Chromium Plating and Chromizing

two methods of the deposition of chromium or chromium alloy on a metal object to impart a number of physical and chemical properties to the metal’s surface, such as high resistance to corrosion, wear, and heat, as well as good mechanical and electromagnetic properties. The application of chromium coatings can be carried out by various methods, depending on the nature of the interaction between an object’s surface and chromium. The two most common are chromium plating and chromizing.

Chromium plating. Chromium plating is the most widespread electroplating process, which came into industrial practice in the 1920’s. Used mainly on objects made of steel and cast iron, it is also used on objects made of alloys based on copper, zinc, nickel, or aluminum. Because the chromium can be passivated, the chemical stability of the chromium coating is high. In view of the difficulties of obtaining a thin, nonporous coating, good corrosion protection may be achieved by depositing a more economical three-layer protective and decorative coating of copper, nickel, and chromium (the chromium layer is 1 micron thick). The chromium deposited on a previously polished surface has a mirror-like shine, with a silvery blue color. To prevent corrosion and for decorative purposes, many parts for automobiles, bicycles, streetcars, and railroad cars are chromium plated, as are parts of measuring instruments, calculating machines, typewriters, clocks, steam and water-main fittings, and medical instruments. A chromium coating also provides high resistance to mechanical wear, especially when it is in the form of “frosted” deposits. To improve the surface hardness and wear resistance, parts that are subjected to friction, such as the cylinders of internal-combustion engines, piston rings, and roller grooves, are chromium plated. In these cases, a layer of more than 1 mm is deposited. Porous chromium-plating methods have been developed, in which the parts being plated are anodized so that pores are formed in the coating to retain lubricant. Sometimes steel objects are subjected to combined chromium plating, which provides both protection from corrosion and improved wear resistance.

The principal electrolytic component in chromium plating is chromic acid. The electrolytes used can be divided into three groups: acidic electrolytes (chromic and sulfuric acids), neutral electrolytes (chromic acid and chromium sulfate), and basic electrolytes (chromic acid, chromium sulfate, and chromium chromate).

Chromizing. Chromizing, in which the chromium is deposited by diffusion, can be accomplished from the solid, vapor, gas, or liquid phase (seeDIFFUSION COATING). It is used on various machine parts and on semifinished products made of steel or alloys based on nickel, molybdenum, niobium, copper, or some other element. Chromizing imparts improved resistance to heat in air or in a gaseous medium containing sulfur and vanadium (up to 1000°C), as well as improved resistance to wear, heat, erosion, fatigue, and corrosion in corrosive media (H₂O₂, HNO₃, and NaCl). It also imparts good electromagnetic properties. Chromizing has several advantages over chromium plating; it not only improves the quality of the objects but also reduces their production costs and aids in environmental protection (by the absence of discharges of harmful electrolytes).

Depending on the properties required, chromizing is carried out at temperatures ranging between 900° and 1250°C. The thickness of the chromium layer varies from 40 microns to 3 mm.