chopping machines. Differential pins are also a large application area.

Automotive

Except for plated plastics, components for the automotive industry historically have provided only a limited market for electroless nickel. With the longer lives and greater reliability now required for automotive components, however, more and more applications for the coating are being developed. Some existing applications are pad holders for disc brakes and brake cylinders, synchromesh gears, piston rods, shock absorbers, steering assemblies, mufflers, exhaust pipes, exhaust manifolds and lock components. Differential pins are also a large application area.

Molds and Dies

The ability of electroless nickel and electroless nickel/PTFE coatings to provide a uniform deposit, even into deep recesses, helps to ensure that the finish on a mold will duplicate the original surface. The natural lubricity of the coating provides smooth flow during injection and quick and easy release of the part. Because of its high hardness at elevated temperatures, electroless nickel minimizes erosion and abrasion of molds and dies and helps to extend their lives. The coating also provides excellent protection against the corrosive fumes produced during molding such plastics as ABS, PVC, polycarbonate, acrylics and materials with thermoplastic additives. Similarly, electroless nickel has been found to provide a superior coating for zinc diecast dies and glass molds.

Electronics

Coaxial connectors, headers, housings and cases, heat sinks, diode cans, shutters, interlocks and memory disks and drums are among the many electronic applications presently plated with electroless nickel. For electronic components the coating is used for its combination of superior corrosion resistance and solderability. Other important considerations with these components are the coating’s uniform thickness and its consistent electrical, thermal and physical properties.

Salvage and Repair

Electroless nickel coatings are very cost effective for salvage and repair. Because of its superior adhesion, uniform and accurately controlled thickness, and excellent wear and corrosion resistance, these coatings are often used to selectively build up worn or mis-machined parts. The cost savings from such repairs are often substantial, since they not only allow mis-machined parts to be used and thus avoid their remanufacture, but this also allows the manufacturing facility to increase the production of new parts and improves productivity.

AMS 2404
AMS 2405
ASTM B656
ASTM B733
AMS MIL-C-26074

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