Electroless Nickel Plating
5/1/1999 by Fred Hochgraf
Electroless Nickel Plating is used to deposit nickel without the use of an electric current. Since gaining commercial use in the 1950s, electroless nickel plating has grown rapidly to be an established industrial process. Electroless nickel is an engineering coating, normally used because of excellent corrosion and wear resistance. Electroless nickel coatings are frequently applied to aluminum to provide a solderable surface and on shafts to improve hardness and wear characteristics. In the case of optics, electroless nickel is used to render a highly reflective, uniform, corrosion-free surface for mirrors used in Infra-Red, Laser and Satellite systems.
Superior quality plating
As compared to an electro-plated nickel surface, electroless nickel provides a superior quality plating surface. It is also a relatively more expensive process. However, it is more homogenous, uniform and almost amorphous in character. As deposited, electroless nickel is not crystalline but has the characteristics of glass. With the exception of a few exotic applications, this is the only example of a metallic "glass" we would normally ever come in contact with.
The mechanical properties of electroless nickel deposits are similar to those of other glasses. They have high strength, limited ductility and a high modulus of elasticity. The ultimate tensile strength is very high and allows the coating to withstand a considerable amount of abuse without damage.
The ductility of electroless nickel coatings varies with composition. High phosphorus, high purity coatings have a ductility of about 1 to 1.5% as percentage of elongation. Although this is less ductile than most engineering materials, it is adequate for most coating applications. Thin films of deposited nickel can be bent completely around themselves without fracture. With lower phosphorus deposits, or with deposits containing metallic or sulfur impurities, ductility is greatly reduced and may approach zero, making the coating vulnerable to abuse.
The electroless nickel plating bath is unstable in that when exposed to an appropriate catalyst the nickel alloy is spontaneously deposited. The coating then becomes its own catalyst. The base metal that is to be plated must be a good, clean catalyst so as to get the process started.
Contamination is sneaky
Some sources of contamination are not easily identifiable. The one that can sneak up and grab you is when something has contaminated the grit blaster; just when you think you are cleaning a part for electroless nickel plating you are actually depositing a film of catalytic poison. Prior to plating it is a good practice to acid etch the substrate, eliminating any poisons. A copper or nickel strike can plate over the poisons in or on the metal. The nickel adhesion will then be as good as the adhesion of the copper strike to the base metal. In most cases plating defects can be traced to poor cleaning methods.
Electroless nickel is excellent for repairing shafts. The hardness and wear resistance are good and the plating can be built up until it is quite thick.
When failures occur
Plating failures usually occur because of contamination on the shaft such as a contaminated grinding wheel used for the final grind before plating or the reuse of contaminated cleaning solvents.
Electroless nickel coatings can be easily soldered and are used in electronic applications to facilitate soldering such light metals as aluminum. For most applications, rosin mildly activated (RMA) flux is specified along with conventional ten-lead solder. On moderately oxidized surfaces, activated rosin (RA) flux is usually required to obtain wetting of the coating.
Electroless nickel is a barrier coating, protecting the substrate by sealing it. Because of its amorphous, glass-like character and passivity, the corrosion resistance of the coating is excellent. In many environments it is superior to that of pure nickel or chromium alloys. Such amorphous alloys have better resistance to attack than equivalent polycrystalline materials because they are free from grain or phase boundaries.
Electroless Nickel - Questions&Answers
Is thermal expansion a problem?
The coefficient of thermal expansion can change by 20% as the percentage of phosphorous is doubled from 6 to 12%. If the thermal expansion coefficients of the coating and the substrate alloy are mismatched, then cyclic strain associated with repetitive heating and cooling can use up the coating's low fatigue strength, resulting in cracking. This effect is greatly exaggerated if the temperature change causes the nickel stress to be tensile.
What are the quality control methods?
For nickel-phosphorous coatings, visual appearance, thickness and adhesion are covered by AMS 2404B, ASTM 656 and MIL-C-26074B. We know of no standards for nickel-boron coatings. At NHML we are called on most often to test for cracks and porosity, evaluate the integrity of the coating, and to give high accuracy measurements of the thickness in critical locations. We also measure the mechanical properties of thin parts made out of electroless nickel and we measure the percentage of phosphorous or boron in the nickel.
How about the cost?
Nickel-phosphorous plating is more expensive than electroplated nickel and nickel-boron can be even more expensive than a nickel-phosphorous coating.
Overall when is the use of electroless nickel dictated?
To plate parts having recesses or holes that electroplating cannot reach. To avoid buildup on outside corners, to avoid skimpy plating on inside corners and when a thick or hard deposit is required.
Are there any new developments?
Proprietary "soups" are co-depositing various particles embedded in the nickel. The particles include diamond, silicon carbide, Teflon and calcium fluoride. The goals of most of these are to enhance wear resistance or to deposit an abrasive coating for small grinding tools.
How is the corrosion resistance of nickel-phosphorous electroless nickel?
Often excellent, often better than metallic nickel. Since the amorphous condition is free from grain boundaries, a whole range of corrosion sensitive situations is eliminated. The surface passivates much the same as nickel. In galvanic corrosion these coatings have the same electropotential as nickel and generally behave much the same as metallic nickel.
How do nickel-boron electroless nickel coatings behave?
As deposited, the hardness is high and with heat treating, the hardness can approach hard chromium. The wear resistance can be excellent. However, corrosion resistance is not as good as a nickel-phosphorous coating since the nickel-boron coating is not wholly amorphous. The residual coating stress is generally tensile, with the amount of tension depending on the chemistry of the bath. The strength and ductility of the nickel-boron coatings are only about 1/5 that of the nickel-phosphorous coatings.
Compared to electroplated nickel, why does electroless nickel tend to be so hard and brittle?
Two reasons: Electroless nickel is actually an alloy. Usually nickel plus phosphorous but sometimes nickel plus boron. The increase in strength and brittleness is just as dramatic as the effect of adding carbon to iron to make steel. The second reason is because it is a glass rather than a crystalline metal.
What metals can electroless nickel be applied to?
You need to know a little about how electroless nickel gets deposited. The plating bath is chemically unstable in that when exposed to an appropriate catalyst, the nickel-phosphorus (or boron) alloy is spontaneously deposited. The coating then becomes its own catalyst, which then allows the thickness to continue to build up.
Just as there are poisons for the catalytic converter in your car, there are poisons for the electroless nickel reaction. The common poisons are lead, tin, cadmium and zinc. Lead is found in leaded-free machining steels and brasses. Tin, zinc and cadmium can be left over from some coatings on steel.
A copper or nickel strike can plate over the poisons in or on the metal to be plated. Careful maintenance of the bath, temperature, pH, % solution, etc. is required to keep it active. If foreign particles are deposited on the growing layer, they may cause pits.
Is coating stress a problem?
Tensile stress in a coating always diminishes tensile strength and since the coating is brittle, cracking can occur. Tensile cracks ruin the corrosion resistance and flakes may break away. Coating performance can be excellent if the stress is compressive.
In nickel-phosphorous coatings, the percentage of phosphorous determines the stress. Below about 11% phosphorus the stress is progressively more tensile while above it is compressive. Above about 12.5% phosphorus there's little to be gained.
Why do nickel-phosphorous coatings sometimes come out porous?
This effect is nearly non-existent now because of improved chemistry of the nickel solution. In the past, low phosphorous content would make it porous, especially if the bath chemistry had been stabilized with heavy metals or sulfur. Coating ductility along with corrosion resistance would also suffer.
What happens if a part coated with nickel-phosphorous electroless nickel is heated?
Above 220 to 260°C (430-500°F) the phosphorous begins to segregate into tiny particles of a nickel-phosphorous compound which further hardens the alloy. Through heat treatment you can take advantage of the coating's hardness and wear resistance. But recognize that the ductility and corrosion resistance is decreased. With optimum phosphorous content of about 11.5%, the hardness can exceed 600 HV after aging at 400°F.
Originally published on 5/1/1999 in Nuts & Bolts, Volume 9
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