Chemical Composition of Chrome Plating

When it comes to hard chrome plating, it turns out it is simply elemental chromium. Hard chrome plating, however, is not what you think of when you think of chrome. It refers to a coating of the element chromium several thousandths of an inch thick that is applied to the moving parts of a vehicle or other machine for performance reasons. Chromium is fairly good at staying smooth and resisting corrosion, so it is useful as a solid lubricant and for resisting damage from wear. Hard chrome plating is used on pistons, cylinders, threads and other machinery.

Decorative chrome plating is the shiny plating that you see on the surfaces of cars and motorcycles. Unlike hard chrome plating, it is only millionths of an inch thick--at least the chromium metal is. Decorative plating includes not just chromium, but also at least one layer of nickel below and often a layer of copper under that. The nickel is what really gives it that smoothness and shine. Chromium is, in fact, much duller on its own, and it serves only to add a slight blue tint and keep the nickel from corroding on the outside. The layers underneath the nickel, whether they are more nickel or copper, also help against corrosion.

Chrome plating, when used as a verb, refers to the process of electroplating the chromium metal onto a surface using a bath of aqueous chromium ions. Because of its chemical composition, you do not want to get close to that bath. It contains hot (over 100 degrees Fahrenheit) chromic acid (CrO3) and sulfuric acid (H2SO4, the same substances that makes you cry when you chop onions), a highly toxic mixture with an extremely high acidity (in the area of pH 1 to 0). A piece of metal to be chrome plated is placed in this bath and an electric current is run from the liquid through the metal plate. Metallic chromium builds up on the surface as the electrons are absorbed off of it by the chromium ions in solution.