Chrome plating

Bright chrome is often used as a decorative feature on consumer products such as cars.

Chrome plating is a finishing treatment utilizing the electrolytic deposition of chromium. The most common form of chrome plating is the thin, decorative bright chrome, which is typically a 10 µm layer over an underlying nickel plate. It imparts a mirror-like finish to items such as metal furniture frames and automotive trim. Thicker deposits, up to 1000 µm, are called hard chrome and are used in industrial equipment to reduce friction and wear and to restore the dimensions of equipment that has experienced wear. Black chrome is a variant in which process conditions are altered to give the plating a dark color.

Chromium plating solutions

There are two types of chromium plating: industrial and decorative. Industrial chromium plating is also referred to as Hard Chrome or Engineered Chrome. There are two types of Industrial chromium plating solutions:

1. Hexavalent chromium baths whose main ingredient is chromic anhydride.
2. 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.

Typical bath composition and operation of the hexavalent bath

• Chromic acid (CrO₃): 250–300 g/l.
• Sulfuric acid: 2.5–3.0 g/l.
• Operating temperature: 45 to 60 °C.
• Plating current: 1.55–3.10 kiloamperes per square meter DC.
• Maximum superimposed AC ripple allowed is 18%, preferred ripple is 5% to 10%.
• Anodes: lead with up to 7% tin or antimony.
• Chromium may be stripped anodically in an aqueous solution of sodium hydroxide.

Quality requirements

Hard chromium layers (over 10 micrometres) can be used in various applications and consequently subject to different types of quality requirements. Hard chromium layers on piston rods of hydraulic cylinders are tested on corrosion resistance in salt spray cabinets. Because these are destructive tests, only representative sample material can be tested. The salt spray solutions and the conditions in the cabinet are normed as well as the evaluation of the result after the test. The duration of the test is depending the agressivity of the test and the standard the industry has developed for this type of materials. In general there is a requirement that the deposit be uniform in thickness on all surfaces, and should be smooth, homogeneous and free from frosty areas, pin holes, pits, nodules, and other defects.

See also

• Stainless steel

Chromium plating specifications, including quality requirements, can be found in the following documents: AMS 2406K, Plating, Chromium Hard Deposit AMS-QQ-C-320, Chromium Plating (Electrodeposited).