Chrome plating

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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 chrome 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): 225–300 g/l.
• Sulfuric acid: 2.25–3.0 g/l, Although it is measured in molarity or % concentration and not 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 or inhibited Hydrochloric acid.

Typical plating process

The component will generally go through these different stages.

• Degreasing to remove heavy soiling.
• Manual cleaning to remove all residual traces of dirt and surface impurities.
• Component placed on a jig or rack.
• Various pretreatments depending on the substrate.
• Placed into the chrome plating vat and allowed to warm to solution temperature.
• Plating current applied and component is left for the required time to attain thickness.

There are many variations to this process depending on the type of substrate being plated upon. Different etching solutions are used for different substrates. Hydrochloric, Hydrofluoric, and Sulphuric acids can be used, and ferric chloride is also popular for the etching of Nimonic alloys. Sometimes the component will enter the chrome plating vat electrically live. Sometimes the component will have a conforming anode either made from lead/tin or platinized titanium. A typical hard chrome vat will plate at about 25 micrometres (0.001 inches) per hour. To put that into perspective a human hair is between 50 to 100 micrometres.

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. Typically for engineering purposes the hardness value will be between 800 - 1000 HV500 when subject to a Vickers hardness test. 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).