Chromate conversion coating is a kind of coating done by the deposition of a layer of oxide on its surface used to, generally, make the materials such as aluminum, zinc, cadmium, copper, silver, magnesium inactive. It is primarily used as a corrosion inhibitor, primer, which is a preparatory coating put on materials before painting, decorative finish, or to keep hold of electrical conductivity. Zinc chromate is one such chromated metal. Chromating is normally done on zinc-plated parts to make them more long-lasting. Zinc chromate is a chemical compound containing the chromate anion, CrO4 2-.The chromate coating imitates the paint, shielding the zinc from white corrosion, a type of zinc corrosion which can make the part several times more hard-wearing depending on the thickness of the chromate layer. Zinc chromate is used industrially in chromate conversion coatings. Zinc chromate as a corrosion resistant agent was used in aluminum alloy parts initially in commercial and military aircrafts.
Zinc chromate was developed by the Ford Motor Company in the late 1920s as a primer coating. Around 1936, zinc chromate was used by the aviation industry and the U.S. military. As zinc chromate is sensitive to light, it is mixed with black pigment to offer some UV protection. The outcome is a green color. The primer was colored to show a second coat, between 1930s and 1940s. Zinc chromate was classically found as the "paint" in the wheel wells of retractable landing gear on U.S. military aircraft, between 1940 and 1950s, to shield the aluminum from corrosion.
Zinc chromate is obtained from the process of chromate coating. The process that is used extensively for zinc chromating is known as “Cronak process” which includes of 5–10 seconds of dipping at room temperature in a solution of 182 g/l sodium dichromate crystals (Na2Cr2O72H2O) and 6 ml/l concentrated sulfuric acid. Chromate coatings are soft and viscous when applied initially, although solidify and turn out to be hydrophobic (tending to repel or fail to mix with water) as time goes by. The thickness of chromate coating alters from a few nanometers to a few micrometers. Generally, Chromating is carried out on zinc-plated parts to make them extremely strong. The defensive effect of chromate coatings on zinc is shown by color, moving ahead from clear/blue to yellow, gold, olive drab and black. Usually, darker coatings offer extra protection from corrosion. The chromate conversion coating on electroplated zinc and cadmium coatings is identified as ISO 4520 . Zinc plating added with chromate conversion on iron and steel parts is given as ASTM B633 type II and III.
Zinc chromate is usually sprayed on in a paint booth, where it acts as a primer to defend zinc plated metal or aluminum alloys having zinc. It works by reacting with the surface of the metal when it gets damp. Once the moisture goes through it, the primer starts to dissolve and produces a shield, or passive layer, between the metal and the primer which protects form corrosion. After some time, the zinc chromate primer dissolves fully and the metal starts to corrode.
• The most common use of zinc chromate is as a corrosion inhibitor.
• Its use as a corrosion resistant agent was used in aluminum alloy parts initially in commercial aircraft, later in military aircrafts also.
• Zinc chromate primer is commonly sprayed in paint booths to prevent corrosion.
• Even though it is used in automotive industries, from 2000, less toxic replacements are preferred due to its toxic effects.
In United States., the Occupational Safety and Health Administration (OSHA) standardize exposure limits and safety equipment needs. The permissible OSHA exposure limit for zinc chromate is 0.1 milligram per cubic meter of air. OSHA needs office observation and medical surveillance of workers to protect them from exposure. Individual protective devices are also needed to avoid the exposure.