|Molar mass||181.403 g/mol|
|Appearance||yellow-green crystals; yellow powder|
|Melting point||316 °C (601 °F; 589 K)|
|Boiling point||732 °C (1,350 °F; 1,005 K)|
|EU classification||not listed|
LD50 (Median lethal dose)
|0.5 to 5 g/kg|
|US health exposure limits (NIOSH):|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
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.
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
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.
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.
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- Davies, J.M. (May 1984). "Lung cancer mortality among workers making lead chromate and zinc chromate pigments at three English factories". British Journal of Industrial Medicine (41): 158–169.
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