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What is chrome and how is it done?

“Plating” as used on bicycles is the process of applying a layer of metal to the surface of another metal by means of electrochemical attraction in an immersion bath.  Chrome plating is only practical on steel bicycle parts. 

There are many steps to the process.  First, the part must be cleaned of all paint, grease or other coatings.  If the part has been plated before, the old plating must be removed.  This is done by immersing the part in a tank of acid and “de-plating” by reversing the current so that metal ions are drawn from the surface of the part to the cathode in the tank. This process leaves a dull etched surface.  Next, the part must be polished to a high sheen.  This is a tedious and labor-intensive process using powerful lathes and other tools specially adapted for frames.   Few people realize that the part must fully polished before the chrome is applied.  The part must then again be meticulously cleaned and then can be immersed in the first of several baths for plating.  Most of the actual plating is done with nickel. Chrome is then applied as a final coat.  We then bake parts to remove any “hydrogen embrittlement” which may have occurred in plating.  Next, masking tape is applied and precisely cut the areas to remain exposed.  We then protect the masking tape and abrasive blast the area to be painted, “frosting” the surface of the chrome to increase paint adhesion.  After painting, the making is removed and the edges trimmed precisely with a knife. 
The plating we do is called “decorative” chrome plating and always involves plating nickel before plating the chrome. The chrome plating in decorative chrome plating is exceptionally thin, measured in millionths of an inch rather than in thousandths. It is still a very hard surface, but simple 'anvil' type hardness measurements don't detect the hardness because the anvil just punches through such a thin coating.

When you look at a decorative chromium plated surface, most of what you are seeing is actually the nickel. The chrome adds a bluish cast (filtering the somewhat yellowish cast of the nickel), and it protects against tarnish, and minimizes scratching. But the point is, without the brilliant leveled nickel undercoating, you would not have a reflective, decorative surface.
Chrome plating is hardly a matter of dipping an article into a tank, it is a long involved process that often starts with tedious polishing and buffing, then cleaning and acid dipping, zincating, and copper plating. This may be followed by buffing of the copper, cleaning and acid dipping again, and plating in two or three different types of nickel plating solution, all before the chrome plating is done.
When an items needs "rechroming", understand what is really involved: stripping the chrome, stripping the nickel (and copper if applicable), then polishing out all of the pits and blemishes, then starting the whole process described above.