Decorative chrome plating on a motorcycle.

Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object. Electroplating is the process of using electrical current to reduce Cations of a desired material from a solution and coat a conductive object Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24 The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across The chromed layer can be decorative, or it may help provide corrosion resistance. Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings

Process

The component will generally go through these different stages.

• Degreasing to remove heavy soiling. Parts cleaning is essential to many industrial processes as a prelude to Surface finishing or to protect sensitive components
• 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. Hydrofluoric acid is a Solution of Hydrogen fluoride in Water. Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. Ferric chloride is also popular for the etching of Nimonic alloys. Iron chloride redirects here For Iron(II chloride see Iron(II chloride. 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:

1. Hexavalent chromium baths whose main ingredient is chromic anhydride. Hexavalent chromium ( Cr(VI) compounds are those which contain the element Chromium in the +6 Oxidation state. The acid Anhydride of Chromic acid, CrO3 is chromium trioxide or chromium(VI oxide; industrially this compound is sometimes sold as "
2. Trivalent chromium baths whose main ingredient is chromium sulfate or chromium chloride. Chromium chloride may refer to Chromium(II chloride, also known as chromous 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. Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing Chromate and Dichromate anions or the
• Sulfuric acid: 2. Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. 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. Direct current ( DC) is the unidirectional flow of Electric charge.
• Maximum superimposed AC ripple allowed is 18%, preferred ripple is 5% to 10%. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant
• Anodes: lead with up to 7% tin or antimony. Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly Tin is a Chemical element with the symbol Sn (stannum and Atomic number 50 Antimony (IPA (Received Pronunciation, /ˈæntɪmoʊni/ (US is a Chemical element with the symbol Sb (stibium meaning "mark" and
• Chromium may be stripped anodically in an aqueous solution of sodium hydroxide or inhibited hydrochloric acid. Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature Hydrochloric acid is the Solution of Hydrogen chloride ( H[[Chlorine Cl]] in water

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. The Vickers hardness test was developed in 1924 by Smith and Sandland as an alternative method to measure the Hardness 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

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). In Metallurgy, stainless steel is defined as a Steel Alloy with a minimum of 11