An Introduction To Chrome Plating

By: James Monahan

Chrome plating is a kind of finishing treatment that utilizes the electrolytic deposition of chromium. The thin, decorative bright chrome is the most common form of chrome plating.

Chrome plating imparts a mirror-like finish to items such as metal furniture frames and automotive trim. Thicker deposits, called hard chrome, are used in industrial equipment to reduce friction and wear and to restore the dimensions of equipment that has experienced wear.

There are two types of chromium plating: industrial and decorative. Industrial chromium plating is also called Hard Chrome or Engineered Chrome.

There are two types of Industrial chromium plating solutions: hexavalent chromium baths whose main ingredient is chromic anhydride and trivalent chromium baths whose main ingredient is chromium sulfate or chromium chloride. Trivalent chromium baths are not yet common, due to restrictions concerning color, brittleness, and plating thickness.

For decorative purposes, the best combination would be chrome and nickel which offers the most protection against corrosion. It will have a mirror finish that will only be as good as the finish you put on the surface before you put on the chrome.

Hard chrome is plated in thickness as required to take advantage of the extremely low chrome coefficient of friction, or for wear build-up for functional purposes.

Micro-finished chrome will provide a coefficient of friction lower than any other metal when used against steel, iron, brass, bronze, babbitt, or aluminium alloys. This is done when chrome is used as a bearing surface.

Chrome is a perfect set-up for longwearing working surfaces because it is much harder than casehardened steel. It is not advisable to use chrome on chrome. Chrome will resist mostly all organic and in organic compounds and acids, except hydrochloric acid.

Chrome plate shall be uniform in thickness on all surfaces. Plate shall be smooth, homogeneous and free from frosty areas, pin holes, pits, nodules, and other defects.

Chrome plating is not a difficult process provided that the part has been properly cleansed and the following requirements met: Preparation of the chromic acid (CrO3) solution, Temperature control of the bath (plating solution),Preparation of lead anodes (peroxide), Agitation method of the bath (bubbles), Plating current density control and duration (controller), Ventilation (for safety)Black chrome can also be plated in the same way and still have similar characteristics to the bright chrome. For aesthetic or anti-reflective applications, it may be preferable in some cases. I have not yet used it, but the formula is as follows:

Having the chrome plating facility right there when needed is very valuable to the home machinist. Of course there is a learning curve and some basic investment in dollars but this investment is mostly non-recurring and will give you the benefit of this process for quite a long time.

In industrial chrome plating the process is electrolysis. In the process chromium metal is deposited on metallic surfaces submerged in a chromic acid plating bath. The part to be plated is made cathodic by connection to the negative terminal of the rectifier.

Inert lead anodes are made electrically positive. When voltage is applied across the two, current flows through the solution and metal is deposited. Proper control of various plating parameters result in bright, hard, adherent deposits.

Coupled with other surface finishes such as anodizing aluminum and the plating of other prime metals such as copper, nickel, silver, or gold which we may yet cover, we can really dress up and protect our creations.

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