Chromium is the protagonist of modern stainless steel and electroplating, and an indispensable alloying element. Steel is its largest outlet: stainless steel typically contains more than 10% chromium, which spontaneously forms a dense passivating oxide film on the steel surface, making the steel rust-free and corrosion-resistant in air, water, acids and alkalis; stainless steel is therefore used in large quantities in tableware, kitchenware, chemical reactors, architectural curtain walls and medical instruments. Adding chromium to tool steel, bearing steel and gear steel raises hardness and wear resistance. Chromium plating is another major use: a bright decorative chromium layer on automotive bumpers, bicycle frames, bathroom fittings and appliance housings is both wear-resistant and reflective, while hard chrome plating serves as a wear-resistant surface layer on hydraulic piston rods, molds and printing rolls. In refractories, chromium ore is used in chrome-magnesite bricks for steelmaking furnaces and glass tanks, resisting high-temperature slag attack. In defense and aerospace, chromium is an important constituent of superalloys and electrical-resistance alloys, and nichrome wire is the heating element of electric furnaces and hair dryers. Chromium’s secure standing rests on that self-healing oxide film, which gives inexpensive iron-based materials expensive corrosion resistance.
Chromium oxide (Cr2O3), commonly called chrome green, is a dark-green pigment with strong tinting power and light and heat resistance, used in paints, ceramic glazes and inks; it is also the raw material for preparing other chromium compounds and metallic chromium, and is used as a green abrasive in grinding and polishing. Chrome yellow, or lead chromate (PbCrO4), was once a vivid, full-bodied yellow pigment commonly used in modern oil painting; because it contains lead and is toxic, it has largely been replaced by organic pigments and eco-friendly chromium pigments. Chromium trioxide (CrO3), also called chromic anhydride, is a dark-red crystal that is the main component of chromium-plating electrolytes and a strong oxidizer used in aluminum anodizing and wood preservation; because hexavalent chromium is toxic, its use has gradually come under strict restriction. Potassium dichromate (K2Cr2O7) is an orange-red strong oxidizer, once used in large quantities for leather tanning, laboratory glassware cleaning solution, organic synthesis and match manufacturing; since hexavalent chromium is a recognized carcinogen and environmental pollutant, its use is being strictly curtailed and replaced worldwide.
