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Zinc is the 23rd most abundant element in the earth's crust. Sphalerite, zinc sulfide, is and has been the principal ore mineral in the world. In the United States, about two-thirds of zinc is produced from ores (primary zinc) and the remaining one-third from scrap and residues (secondary zinc). Zinc is necessary to modern living, and, in tonnage produced, stands fourth among all metals in world production—being exceeded only by iron, aluminum, and copper. Zinc uses range from metal products to rubber and medicines. About three-fourths of zinc used is consumed as metal, to protect iron and steel from corrosion mainly as a coating (MetalCoatings/Zinc-coatings.htm) , as alloying metal to make bronze and brass, as zinc-based die casting alloy, and as rolled zinc. The remaining one-fourth is consumed as zinc compounds mainly by the rubber, chemical, paint, and agricultural industries. Zinc is also a necessary element for proper growth and development of humans, animals, and plants; it is the second most common trace metal, after iron, naturally found in the human body.
In a normal atmosphere, zinc forms a basic zinc carbonate film that greatly retards its corrosion rate, which is similar to the aluminum oxide film that forms on aluminum which accounts for its low corrosion rate. Zinc, when connected with metals below it in the electrochemical series, will sacrificially protect that metal, which accounts for its wide usage in the galvanized steel industry.
The behavior of zinc and zinc coatings during atmospheric exposure has been closely examined in tests conducted throughout the world. The performance of zinc in a specific atmospheric environment can be predicted within reasonable limits. (reference)
Precise comparison of corrosion behavior in atmospheres is complex because of the many factors involved, such as prevailing wind direction, type and intensity of corrosive fumes, the amount of sea spray, and the relative periods of moisture or condensation and dryness. However, it is generally accepted that the corrosion rate of zinc is low; it ranges from 0.13 µm/yr in dry rural atmospheres to 0.013 mm/yr in more moist industrial atmospheres.
Zinc is more corrosion resistant than steel in most natural atmospheres, the exceptions being ventilated indoor atmospheres where the corrosion of both steel and zinc is extremely low and certain highly corrosive industrial atmospheres. For example, in seacoast atmospheres the corrosion rate of zinc is about 1/25 that of steel.
Zinc owes its high degree of resistance to atmospheric corrosion to the formation of insoluble basic carbonate films. Environmental conditions that interfere with the formation of such films may attack zinc quite rapidly. The important factors that control the rate at zinc corrodes in atmospheric exposures are:
In dry air, zinc is slowly attacked by atmospheric oxygen. A thin, dense layer of oxides formed on the surface of the zinc, and outer layer then forms on top of it. Although outer layer breaks away occasionally, the under layer remains and protects the metal restricting its interaction with the oxygen. Under these conditions, which occur in some tropical climates, the zinc oxidizes very slowly.
The rate of drying is also an important factor because a thin moisture film with higher oxygen concentration promotes corrosion.
Atmospheric corrosion has been defined to include corrosion by air at temperatures between -18 to 70°C in the open and in enclosed spaces of all kinds. Deterioration in the atmosphere is sometimes called weathering. This definition encompasses a great variety of environments of differing corrosivity. The factors that determine the corrosivity of an atmosphere include industrial pollution, marine pollution, humidity, temperature (especially the spread between daily highs and lows that influence condensation and evaporation of moisture) and rainfall.
The atmosphere, as far as corrosion is concerned, is not a simple invariant environment. The influence of these factors on the corrosion of zinc is related to their effect on the initiation and growth of protective films.
Corrosion of Zinc in Water
The corrosion of zinc in water is largely controlled by the impurities present in the water. Naturally occurring waters are seldom pure. Even rainwater, which is distilled by nature, contains nitrogen, oxygen, CO2, and other gases, as well as entrained dust and smoke particles. Water that runs over the ground carries with it eroded soil, decaying vegetation, living microorganisms, dissolved salts, and colloidal and suspended matter. Water that seeps through soil contains dissolved CO2 and becomes acidic. Groundwater also contains salts of calcium, magnesium, iron, and manganese. Seawater contains many of these salts in addition to its high NaCl content. () reference
All of these foreign substances in natural waters affect the structure and composition of the resulting films and corrosion products on the surface, which in turn control the corrosion of zinc. In addition to these substances, such factors as pH, time of exposure, temperature, motion, and fluid agitation influence the aqueous corrosion of zinc.
As in the atmosphere, the corrosion resistance of a zinc coating in water depends on its initial ability to form a protective layer by reacting with the environment. In distilled water, which cannot form a protective scale to reduce the access of oxygen to the zinc surface, the attack is more severe than in most types of domestic or river water, which do contain some scale-forming salts.
The scale-forming ability of water depends principally on three factors: the hydrogen ion concentration (pH value), the total calcium content and the total alkalinity. If the pH value is below that at which the water would be in equilibrium with calcium carbonate (CaCO3), the water will tend to dissolve rather then to deposit scale. Waters with high content of free CO2 also tend to be aggressive toward zinc.
Corrosion in dissolved salts, acids and bases
Zinc is not used in contact with acid and strong alkaline solutions, because it corrodes rapidly in such media. Very dilute concentrations of acids accelerate corrosion rates beyond the limits of usefulness. Alkaline solutions of moderate strength are much less corrosive than corresponding concentrations of acid, but are still corrosive enough to impair the usefulness of zinc. (reference)
Zinc-coated steel is used in handling refrigeration brines that may contain calcium chloride (CaCl2). In this case, the corrosion rate is kept under control by adding sufficient alkali to bring the pH into the mildly alkaline range and by the addition of inhibitors, such as sodium chromate (Na2CrO4). Certain salts, such as the dichromates, borates, and silicates, act as inhibitors to the aqueous corrosion of zinc.
Many organic liquids that are nearly neutral in pH and substantially free from water do not attack zinc. Therefore, zinc and zinc-coated products are commonly used with gasoline, glycerine, and inhibited trichlorethylene. The presence of free water may cause local corrosion because of the lack of access to oxygen. When water is present, zinc may function as a catalyst in the decomposition of such solutions as trichlorethylene with acid attack as the result. Some organic compounds that contain acidic impurities, such as low-grade glycerine, attack zinc. Although neutral soaps do not attack zinc, there may be some formation of zinc soaps in dilute soap solutions. (reference)
Zinc may be safely used in contact with most common gases at normal temperatures if water is absent. Moisture content stimulates attack. Dry chlorine does not affect zinc. Hydrogen sulfide (H2S) is also harmless because insoluble zinc sulfide (ZnS) is formed. On the other hand, SO2 and chlorides have a corrosive action because water-soluble and hygroscopic salts are formed. (reference)
Zinc corrodes very little in ordinary indoor atmospheres of moderate relative humidity. In general, a tarnish film begins to form at spots where dust particles are present on the surface: the film then develops slowly. This attack may be a function of the percentage of relative humidity at which the particles absorb moisture from the air. (reference)
Rapid corrosion can occur where the temperature decreases and where visible moisture that condenses on the metal dries slowly. This is related to the ease with which such thin moisture films maintain high oxygen content because of the small volume of water and large water/air interface area.
Coatings of metallic zinc are generally regarded as the most economical means of protecting against corrosion. Seven methods of applying a zinc coating to iron and steel are in general use: hot dip galvanizing, continuous-line galvanizing, electro-galvanizing, zinc plating, mechanical plating, zinc spraying, and painting with zinc-bearing paints. (reference)
See also: Aluminum, Aluminum alloys, Brass, Bronze, Cadmium, Chromium, Cobalt, Copper, Gold, Iron, Lead, Magnesium, Molybdenum, Nickel, Nickel alloys, Silver, Stainless steels, Steel, Tantalum, Tin, Titanium, Zinc, Weathering steel
Numerous pages on this web site refer to hygroscopic corrosion: Avionics, Chlorides, Coatings, Contaminants, Design, Electronics, Quiz, Relative humidity, Sorption behavior, Temperature fluctuations, Time-of-wetness, Weld flux, Zinc