Why Are Some Metals Chromium Plated?
Some metals are chromium-plated to make them look attractive and to prevent them from corroding or rusting. Chromium is a silver-white, hard, brittle metal which was discovered in 1798 by N. L. Vauquelin. Its non-corrosive, high-strength, heat-resistant characteristics are utilized in alloys and as an electroplated coating. In electroplating, the article to be plated is connected to the negative terminal of a battery and placed in a solution known as electrolyte.
Direct electric current is introduced through the anode or positive terminal, which usually consists of the metal with which the article is to be coated. Metal slowly leaves the anode and forms a deposit on the article. The electrolyte for chromium contains chromic acid and sulphuric acid. It deposits a bright top layer but this is not the only important part of the electroplating. The chromium is only about 0.00002 inches thick.
Under it lays a thick layer of nickel and beneath that again may be a layer of copper. Many household appliances are chromium-plated and so are the bright parts of an automobile. Tools, chemical equipment, electric appliances, gears, packing machinery, and hundreds of other articles are similarly treated to give them brightness, beauty or resistance to wear and rust.
Electroplated and polished chromium is bright bluish-white with a reflecting power which is 77 percent that of silver. Most bright decorative items affixed to cars are referred to as “chrome,” meaning steel that has undergone several plating processes to endure the temperature changes and weather that a car is subject to outdoors. Triple plating is the most expensive and durable process, which involves plating the steel first with copper and then nickel before the chromium plating is applied.
Prior to the application of chrome in the 1920s, nickel electroplating was used. In the short production run prior to the US entry into the Second World War, the government banned plating to save chromium and automobile manufacturers painted the decorative pieces in a complementary color. In the last years of the Korean War, the US contemplated banning chrome in favor of several cheaper processes (such as plating with zinc and then coating with shiny plastic).
In 2007, a Restriction of Hazardous Substances Directive (RoHS) was issued banning several toxic substances for use in the automotive industry in Europe, including hexavalent chromium, which is used in chrome plating. However, chrome plating is metal and contains no hexavalent chromium after it is rinsed, so chrome plating is not banned.
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.)
Chrome plating is more reflective (brighter), bluer (less pale, grayish, or yellowish), and more specular (the reflection is deeper, less distorted, more like a mirror) than other finishes. Put one end of a tape measure against a bright finish, and see how many inches of numbers you can clearly read in the reflection — you can see skywriting clearly reflected in top quality chrome plating. And there’s a hard to define “glint” to chrome plating that almost nothing else has.
Decorative 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.
When you look at a decorative chrome plated surface, such as a chrome plated wheel or truck bumper, most of what you are seeing is actually the effects of the nickel plating. The chrome adds a very slightly bluish cast (compared to the slightly yellowish cast of nickel), protects the nickel against tarnish, minimizes scratching, and symbiotically contributes to corrosion resistance. But the point is, without the brilliant leveled nickel undercoating, you would not have a rust-resistant, reflective, decorative surface. By the way, there is no such thing as “decrotif chrome plating”. That is just a misspelling of ‘decorative’.