Zinc is a versatile element that has numerous uses in various industries. It forms several compounds with distinct properties, including zinc oxide, zinc chloride, and zinc sulfide. Learn more about the uses and properties of zinc and its compounds in this article.
The major use of zinc is as a protective coating for steel, to which it is applied by some method such as hot dipping (galvanizing), electroplating, or spraying (using molten zinc). The first has become the most important method since World War II because of the rapid installation of high speed continuous galvanizing lines for steel strip. Zinc is more active in a galvanic couple than the coated iron and steel, and consequently is attacked while the coated metal is protected.
Another application, similar in principle, utilizing the electropositive character of zinc, is the provision of galvanic protection for underground and underwater installations of iron and steel such as storage tanks, pipelines, piers, and ships’ hulls. Cast anodes of zinc are bonded to the structure to be protected. A slight galvanic countercurrent is generated due to the greater electrochemical activity of the zinc, thus protecting the iron or steel at the expense of the zinc which undergoes a controlled long term corrosion. This is called cathodic protection.
The use of zinc and zinc-base alloys for making low cost, high quality, high finish die castings has increased rapidly and now consumes approximately as much zinc as is used for galvanizing. Zinc die castings are used extensively in automobile and building hardware, electrical appliances, tools, business machines, and toys.
Another, though decreasingly, important use of zinc is in alloying with copper to form brass, which contains from 3 to 45 percent zinc, usually about 30 percent. Still another use is the manufacture of rolled zinc to make sheet strip, ribbon, foil, plate, rod, and wire. Typical applications are dry-cell batteries, extruded cases for radio condensers and tube shields, weather stripping, roof flashing, photoengraving plates, and anodes for cathodic protection referred to previously.
Secondary zinc finds various applications according to its analysis, the chief outlets being conversion to slab zinc, zinc dust, or brass and bronze alloys. Zinc dust, because of its physical condition, is quite reactive, and its applications depend primarily on this fact. It varies from about 95 percent to more than 99 percent metallic zinc, with zinc oxide as the chief impurity, and is sold under close specifications as to metallic content, evenness of grading, and fineness of particles. Chief outlets are in the chemical, textile, dye, paper, and metallurgical industries. It is used also in corrosion-resistant paints and coatings. When high purity is required in the zinc dust, refined zinc may be the source of the product.
The most important commercial compound of zinc is the oxide (ZnO). The method of production is closely akin to the retort process used for the metal, since the zinc must first be produced in vapor form, whether it is to be condensed to metal or oxidized. If the zinc vapor is oxidized directly without being condensed, the product is known as American- or direct-process zinc oxide. If the zinc vapor is first condensed to metal before final vaporization and oxidation, it is known as French- or indirect-process oxide. The grades produced by each process are numerous and are controlled to give the properties desired for specific applications.
Zinc oxide is a white compound with a high refractive index, and that fact, together with its high toxicity for mildews and fungi, accounts for its wide use as a paint pigment. Still more important from the standpoint of the quantity required is the use of zinc oxide in rubber, both natural and synthetic. It aids in the curing process, increases the mechanical strength, reduces heat generation, and facilitates heat dissipation. The Usual addition is about 3 to 5 percent. Other important uses of zinc oxide are in ceramics, coated fabrics and textiles, floor covering, cosmetics, and pharmaceuticals. Another article of commerce, but of much less importance, is leaded zinc oxide (5 percent or more of lead), used almost entirely in the paint-making industries.
Other than the oxide, the two most important commercial compounds of zinc are the sulfate (ZnSO4) and the chloride (ZnCl2). The chief uses of zinc sulfate are in the manufacture of rayon and for agricultural purposes—for example, in the citrus industry as a trace fertilizer and in sprays for controlling certain plant diseases. It is used also in the manufacture of chemicals, flotation reagents, and glues, and in the medicinal, rubber, and mineral industries. Zinc chloride is used for preserving and fireproofing wood, in soldering and tinning fluxes, for various purposes in the processing of textiles, and in battery making and galvanizing. Other zinc compounds of lesser commercial importance are the acetate, borate, carbonate, chromate, silicate, and sulfide, and certain organic compounds used in driers and metallic soaps such as the palmitate, stearate, oleate, and naphthenate.
The pigment lithopone should be mentioned also, although its importance has declined markedly since 1929. It is a coprecipitated pigment of zinc sulfide and barium sulfate made by reacting solutions of zinc sulfate and barium sulfide. It finds use in such industries as paint, floor covering, coated fabrics and textiles, and rubber.