What is 'Chrome'
Chrome is slang for Chromium, one of the 92 naturally occurring chemical elements. Chrome is a metal, but it is not useful as a solid, pure substance. Things are never made of solid chrome. Rather, when you hear that something is chrome, what is really meant is that there is a thin layer of chrome, a plating of chrome, on the object (the bulk of the object usually being steel, but sometimes aluminum, brass, copper, plastic, or stainless steel).A cause of occasional confusion is the fact that people may tend to describe any shiny finish as "chrome" even when it really has nothing to do with chromium. For example, brightly polished aluminum motorcycle parts, electropolished stainless steel boat rigging, vacuum metallized mylar balloons and helmets, semi-shiny painted wheels, and nickel plated oven racks are sometimes called 'chrome' by the lay person.Indeed it's not always easy to tell real chrome plating from other finishes if the parts are not side by side. When a chrome plated finish sits right next to another bright finish though, the other finish usually won't compare very favorably :-)
What's the difference between "Chrome Plating", "Chrome Electroplating", "Chrome Dipping", "Chroming", etc.?Nothing. Chrome is always applied by electroplating, it is never melted onto parts in the fashion of chocolate on strawberries, sprayed on like paint, or applied in any other way than by electroplating. Note the previous paragraph, though, that everything that is somewhat reflective is not necessarily real chrome plating.
Is all chrome plating about the same, then?Not quite. There are two different general applications for chrome plating: "hard chrome plating" (sometimes called 'engineering chrome plating' or 'functional chrome plating') and "nickel-chrome plating" (sometimes called 'decorative chrome plating').
Hard Chrome PlatingMost people would not be very familiar with hard chrome plating. Hard chrome plating is chrome plating that has been applied as a fairly heavy coating (usually measured in thousandths of an inch) for wear resistance, lubricity, oil retention, and other 'wear' purposes. Some examples would be hydraulic cylinder rods, rollers, piston rings, mold surfaces, thread guides, gun bores, etc. 'Hard chrome' is not really harder than other chrome plating, it is called hard chromium because it is thick enough that a hardness measurement can be performed on it, whereas decorative chrome plating is only millionths of an inch thick and will break like an eggshell if a hardness test is conducted, so its hardness can't really be measured directly.Hard chrome plating is almost always applied to items that are made of steel, usually hardened steel. It is metallic in appearance, and can be shiny, but is not necessarily decorative. Hard chrome plating is not a finish that you would want on a wheel or bumper.
Hard chrome plated components, courtesy of U.S. Chrome Corporation of New York There are variations even within hard chrome plating, with some of the coatings optimized to be especially porous for oil retention, others "thin dense chrome", and so on.Many shops who do hard chrome plating do no other kind of plating at all, because their business is designed to serve only engineered, wear-type, needs. If a shop says they do 'hard chrome only', they have no service that most consumers would be interested in.
Decorative Chrome PlatingDecorative chrome plating is sometimes called nickel-chrome plating because it always involves electroplating nickel onto the object before plating the chrome (it sometimes also involves electroplating copper onto the object before the nickel, too). The nickel plating provides the smoothness, much of the corrosion resistance, and most of the reflectivity. The chrome plating is exceptionally thin, measured in millionths of an inch rather than in thousandths.
"Sacrificial" vs. "Barrier Layer" Coatings and Why Quality is Crucial for Chrome PlatingFirst an aside: Some readers may be familiar with the replaceable zinc anodes used on ships and outboard motors to protect the hull or motor from from corroding. What the zinc anodes do is sacrifice themselves to protect the steel. Zinc is "anodic" to steel, and what that means is that when the steel is under attack and about to lose electrons (which would cause the steel to oxidize and convert from solid metal to rust), electrons will flow from the zinc to the steel to maintain the balance and protect it, so the zinc corrodes instead of allowing the steel to corrode. Galvanized roofing materials are coated with zinc, and function the same way: the steel is pretty safe from corrosion as long as there is some zinc left on it to sacrifice itself. Now, could you use a nickel anode or chrome anode instead of a zinc anode to protect a boat's steel hull from corroding? Absolutely Not! Steel is "anodic" to the nickel, instead of the other way 'round. The current flows the wrong way. The steel will sacrifice itself to protect the nickel and chrome. So now imagine a steel item that is plated with nickel and chrome but there is porosity or pinholes in the nickel plating ... the steel will rust away, sacrificing itself to try to protect the nickel! If you've seen a 50-year old junked rat-trap of a truck or car, you may have seen bumpers with thin pieces of curling nickel chrome, and almost no steel, where the bumper used to be. ^- 1. Bicycle fender with low quality nickel-chrome plating, rotting away in 3 months as the steel sacrificially corrodes to protect the nickel-chrome plating
2. Bar rack designed for indoor use ^ after one day outdoors in the rain
3. New bathroom accessories on display at a "big box" store, with tiny little rust pits before you even buy them -^Unlike "sacrificial" coatings like zinc plating or galvanizing where porosity or a hole or bare edge may be no big deal, porosity in nickel-chrome plating is a disaster that doesn't merely fail to protect the steel, but greatly accelerates the corrosion of the steel. Chrome plating is a "barrier layer" plating; once the barrier is breached by a pinhole, very rapid rusting is unavoidable. A low quality chrome plating job with pinholes or porosity is much much worse than no plating at all; the plating electrochemically forces the underlying steel to rust. Poor quality plating that is already showing tiny signs of rusting when you buy it is a manufacturing defect; return it, and don't let them harass you about "chemicals in your bathroom"! It is they who have chosen to distribute defective items; costly returns are the only way you can influence their policy of selling garbage.
Buzzwords: "Show chrome", "Triple Chrome Plating", "Double Nickel-Chrome""Show chrome" means chrome that is good enough to be on a winning entry in a car show. Although most OEMs rely on the "self-leveling" property of nickel plating to give sufficient reflectivity to roughly polished steel, chrome-lovers believe that the key to "show chrome" is to copper plate the item first and then buff the copper to a full luster before starting the nickel plating.Whether you start with bare steel or buffed copper, at least two layers of plating follow -- a layer of nickel and a layer of chrome. But high quality plating usually requires two layers of nickel.Salespeople are always looking for advantage, and they will use any good-sounding terms they can get away with! There are no laws that define what triple chrome plating actually means, so salespeople will be prone to call their service "triple chrome plating" if there are a total of 3 layers of any kind of plating, or "quadruple chrome plating" if there are 4. So those terms mean little.By the way, tri-chrome is not an abbreviation for triple chrome, and hex chrome does not mean six layers. But more on that later . . .The most important issue for durable chrome plating for outdoor exposure such as on a vehicle is that it should have at least two layers of nickel plating before the chrome: namely semi-bright nickel followed by bright nickel. The reason for this involves the anodic corrosion issues we discussed. The bright nickel is anodic to the semi-bright nickel, and sacrificially protects it, spreading the corrosion forces laterally instead of allowing them to penetrate through to the steel. OEMs demand very close control of this factor, and there is a test (the Chrysler developed STEP test) which large shops run daily to insure the right potentials. Careful control of this issue is probably the principal reason that today's chrome plating greatly outlasts the chrome plating of earlier times. If a restoration shop offers only single layer nickel plating, they must apply it really really heavy if corrosion resistance is to be guaranteed, because any porosity or pinhole will doom the underlying steel.Experts argue whether copper plating provides any additional corrosion resistance at all, but with or without copper plating, chrome on top of a single thin layer of nickel will not hold up to the severe exposure of a vehicle! Industry professionals call the two layers of nickel "duplex nickel plating", and that would be a much better term to use than "triple chrome" and such.