Decorative chrome plating on a motorcycle.
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
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.
- 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 25 micrometres (0.001 inches) per hour. To put that into perspective a human hair is between 50 to 100 micrometres.
Bright chrome is often used as a decorative feature on consumer products such as cars.
Chromium plating solutions
Industrial chromium plating is also known as "Hard Chrome" or "Engineered Chrome". 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.citation needed
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 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.
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.
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
References
Further reading
Chromium plating specifications, including quality requirements, can be found in the following documents: AMS 2406K, AMS 2438, Plating, Chromium Hard Deposit AMS-QQ-C-320 retired by AMS 2460, Chromium Plating (Electrodeposited).
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