Atmospheric Corrosion. 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.
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:
- The duration and frequency of moisture
- The rate at which the surface dries
- The extent of industrial pollution of the atmosphere.
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 corrosivities. 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.
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.
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.
Organic Compounds. 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.
Gases. 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.
Indoor Exposure. 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.
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.
Zinc Coating Processes
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.
There is usually at least one process that is applicable to any specific purpose. Because the processes are complementary, there are rarely more than two processes to be seriously considered as the best choice for a particular application.
Hot Dip Galvanizing. In hot dip galvanizing, the steel or iron to be zinc coated is usually completely immersed in a bath of molten zinc. It is by far the most widely used of the zinc coating processes and has been practiced commercially for almost two centuries. The modern hot dip galvanizing process is conducted in carefully controlled plants by applying the results of scientific research, and it is far removed from that of years ago although it is still dependent on the same basic principles.
Continuous Galvanizing. This process, known as the Sendzimir process, uses a small amount of aluminum in the zinc bath and produces a coating with essentially no iron-zinc alloy and with sufficient ductility to permit deep drawing and folding without damage to the coating.
The Galvanizing Process. Before the iron or steel pans are dipped in the molten zinc, it is necessary to remove all scale and rust. This is usually done by pickling in an inhibited acid. To remove molding sand and surface graphite from iron castings shot- or gritblasting is generally used, usually followed by a brief pickling operation.
In the dry galvanizing process, the work is prefluxed by dipping in a flux solution of zinc ammonium chloride, then passed to a low-temperature drying oven. It is then ready for dipping into the molten zinc bath, the surface of which is kept relatively clear of flux. In contrast, in wet galvanizing, the pickled articles are dipped into the molten zinc bath through a substantial flux blanket.
Electrogalvanizing. The additional development of continual electrogalvanizing lines added another dimension to zinc-coated steel, that is very thin, formable coatings ideally suited to deep drawing or painting Zinc is electrodeposited on a variety of milled products by the steel industry: sheet, wire, and in some cases, pipe. Electrogalvanizing at the mill produces a thin, uniform coat of pure zinc with excellent adherence.
Electrogalvanized steel is produced by electrodepositing an adhering zinc film on the surface of sheet steel or wire. These coatings art not as thick as those produced by hot dip galvanizing and are mainly used as a base for paint.
