Chromate conversion coating is a type of
conversion coatingConversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
applied to passivate aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow
corrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
. The process uses various toxic
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
compounds which may include
hexavalent chromiumHexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate. Approximately of hexavalent chromium were produced in 1985. Other hexavalent chromium compounds are chromium...
. The industry is developing less toxic alternatives in order to comply with substance restriction legislation such as RoHS. One alternative is trivalent chromate conversion which is not as effective but less environmentally damaging.
Chromating is commonly used on zinc-plated parts to make them more durable.
Chromate conversion coating is a type of
conversion coatingConversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
applied to passivate aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow
corrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
. The process uses various toxic
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
compounds which may include
hexavalent chromiumHexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate. Approximately of hexavalent chromium were produced in 1985. Other hexavalent chromium compounds are chromium...
. The industry is developing less toxic alternatives in order to comply with substance restriction legislation such as RoHS. One alternative is trivalent chromate conversion which is not as effective but less environmentally damaging.
Chromating is commonly used on zinc-plated parts to make them more durable. The chromate coating acts like a paint, protecting the zink from white corrosion, this can make the part several times more durable depending on chromate layer thickness. It cannot be applied directly to steel or iron, and does not enhance zinc's cathodic protection of the underlying steel from brown corrosion. It is also commonly used on
aluminium alloyAluminium alloys are mixtures of aluminium with other metals , often with copper, zinc, manganese, silicon, or magnesium. They are much lighter and more corrosion resistant than plain carbon steel, but not as corrosion resistant as pure aluminium...
parts in the aircraft industry where it is often called
chemical film, or the well known brand name
Alodine. It has additional value as a
primerA primer is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted....
for subsequent organic coatings, as untreated metal, especially aluminium, is difficult to paint or glue. Chromated parts retain their electrical conductivity to varying degrees, depending on coating thickness. The process may be used to add color for decorative or identification purposes.
Chromate coatings are soft and gelatinous when first applied but harden and become
hydrophobicIn chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
as they age. Curing can be accelerated by heating up to 70°C, but higher temperatures will gradually damage the coating over time. Some chromate conversion processes use brief degassing treatments at temperatures of up to 200°C, to prevent hygdrogen embrittlement of the substrate. Coating thickness vary from a few nanometers to a few micrometers thick.
The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black. Darker coatings generally provide more corrosion resistance. Chromate conversion coatings are common on everyday items such as hardware and tools and usually have a distinctive yellow color.
Phosphate coatings on iron and steel may also be treated with a chromic acid rinse to enhance the phosphate coating.
Standard specifications
ISO 4520 spesifies chromate conversion coatings on electroplated zinc and cadmium coatings.
MIL-DTL-5541FThe military specification referred to as MIL-C-5541, "Military Specification, Chemical Conversion Coatings on Aluminum and Aluminum Alloys" covers chemical conversion coatings formed by the reaction of chemical conversion materials with the surfaces of aluminum and aluminum alloys. The current...
specifies chromate conversion of aluminium alloy parts.
ASTM B633 Type II and III specify zinc plating plus chromate conversion on iron and steel parts.
Composition
The composition of chromate conversion solutions varies widely depending on the material to be coated and the desired effect. Most solution compositions are proprietary.
The widely used Cronak process for zinc and cadmium consists of 5-10 seconds of immersion at room temperature in a solution of 182 g/l sodium dichromate crystals (Na
2Cr
2O
72H
2O) and 6 ml/l concentrated sulfuric acid.
Iridite 14-2, a chromate conversion coating for aluminium, contains
chromium(IV) oxideChromium dioxide or chromium oxide is a synthetic magnetic substance once widely used in magnetic tape emulsion. With the increasing popularity of CDs and DVDs, the use of chromium oxide has declined. However, it is still used in data tape applications for enterprise-class storage systems...
barium nitrateBarium nitrate with chemical formula Ba2 is a salt of barium and the nitrate ion.Barium nitrate exists as a white solid at room temperature. It is soluble in water, and like other soluble barium compounds, is toxic and should be handled with care...
and sodium silico fluoride.
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