Chrome plating

Chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, provide corrosion resistance, or increase surface hardness.

Process

• Degreasing to remove heavy soiling.
• Manual cleaning to remove all residual traces of dirt and surface impurities.
• 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 sulfuric acids can be used. 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 per hour.

The chrome plating chemicals are very toxic. Disposal of chemicals is regulated in most countries.

Industrial chrome

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 , 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:

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.

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.

Automotive use

Formerly most decorative items affixed to cars were referred to as "chrome", by which phrase was actually meant steel that had undergone several plating processes to endure the temperature changes and weather that a car was subject to outdoors. The most expensive and durable process involved plating the steel first with copper, and then nickel, before the chromium plating was applied.

Prior to the application of chrome in the 1920s, nickel was used. In the US for the short production run prior to the entry into the Second World War, plating was banned to save chromium and the decorative pieces were painted in a complementary color. In the last years of the Korean War, the banning of chrome was contemplated and several cheaper processes (such as plating with zinc and then coating with shiny plastic) were considered.

The largest plated parts were the bumpers of the cars. These were done by specialty shops, which had a different sequence of labor relations. Just after the Second World War, some American cars initally came off the assembly line with wooden bumpers -- not due to economy or rarity of the metal, but due to labor action at the plating shops. No doubt when the shops got on-line again these would be replaced.

See also

• Stainless steel

References

Further reading

• SAE AMS 2406
• SAE AMS 2438
• SAE AMS 2460 - Plating, Chromium