Zinc chromate

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 File:Zinkchromat.jpg|zinc chromate (powder) File:Alfred T. Palmer - Assembling the North American B-25 Mitchell at Kansas City, Kansas (USA).jpg|B-25 Mitchell bombers, painted with zinc chromate undercoat, being assembled, 1942

Uses

Zinc chromate's main use is in industrial painting as a coating over iron or aluminium materials. It was used extensively on aircraft by the US military, especially during the 1930s and 1940s. It 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 US 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 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. The degradation of zinc yellow in Seurat's painting was thoroughly investigated and these findings were subsequently employed in a digital rejuvenation of the painting.

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 its catastrophic loss.

Toxicity

Recent studies have shown that not only is zinc chromate highly toxic, it is also a carcinogen because it contains Cr(VI). 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.

References

References

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2. "OHSA Guideline for Zinc Chromate". [[Occupational Safety and Health Administration]].
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6. (1995). "Paint and Coating Testing Manual". American Society for Testing and Aircraft Materials.
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11. (2011). "Electron energy loss spectroscopy elucidates the elusive darkening of zinc potassium chromate in Georges Seurat's A Sunday on La Grande Jatte–1884". Analytical and Bioanalytical Chemistry.
12. [http://colourlex.com/project/digital-restoration/ Digital restoration of paintings], ColourLex
13. (2006). "Rejuvenating the color palette of Georges Seurat's A Sunday on La Grande Jatte—1884: A simulation". Color Research & Application.
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15. J.A. Hickman. (1997). "Polymeric Seals and Sealing Technology". iSmithers Rapra Publishing.
16. (15 February 2011). "Chronic exposure to zinc chromate induces centrosome amplification and spindle assembly checkpoint bypass in human lung fibroblasts". Chemical Research in Toxicology.
17. "OHSA Chemical Sampling Information for Zinc Chromate". [[Occupational Safety and Health Administration]].
18. (May 1984). "Lung cancer mortality among workers making lead chromate and zinc chromate pigments at three English factories". British Journal of Industrial Medicine.