Zinc chromate, Zn Cr O₄, is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

, a salt containing the chromate

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

, appearing as odorless yellow powder or yellow-green crystals, but, when used for coatings, pigments are often added. It is used industrially in

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 ...

s, having been developed by the

Ford Motor Company Ford Motor Company (commonly known as Ford) is an American multinational corporation, multinational automobile manufacturer headquartered in Dearborn, Michigan, United States. It was founded by Henry Ford and incorporated on June 16, 1903. T ...

in the 1920s.

Production

A process known as the

Cronak process The Cronak process is a conventional chromate conversion coating process developed in 1933 by The New Jersey Zinc Company. It involves immersing a zinc or zinc-plated article for 5 to 15 seconds in a chromate solution, typically prepared from sod ...

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 Sodium dichromate is the inorganic compound with the formula Na2 Cr2 O7. However, the salt is usually handled as its dihydrate Na2Cr2O7·2 H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compoun ...

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, ...

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 The North American B-25 Mitchell is an American medium bomber that was introduced in 1941 and named in honor of Brigadier General William "Billy" Mitchell, a pioneer of U.S. military aviation. Used by many Allied air forces, the B-25 served ...

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 Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

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 ''A Sunday Afternoon on the Island of La Grande Jatte'' () was painted from 1884 to 1886 and is Georges Seurat's most famous work. A leading example of pointillist technique, executed on a large canvas, it is a founding work of the neo-impress ...

''. 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 An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more part ...

between sections of the failed

solid rocket booster A solid rocket booster (SRB) is a solid propellant motor used to provide thrust in spacecraft launches from initial launch through the first ascent. Many launch vehicles, including the Atlas V, SLS and Space Shuttle, have used SRBs to give laun ...

Space Shuttle Challenger Space Shuttle ''Challenger'' (OV-099) was a Space Shuttle orbiter manufactured by Rockwell International and operated by NASA. Named after HMS Challenger (1858), the commanding ship of a Challenger expedition, nineteenth-century scientific exp ...

. 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 A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

because it contains

Cr(VI) 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 ...

. 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

{{DEFAULTSORT:Zinc Chromate chromate

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...