Industrial chromium plating, also known as hard chrome or engineered chrome, is used to reduce friction, add wear resistance, or increase corrosion resistance. It is very hard, measuring between 66 to 70 HRC. Hard chrome tends to be thicker than the decorative treatment, typically ranging from 0.075 to 0.25 millimetres (0.0030 to 0.0098 in), but thinner and thicker layers are not uncommon. Surface defects and roughness are amplified, because hard chrome does not have a leveling effect. Hard chromium plating is subject to different types of quality requirements depending on the application, for instance, the plating on hydraulic piston rods are tested for corrosion resistance with a salt spray test.
There are two types of industrial chrome plating solutions:
- Hexavalent chromium baths whose main ingredient is chromic anhydride.
- 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.
A typical bath composition of a hexavalent chromium bath is as follows:
- Chromic acid (CrO3): 225–300 g/l
- Sulfuric acid: 2.25–3.0 g/l, although it is usually measured in molarity or percent concentration
- Operating temperature: 45 to 60 °C
- Plating current: 1.55–3.10 kiloamperes per square meter DC
- Anodes: lead with up to 7% tin or antimony
- Maximum superimposed AC ripple allowed is 18%, preferred ripple is 5% to 10%
- Chromium may be stripped anodically in an aqueous solution of sodium hydroxide or inhibited hydrochloric acid.