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Zinc chromate, which has the chemical formula ZnCrO4, is a chemical compound used to protect metals from corrosion. It was developed by the Ford Motor Company in the late 1920s as a primer coating. By 1936, it was used by the aviation industry and the U.S. military. While this coating is still in use, due to health risks and skin irritation, less toxic substances and other organic compounds are substituted when possible.
Commonly sprayed on in a paint booth, zinc chromate primer is used to protect zinc plated metal or aluminum alloys containing zinc. It works by interacting with the surface of the metal when it gets wet. The primer begins to dissolve when moisture penetrates it and creates a shield, or passive layer, between the metal and the primer which prevents corrosion. Over time, the primer dissolves completely and the metal begins to corrode.
While the chemical composition of zinc chromate is important, there are no standards for color. The natural color of the compound is greenish-yellow, but a variety of pigments are added during the manufacturing process to give it the color required by customers. One of the most common colors is rusty red, used on automobiles. The aerospace industry commonly uses yellows and greens.
Since this compound is sensitive to light, it is mixed with black pigment to provide some UV resistance. The result is a green color. During the 1930s and 1940s, the primer was colored to indicate a second coat. Untinted or yellow primer indicated a single coat, while a tinted primer would be one of the other colors.
Zinc chromate is one of a collection of compounds known as hexavalent chromium. These chromium compounds are carcinogenic and long-term exposure has been shown to cause lung cancer. It is still used in the automotive and aerospace industries, but since 2000, less toxic substitutes are used when possible.
In the U.S., the Occupational Safety and Health Administration (OSHA) regulates exposure limits and safety equipment requirements. The OSHA personal exposure limit for zinc chromate is 0.1 milligram per cubic meter of air. OSHA requires workplace monitoring and medical surveillance of workers to protect them from exposure. Personal protective equipment is also required to prevent exposure.
There are safer substitutions for zinc chromate corrosion inhibitors. These include zinc and calcium phosphates. These compounds eliminate the dangers of chromates and still work well for preventing or delaying corrosion.