Finishing Friday: A Nickel for your Thoughts

More than four decades ago, Neil Young stated the obvious: rust never sleeps. Granted, the so-called Godfather of Grunge was referring to the rise of punk music, but it’s an unfortunate fact that pretty much all metals suffer from oxidation, a.k.a. rust. As proof, stick a chunk of steel out in the yard for a few weeks. You’ll soon gain a first-hand understanding of why the vast majority of machined, stamped, and fabricated parts receive some kind of surface protection before being placed into service.

We explored one such protection, conversion coating, in a previous blog article, Finishing Friday: Conversion Coating. Today we look at another finishing option that’s equally important: nickel plating. Two basic types exist—electroless and electrolytic—although you’ll soon discover that it’s the elimination of electrical current (a requirement in most plating and surface treatment operations) that makes the first of these more uniform, wear and corrosion resistant, and often the preferred choice.

The Devil’s copper

Before diving into the pros and cons of each, let’s first talk about the metal common to both: nickel. According to the Nickel Institute, this important element is number 28 on the periodic table, and is a hard, highly ductile, silvery-white metal that offers excellent resistance to corrosion and heat, used widely in the production of stainless steel and nickel-based superalloys. The ubiquitous coin shown in the header image and probably also in your pocket is made of cupronickel (CuNi), an alloy that is roughly 75% copper.

Nickel was first identified in 1751 by Axel Cronstedt, the Swedish chemist who also discovered tungsten. Until that time, miners would refer to the reddish-brown ore as Kupfernickel, or the Devils’ copper, for its annoying resemblance to—and complete lack of—copper. Regardless, modern air travel would be impossible without nickel, as would battery-powered electric vehicles and, for that matter, Earth, as our planet’s core consists primarily of nickel and iron, with nickel being its fifth most common element.

Taking a bath

Because of its many desirable properties, nickel is also a great metal for plating parts. That’s true regardless of the method, but as you’ll see, electrolytic plating suffers from some notable shortcomings. As the name implies, this 200-year-old process works by placing manufactured parts into a chemical bath (the cathode) containing a series of nickel bars (the anodes). As electricity flows through the anodes, they gradually dissolve, freeing nickel ions to migrate into the bath and attach to the workpiece.

On the other hand, electroless nickel plating eliminates the need for electrical current. A bath is still required, but the process relies on carefully controlled chemical reactions to deposit nickel ions onto the workpiece. And where electrolytic nickel plating is notorious for its uneven buildup, especially on part corners and complex part geometries, its electroless counterpart provides very uniform and consistent thicknesses.

Thanks to the addition of another important element—phosphorus—to the process (electroless nickel plating generally contains between 2% to 14% by weight of phosphorus), corrosion resistance is also greater, as is hardness (electroless nickel is nearly as hard as chrome plating), lubricity, and ductility. Finally, electroless nickel is suitable for a much wider range of metals than electrolytic.

Pick one

That’s not to say that electrolytic nickel plating doesn’t have its uses. It does. Since there’s no phosphorus in the chemical bath, this far older and more mature process creates a plated surface made of pure nickel plating, improving conductivity. In addition, electrolytic nickel plating provides heat resistance of 1,000°C (1,832°F), nearly twice that of electroless.

Given these advantages, the question then becomes: why not use the electroless process for everything? As with so many manufacturing decisions, the answer comes down to cost. It wasn’t until the last few decades that electroless nickel’s more elaborate chemistry and process control requirements have become quite manageable, making it a widely available and cost-competitive alternative. Secondly, nickel is an expensive metal. Electroless nickel offers greater uniformity over electrolytic, not to mention the ability to deposit less metal while achieving the same results. Add to that reduced electrical use, making electroless is the more environmentally friendly alternative. Nowadays, electroless is often preferred over its great-great-grandfather, electrolytic nickel plating.

Regardless of which plating process you decide to use, there are some important things to consider before putting parts in the drink. First off, rough surfaces and burrs are a no-no with both, leading to poor adhesion and ugly surfaces. As with all finishing processes, the parts should be clean, free of manufacturing lubricants, grime, and cleanser residue, and if they’re welded, the spatter removed and the welds made clean and smooth. And for manufacturers in humid environments, or where the parts have sat long enough that corrosion is evident, bead blasting or chemical pre-treatment might be required.

The fine details

There’s lots more to nickel plating. Chemical geeks and plating experts alike will offer up terms like Watts bath, dog bone effects, and calcium sulfamates. They’ll then point out the fact that nickel-plated surfaces can be made dull, satin, or bright, and that electroless nickel is often used in conjunction with other plating processes or as a “strike” coat for adhesion.

If you want to learn more, cruise over to the SAE International website, where you’ll find a variety of relevant resources, including the AMS 2403, “Plating, Nickel General Purpose” specification, which you can purchase for a moderate fee. That, or simply give us a call here at Prismier. We’ll be happy to walk you through your plating options, nickel or otherwise.