2.2 - Electroless Nickel

Maybe some of you remember the magnet experimient from your general science classes years ago. When you placed iron filings on a piece of paper near a magnet, the iron filings traced the path of the magnetic field - and resulted in some lines being more dense than others.

Electrolytic Nickel is an application by which direct electric current is passed through a part while it sits in a nickel solution, usually nickel sulfate. The part being applied with electrolytic nickel resembles the magnet: like nickel, the iron filings gather on the edges in greater quantities than the recesses. Hence, a complaint often arises from quality assurance personnel that, “I only wanted .0002/3” of plated nickel...you gave me .0006/7”. In reality, a machined part will exhibit both thicknesses - the lower amount on flat surface, recesses, and bores; and the larger amounts on edges or protruding threads on screws.

Electroless Nickel is a coating applied with heat, nickel sulfate, and a catalyst of hypophosphate chemicals (not electric current). This particular application will help solve the uneven buildup problem. The nickel solution, which is 90% nickel and 10% phosphorus, literally jumps out of the plating solution and onto the machined part in a uniformly distributed pattern. Unless some recesses are just too small to overcome surface tension, the part will have as much plating on the edges as anywhere else.

Once a part has been machined correctly, the need for re-machining after electroless nickel is often eliminated. As a result, the savings in machine time and aggravation are more than enough to offset the higher costs of the electroless nickel process. Of course, there are other good reasons as well. For instance, the Food Industry prefers electroless nickel because of its purity, whereas electrolytic nickel is highly toxic when co-deposited with impurities like copper and cadmium. In addition, electroless nickel has excellent release properties, so it facilitates cleanup - not only on food contact surfaces, but also on hardened steel molds. If .0015” of electroless is applied to a machined part, the Military could set it in the ocean practically forever without the appearance of corrosion. Also, the Petroleum Industry likes electroless nickel because of its tenacious wear properties in hostile underground environments of CO₂ and H₂S.

Lastly, the as-plated hardness of electroless nickel/phosphoric coatings are in the range of 48 to 52 HRE. This is equivalent to many hardened steel alloys. With heat treatment after plating, this finish can be easily raised to a 58 to 62 HRE.

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