Zinc coating continued

Nowadays, however, zinc-coated steel sheets, either continuously galvanised or electroplated, are often used as a basis material for overcoating with plastic materials or paints. The coatings are usually applied continuously and have a range of uses both externally and internally. Many surface finishes are obtainable, e.g. plain or embossed, and in an extensive range of colours, to suit almost any requirement . 

BS2989 1992, Specification for continuously hot-dip zinc coated and iron-zinc alloy coated steel flat products tolerances on dimensions and shape ... 

Proceedings First International Zinc Coated Sheet Conference, ZDA, London, (1985) Proceedings Second International Zinc Coated Sheet Conference, ZDA, London (1988) Proceedings Third International Zinc Coated Sheet Conference, EGGA, London (1991) Bibliography — Hot Dip Galvanizing ZDA, London (1976) (Continuously updated in Zinc Abstracts and, since 1986, Zincscan)... 

In the Forth, Severn and many other suspension bridges, zinc coatings have an important function. The whole main structure is of steel and has been zinc-sprayed on the external surfaces, while the main cable and hanger ropes have been coated by continuous hot-dip galvanising. Case histories of galvanised multi-truss bridges cover more than 30 years. 

PROTECTIVE COATING. A film or thin layer of metal glass of paint applied to a substrate primarily to inhibit corrosion, and secondarily for decorative purposes, Metals such as nickel, chromium, copper, and tin are electrodeposited on the base metal paints may be sprayed or brushed on. Vitreous enamel coatings are also used these require baking. Zinc coating are applied by continuous bath process in which a strip of ferrous metal is passed through molten zinc. 

The process has been continuously improved [71] but so far a breakthrough in the application of electroplated aluminum is still pending. The problem is the price, compared to zinc coatings. Electroplated aluminum is more expensive than zinc by a factor of 3 to 5. Therefore, electroplated aluminum is restricted to some high-tech applications, for example, special optical surfaces. As a corrosion-protective coating, the material is not yet an alternative to the established systems. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

The change of polarity between the steel substrate and zinc coating when the temperature is increased to 60-70°C may cause corrosion of steel on bare regions. Therefore a continuous coating is required at higher temperatures. 

Steel sheet, strip, wire, and tube are all galvanized in the semifabricated state. Fabrication practice is the same as with ungalvanized material, and the only additional requirement is to restore the coating (see later) to areas from which it has been removed by, for example, machining during fabrication. Similar comments apply to continuously electroplated sheet, strip, and wire. The other zinc coatings are normally applied after fabrication. 

More than 10 years experience with thermally sprayed coatings of zinc-15% aluminum is now available. As a result of the two-phase structure (a zinc-rich and an aluminum-rich phase) of such coatings, the oxidation of the active zinc-rich phase occurs within the more inert aluminum-rich phrase. Oxidation products are encapsulated in the porous layer formed by the latter and do not build up a continuous surface layer as with pure zinc coatings. Consequently, no thickness or weight loss is observed even after several years of exposure in atmospheric field testing. 

Kruse et al. (1983) tested zinc tubes and steel pipe specimens with different zinc coatings. All specimens were corroded in Dortmund tap water in 11 test lines containing hot water at approximately 60°C and cold water, with both discontinuous and continuous flow in certain tests there were occasional additions of polyphosphates or orthophosphates. One test line was operated with warm water from a water heater, with cathodic protection of the reservoir by the Guldager method. Visual inspection, as well as the deter-... 

Anon. (1988). Continuous Hot-Dip 55% Aluminium-Zinc Coated Steel Sheet. Technical brochure, BIEC International, Siegen, Germany, 48 pp. 

Skenazi, A. F., Davin, A., Coutsouradis, D., and Goodwin, F. E. (1985). The influence of alloying elements in zinc-aluminium baths for continuous hot dip galvanizing. 1st Int. Conf. on Zinc Coated Steel Sheet, ZDA, London, pp. OA/1-5. 

There is evidence that surface coatings containing zinc metal can adsorb and retain iodine from water at a high pH (9 -10). Continued retention of the iodine requires that the water in contact with the zinc coating be maintained at a high pH. Hence, for this mitigation strategy to be effective, it must be combined with the use of pH buffer to control the pH of the water in containment. 

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