The metal and its alloys

Chromium plating

Chromium surfaces are produced on other metals by electroplating and chromizing. There are two types of electroplating: decorative and “hard.” Decorative plate varies between 0.000254 and 0.000508 millimetre (0.00001 and 0.00002 inch) in thickness and is usually deposited over nickel. “Hard” plating is used because of its wear resistance and low coefficient of friction. For these types of plating, solutions of chromic acid (CrO₃) are used.

In one method of chromizing, chromium is condensed on the surface from the vapour and is diffused into the metal by heating. In another method a chromium layer is fused on the surface and diffused in. Electron-beam deposition of chromium onto the surface followed by diffusion into the metal has also been used. Salt-bath chromizing using chromium chloride (CrCl₂) has been tried.

Steels

The greatest consumption of ferrochromium is in the manufacture of stainless steel. Formerly, much of the alloy had to be of the low-carbon type, but since the advent of the argon-oxygen decarburization process, which allows steelmakers to burn off impurities such as silicon and carbon without also removing too much chromium, the demand has shifted to charge ferrochromium. Refined ferrochromium is now used principally as a trimming agent.

Stainless steels have a high resistance to oxidation and atmospheric corrosion, mainly because of the presence of chromium, which, at levels varying between 10 and 26 percent, forms a protective oxide film on the surface of the steel. The low-carbon ferritic stainless steels cannot be hardened by heat treatment; ferritic varieties containing 17 to 18 percent chromium are used in automobile trim and in equipment for handling nitric acid. High-carbon martensitic stainless steels are used when hardness and abrasion resistance are desired; steels of this type with 13 percent chromium are made into cutlery. Nickel and manganese can be added to high-chromium (16 to 26 percent) stainless steels to form the austenitic types, of which the 18-percent-chromium–8-percent-nickel variety is probably the best known. In addition to their resistance to oxidation and corrosion, austenitic steels maintain their strength at high temperatures better than do the plain chromium steels. Sometimes molybdenum, tungsten, niobium, or titanium are added to improve strength and corrosion resistance or to stabilize the carbides present. Steels of this type containing up to 26 percent chromium have excellent oxidation resistance at high temperatures; they are used in furnace parts, burner nozzles, and kiln linings.

Up to 2 percent chromium is added to low-alloy steel to improve hardenability, wear resistance, and high-temperature strength. Such steels, containing chromium in combination with other elements, such as molybdenum, nickel, manganese, and vanadium, are used for springs, roller and ball bearings, dies, rails, and high-strength structures. Steels containing 6–10 percent chromium have increased corrosion and oxidation resistance and are used in the form of tubes in the oil industry.

Nonferrous alloys

Chromium is added to cobalt alloys in amounts up to 25 percent to obtain corrosion resistance and hardness. Cobalt-chromium-tungsten alloys are used for cutting tools and hard facings.

Nickel-chromium superalloys with up to 60 percent chromium and sometimes a little iron are used for high-temperature applications. Chromium is also added to aluminum alloys in quantities up to 0.5 percent to improve their strength and corrosion resistance.

Refractory applications

The use of chromite as a refractory is next in importance to the metallurgical applications of chromium. A typical analysis of a chromite suitable for refractory purposes is 38 to 48 percent Cr₂O₃, 12 to 24 percent Al₂O₃, 14 to 24 percent Fe₂O₃, 14 to 18 percent MgO, and less than 10 percent SiO₂. The usefulness of chromite as a refractory is based on its high melting point of 2,180 °C (3,960 °F), moderate thermal expansion, the stability of its crystalline form at elevated temperatures, and its neutral chemical behaviour.

Bricks of 100 percent chromium ore have been largely replaced by bricks composed of mixtures of chromite and added magnesia for greater refractoriness, volume stability, and resistance to spalling. One of the refractories used in the fused-cast condition is composed of 80 percent alumina and 20 percent chromite. This product is highly resistant to the corrosive action of a variety of fluxes, slags, and glasses.

Pigments

Pigments account for about one-third of the primary production of chromium chemicals. Chrome oxide green, which is nearly pure Cr₂O₃, is the most stable green pigment known. It is used for colouring roofing granules, cements, and plasters. It is also employed as a fine powder for polishing. Chromium yellow varies greatly in the shades available and is essentially lead chromate, or crocoite. This pigment makes an excellent paint for both wood and metal. Zinc yellow, a basic zinc chromate, is used as a corrosion-inhibiting primer on aircraft parts fabricated from aluminum or magnesium. Molybdate orange is a combination of lead chromate with molybdenum salts. Chrome green is a mixture of lead chromate with iron blue. This pigment has excellent covering and hiding power and is widely used in paints.

Tanning

About 25 percent of the chromium chemicals produced go into chrome tanning of leather. This process uses chrome reagents in the form of basic chromic sulfates that, in turn, are produced from sodium dichromate. This reagent is produced by heating the ore with soda ash and then leaching out soluble chromate, which is then converted to the dichromate by treatment with sulfuric acid.

Surface treatment

More than one-fourth of the production of primary chromium chemicals is employed in metal-surface treatments and corrosion control. Such applications include chromium plating, chromizing, anodizing of aluminum, and treatment of zinc and magnesium. Chromium chemicals are used in dips for iron, steel, brass, and tin and also as inhibitors for brines and for recirculating water systems.