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Electroless Nickel

Electroless Nickel is a hard, silver coloured coating comprised of nickel alloyed with between 4 and 14% phosphorous. It is deposited by immersion of parts in a solution of nickel salts and reducing agents at a temperature of 90°C. Although it has the appearance of an electroplated coating (e.g. like Hard Chrome), the process is purely chemical, so that deposition is evenly distributed over the part, including internal and external corners.

The bath chemistry changes with use, so that periodic additions are made to maintain the composition and optimise the deposition characteristics. This is important to the coating properties, see below.

Variations

There are four main types of Electroless Nickel.

In general, designers will usually specify the medium phosphorous variety. It is the easiest process to control, the most economical and has the fastest deposition rate. (Users should consult Poeton for advice on their specific application needs.)

Thickness and coverage

A coating thickness of 25 is the normal deposit, with 50 sometimes applied for more demanding wear or corrosion applications. Above that thickness, the surface roughness increases and the deposition rate falls. Coverage is very even, unlike electroplated coatings, so that complex parts such as screw threads will be replicated.

Designers should allow for a variation in thickness over a part of +/- 10% of the deposit (e.g. 2.5 in a 25 deposit). Through holes and bores are coated evenly, provided that the electrolyte has access, and is agitated. To coat down a blind hole, including the base, may require some special jigging, and Poeton will advise on feasibility and cost. Areas that do not require coating can be easily masked with wax and tape.

Coating structure

The coating structure varies from microcrystalline for low phosphorous versions to amorphous for the highest phosphorous variety (giving best corrosion properties), and a mixture of microcrystalline and amorphous for the the medium phosphorous type, the most common. Heat-treatment (see below, for the medium phosphorous variety) precipitates a Ni3P dispersion in the nickel matrix, greatly increasing the hardness.

Coatings possess a residual stress, compressive when the bath is new and progressively tensile (which reduces the corrosion resistance) as the bath ages. Best results (as with the Apticote 400 process), maintaining a compressive stress throughout the bath life, is achieved by automated monitoring of the bath chemistry and continuous metered makeup additions, rather than periodic, volume additions.

Hardness

As plated, medium phosphorous Electroless Nickel has a hardness of 500-550 VPN. This can be increased by heat-treatment (which precipitates a Ni3P dispersion in the nickel matrix) to values up to 1000 VPN, thereby increasing the wear resistance. The optimum heat-treatment is 1 hour at
400°C.

The low phosphorous variety is harder as-plated (700-750 VPN). The high phosphorous version is not usually heat-treated it is left as-plated for its superior corrosion protection.

Colour

As-plated, the coating has a silver colour. Heat treatment at 400oC to harden the coating will give it a slight straw-coloured hue.

Corrosion

Electroless Nickel is a barrier coating, preventing a corroding media attacking a vulnerable substrate. It is not a sacrificial coating like cadmium or zinc. Corrosion protection is greatly reliant on the integrity of the coating, being free from cracks and porosity, and with a residual compressive stress. In the Apticote 400 process, this advantageous compressive stress is maintained throughout the bath life by continuous monitoring and chemical additions.

Some comparative salt-mist endurance results:

Wear resistance

Electroless Nickel provides good low-stress abrasion resistance, as illustrated in the Taber Abrasion (SiC wheels at 100g load) results below (relative wear rates, low is best).

Substrates

Electroless Nickel can be deposited on most metals and alloys, including steels, copper alloys, aluminium alloys and titanium. Deposition on non-conductors, such as plastics, requires an ionic palladium activator.

Machining

Electroless nickel is not usually machined, since it provides even coverage. However, for precision bores, it can be honed, and for reflective surfaces such as mirrors it can be super-finished or highly polished.

Surface finish

Electroless nickel replicates the substrate texture, so that dictates the surface finish in most applications. If it is applied to a highly polished surface, the coating will retain its own fine texture, typically 0.1 Ra.

Physical properties

Poeton Apticote 400N Electroless Nickel has the following properties.

Mechanical properties

The loss in fatigue strength of a high tensile steel component after Electroless Nickel plating is less than with an electroplated coating, such as Hard Chrome.

Some values for fatigue strength (notched) are:

Some mechanical properties of the coating are:

Industrial uses

Just some of the industrial applications of electroless nickel are:

Specifying a coating

When placing an order for Poeton Electroless Nickel, be sure to specify your requirements, specifically:

Other variations around electroless nickel

Electroless nickel/PTFE is a common variant; where sub-micron sized PTFE particles are held in suspension in the plating bath, and are co-deposited in the nickel/phosphorous coatings. The resultant coating (see Apticote 450) gives low friction and is used in a wide variety of applications, including mould tools, connectors and fasteners, circuit breakers, valve seats and pump bearings, machine tools, medical moulds, cylinder liners, clutches and splines, and spindles.

Two other variations on co-deposition are electroless nickel/SiC and electroless nickel/diamond, coatings used commonly in the textile industry for wear resistance and traction, when spinning.

Electroless nickel coatings are also used in combination with sprayed polymers, forming a layered structure (see Apticote 460). Such coatings impart good low friction and non-stick properties, as well as corrosion protection. They are used extensively in the food and medical industries.

Environmental aspects

There is a lack of good data on the effects of Ni on the environment and workers. The Nickel Institute looks at the recent classification of Ni compounds made by the EU and suggests that they are not justified by any known research. They see the 'REACH' initiative by the EU as very important to Ni, because it will initiate a more rigorous programme of testing, one that could place Ni in a less threatening category. It will be several years before these questions are resolved.

Contact us for complete information regarding EU directives on the use of electroless nickel, Health & Safety information and reputable sources of independent advice.

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