Zinc chromate

Zinc chromate

Names
IUPAC name Zinc chromate
Identifiers
CAS Number	13530-65-9 N
PubChem	26089
RTECS number	GB3290000
Properties
Chemical formula	ZnCrO4
Molar mass	181.403 g/mol
Appearance	yellow-green crystals; yellow powder
Density	3.43 g/cm3
Melting point	316 °C (601 °F; 589 K)
Boiling point	732 °C (1,350 °F; 1,005 K)
Solubility in water	insoluble
Vapor pressure	{{{value}}}
Related compounds
Other anions	Zinc dichromate
Other cations	Potassium chromate Sodium chromate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

B-25 Mitchell bombers, painted with zinc chromate undercoat, being assembled, 1942

Zinc chromate, ZnCrO₄, is a chemical compound containing the chromate anion, appearing as odorless yellow powder or yellow-green crystals, but when it is used for coatings pigments are often added. It is used industrially in chromate conversion coatings, having been developed by Ford Motor Company in the 1920s.

Production

A process known as the Cronak process is used to create zinc chromate for use in industry. This process is done by putting zinc or a zinc plated metal in a solution of sodium dichromate and sulfuric acid for a few seconds. Zinc chromate can also be synthesized by using neutral potassium chromate (K₂CrO₄) and zinc sulfate (ZnSO₄), which forms a precipitate.

K₂CrO₄ + ZnSO₄ → ZnCrO₄ + K₂SO₄

Uses

Zinc Chromate’s main use is in industrial painting as a coating over iron or aluminum materials. It was used extensively on aircraft by the U.S. military, especially during the 1930s and 1940s, but is also used in a variety of paint coatings for the aerospace and automotive industries. Its use as a corrosion resistant agent was applied to aluminium alloy parts first in commercial aircraft, and then in military ones. During the 1940 and 1950s it was typically found as the "paint" in the wheel wells of retractable landing gear on U.S. military aircraft to protect the aluminium from corrosion. This compound was a useful coating because it is an anti-corrosive and anti-rust primer. Since it is highly toxic it also destroys any organic growth on the surface. Zinc chromate is also used in spray paints, artists’ paints, pigments in varnishes, and in making linoleum.

When used as a pigment, it is known as Zinc Yellow, Buttercup Yellow or Yellow 36. It is rarely used in art because the pigment degenerates into a brown color. This effect can be seen in Georges Seurat's famous painting: A Sunday Afternoon on the Island of La Grande Jatte.

Zinc chromate putty was used as sealant in addition to two O-rings between sections of the failed solid rocket booster on Space Shuttle Challenger. Blowholes in this putty may have been a minor contributor to the loss of the shuttle.

Toxicity

Recent studies have shown that not only is zinc chromate highly toxic, it is also a carcinogen. Exposure to zinc chromate can cause tissue ulceration and cancer. A study published in the British Journal of Industrial Medicine showed a significant correlation between the use of zinc chromate and lead chromate in factories and the number of cases in lung cancer experienced by the workers. Because of its toxicity the use of zinc chromate has greatly diminished in recent years.