Chromates are chemical compounds containing chromium and oxygen, and they show up across a surprisingly wide range of industries. Their primary uses include making pigments, plating metals to resist corrosion, tanning leather, preserving wood, and serving as catalysts in chemical manufacturing. Pigment production alone accounts for about 25.7% of all chromium used by chemical industries, with metal finishing (electroplating) close behind at 20.9%.
Metal Finishing and Corrosion Protection
One of the most important uses of chromates is protecting metal surfaces from rust and degradation. In electroplating, a thin layer of chromium is deposited onto metal parts using chromic acid. This creates either a decorative shine or a hard, corrosion-resistant surface, depending on the application. You’ll find chrome-plated finishes on everything from automotive trim to industrial machinery and plumbing fixtures.
Chromate conversion coatings work differently from plating. Instead of adding a layer of metal, these coatings chemically react with the surface of aluminum or other metals to form a protective film. What makes chromate coatings uniquely effective is that the hexavalent chromium in the film acts as a self-healing inhibitor. If the coating gets scratched, chromate ions migrate to the damaged area and protect the exposed metal. This “active” protection is why chromate coatings have been the gold standard in aerospace and military applications for decades.
Zinc chromate also plays a role as a corrosion-inhibiting pigment in primers and paints, particularly for metal structures exposed to harsh environments like bridges, ships, and aircraft.
Pigments Across the Color Spectrum
Chromate-based pigments produce an impressive range of vivid colors, from pale greenish yellows to deep oranges and reds. Lead chromate is the base compound, and by adjusting its chemistry, manufacturers can create distinct shades. Pure lead chromate produces a rich reddish yellow known as middle chrome. Mixing it with lead sulfate shifts the color toward a cooler, greenish yellow. Heating lead chromate with an alkali produces chrome orange and chrome red.
The specific pigments include primrose chrome (a pale greenish yellow containing 45% to 55% lead sulfate), lemon chrome (a slightly redder greenish yellow), and orange chrome (pure basic lead chromate with no sulfate). Chrome green is made by blending chrome yellow with Prussian blue. These pigments are valued for their excellent opacity, bright shades, and deep color saturation, making them popular in paints, inks, and plastics. Lead chromate pigments were traditionally common in automotive paints and pipe coatings, though environmental and health concerns are pushing many manufacturers toward lead-free and chromate-free alternatives.
Leather Tanning
About 15.7% of industrial chromium goes into leather tanning. The process uses basic chromium sulfate, a trivalent chromium salt, rather than hexavalent chromate directly. The chromium forms strong chemical bonds with collagen, the protein that gives rawhide its structure. These bonds stabilize the collagen against heat and bacterial breakdown, transforming raw animal skin into durable, flexible leather.
Chrome-tanned leather has distinct advantages over leather produced by vegetable or other tanning methods. It’s softer, more versatile, accepts dyes more easily, and holds up better against water and temperature changes. The vast majority of the world’s leather, from shoes to handbags to car interiors, is chrome-tanned.
Wood Preservation
Chromated copper arsenate (CCA) is a pesticide blend of chromium, copper, and arsenic that protects wood from termites, fungi, and other organisms that break down timber. The chromium component helps fix the preservative chemicals into the wood fibers so they don’t leach out easily. CCA-treated wood is used for utility poles, fence posts, pilings, foundation support beams, shingles, and other structural products that need to withstand long-term outdoor exposure.
In the United States, CCA was voluntarily phased out of most residential uses (like playground equipment and deck lumber) in 2003, but it remains approved for commercial and industrial wood products under EPA-regulated labeling requirements.
Textile and Chemical Manufacturing
Chromate compounds serve several niche roles beyond the major industries. Textile manufacturers use sodium dichromate and other chromium salts to improve colorfastness in fabrics, helping dyes resist fading from washing and light exposure. Chromium compounds also work as oxidizing agents to develop certain textile dyes. In the oil and gas industry, chromium lignosulfonates are added to drilling muds to reduce corrosion on drill strings. Various chromium compounds also serve as catalysts in chemical reactions, speeding up processes without being consumed.
Health Risks of Hexavalent Chromium
The very property that makes chromates useful, their chemical reactivity, also makes them hazardous. Hexavalent chromium (the form found in most chromate compounds) is a known human carcinogen. Workers exposed to chromate dust or fumes face elevated risks of lung cancer, and the compound can irritate the nose, throat, and lungs even at low concentrations. Repeated exposure can damage the lining of the nasal passages, eventually causing ulcers or even perforation of the wall between the nostrils.
Skin contact is also a concern. Prolonged exposure to hexavalent chromium can cause a condition called chrome ulcers: painless, crusted lesions with pitted sores underneath. Some people develop an allergic sensitization, meaning even tiny amounts of chromium on the skin trigger a serious rash. Direct eye contact with chromic acid or chromate dust can cause permanent eye damage. OSHA sets the workplace exposure limit at 5 micrograms per cubic meter of air, measured as an 8-hour average.
Environmental Contamination and Cleanup
Hexavalent chromium is highly soluble in water, which means it travels easily through soil and into groundwater supplies. Industrial runoff from electroplating facilities, tanneries, and wood treatment plants has contaminated groundwater at sites around the world. Unlike trivalent chromium, which is relatively immobile and far less toxic, hexavalent chromium persists in water and poses ongoing risks to drinking water sources.
The primary cleanup strategy involves converting hexavalent chromium back into its trivalent form, which is insoluble and settles out of water as a harmless solid. Microbes naturally do this when given an energy source. Bacteria like Geobacter and Pseudomonas can reduce hexavalent chromium using organic carbon as fuel, but groundwater typically lacks enough organic material to support efficient cleanup. Newer approaches use elemental sulfur as an alternative energy source. In lab-scale reactors, sulfur-based biological systems have achieved removal rates of over 6 milligrams per liter per hour under low-nutrient conditions, making them a promising option for contaminated sites where adding organic material isn’t practical.
The Shift Toward Chromate-Free Alternatives
Given the health and environmental costs, industries are actively seeking replacements for chromate-based products. In aerospace and defense, the U.S. Department of Defense has funded extensive research into non-chromate corrosion inhibitors. Leading alternatives include trivalent chromium process (TCP) coatings, which use the less toxic form of chromium, along with inhibitors based on molybdate, silicate, zinc, and rare earth metals like cerium, praseodymium, and lanthanum. Some of these non-chromate systems are already specified for use on military hardware.
In the pigment industry, organic and inorganic alternatives are replacing lead chromate in automotive paints and consumer products. The leather industry is exploring vegetable-based and synthetic tanning agents, though chrome tanning remains dominant because no alternative yet matches its combination of quality, speed, and cost. The transition away from chromates is well underway, but their unique performance characteristics mean full replacement across all industries will take time.