Chrome plating (less commonly chromium plating) is a technique of

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

a thin layer of

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

onto a

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

object. A chrome plated part is called ''chrome'', or is said to have been ''chromed''. The chromium layer can be decorative, provide

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

resistance, facilitate cleaning, and increase surface hardness. Sometimes a less expensive substitute for chrome, such as nickel, may be used for aesthetic purposes. Chromium compounds used in electroplating are

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

. In most countries, their disposal is tightly regulated. Some fume suppressants used to control the emission of airborne chromium from plating baths are also toxic, making disposal even more difficult.

Process

The preparation and chrome plating of a part typically includes some or all of these steps: * Surface preparation * Manual cleaning to remove dirt and surface impurities * Removal of remaining organic contaminants using emulsion cleaning,

alkaline In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The ...

cleaning, anodic electrocleaning, or solvent cleaning by immersion, spray, manual application, or vapor condensation * Rinsing *

Activation In chemistry and biology, activation is the process whereby something is prepared or excited for a subsequent reaction. Chemistry In chemistry, "activation" refers to the reversible transition of a molecule into a nearly identical chemical or ...

or electroetching * Rinsing (not necessary if the activation and plating steps are done in the same bath) * Immersion in the chrome plating bath, where the part is allowed to warm to solution temperature * Application of plating current for the required time to attain the desired thickness * Rinsing There are many variations to this process, depending on the type of substrate being plated. Different substrates need different etching solutions, such as hydrochloric, hydrofluoric, and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

Ferric chloride Iron(III) chloride describes the inorganic compounds with the formula (H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated f ...

is also popular for the etching of nimonic alloys. Sometimes the component enters the chrome plating vat while electrically live. Sometimes the component has a conforming anode made from lead/tin or platinized titanium. A typical hard chrome vat plates at about per hour. Some common industry specifications governing the chrome plating process are AMS 2460, AMS 2406, and MIL-STD-1501.

Hexavalent chromium

Hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is any chemical compound that contains the element chromium in the +6 oxidation state (thus hexavalent). It has been identified as carcinogenic, which is of concern since approximately of ...

plating'', also known as ''hex-chrome'', ''Cr⁶⁺'', and ''chrome(VI)'' plating, uses

chromium trioxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula . It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple solid ...

(CrO₃, also known as chromic anhydride) as the main ingredient. Hexavalent chromium plating solution is used for both decorative and hard plating, as well as bright dipping of copper alloys, chromic acid anodizing, and

chromate conversion coating Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a pri ...

. A typical hexavalent chromium plating process is: # Activation bath # Chromium bath # Rinse # Second rinse The activation bath is typically a tank of chromic acid with a reverse current run through it. This etches the work-piece surface and removes any scale. In some cases, the activation step is done in the chromium bath. The chromium bath is a mixture of chromium trioxide and

, the ratio of which varies greatly between 75:1 to 250:1 by weight. This results in an extremely acidic bath (pH 0). The temperature and current density in the bath affect the brightness and final coverage. For decorative coating the temperature ranges from , but for hard coating it ranges from . Temperature is also dependent on the current density, because a higher current density requires a higher temperature. Finally, the whole bath is agitated to keep the temperature steady and achieve a uniform deposition.

Disadvantages

One functional disadvantage of hexavalent chromium plating is low cathode efficiency, which results in bad

throwing power Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

. This means it leaves a non-uniform coating, with more on edges and less in inside corners and holes. To overcome this problem the part may be over-plated and ground to size, or auxiliary anodes may be used around the hard-to-plate areas. Hexavalent chromium is also considerably more toxic than trivalent chromium, rendering it a major health risk both in manufacturing and disposal if not handled with care.

Trivalent chromium

''Trivalent chromium plating'', also known as ''tri-chrome'', ''Cr³⁺'', and ''chrome(III)'' plating, uses chromium sulfate or

chromium chloride Chromium chloride may refer to: * Chromium(II) chloride, also known as chromous chloride * Chromium(III) chloride, also known as chromic chloride or chromium trichloride * Chromium(IV) chloride, unstable {{Short pages monitor