Protective coatings galvanizing

Zinc is widely used as a protective coating (galvanizing) and forms many useful alloys, such as brass. Zinc is not known to be toxic in any form, but cadmium and mercury are exceedingly toxic and must be handled accordingly. 

Zinc is often used as a protective coating over iron to form galvanized iron. In industrial settings exposed to SOj and humidity, this zinc coating is subject to sufficient corrosion to destroy its protective capacity. Haynie and Upham (5) used their results from a zinc corrosion study to predict the useful life of a zinc-coated galvanized sheet in different environmental settings. Table 9-2 shows the predicted useful life as a function of SO concentration. 

Primers for protective coatings may be divided into three broad classes based on the mechanism of substrate protection barrier primers that function by preventing the ingress of moisture and electrolytes, primers that protect the substrate galvanically in the presence of electrolytes, and primers that contain electrochemical inhibitors to passivate the substrate. Each of these approaches requires a distinct primer film structure due to the different mechanisms of protection. 

To protect buried metals from premature breakdown it must suffice to say that protective coatings and other methods must be applied against factors such as the effects of galvanic currents, composition of the moisture in the ground, humus acids, bacteria, etc. (See Section 14.8.)... 

A type of assembly calculated to favour maximum galvanic action was developed by the Bell Telephone Laboratories and is illustrated in Fig. 19.32. Here, the less noble metal is in the form of a wire wound in the grooves of a threaded specimen of the metal believed to be more noble. Good electrical contact is achieved by means of set screws covered with a protective coating. This assembly favours accumulation of corrosive liquids around the wire in the thread grooves. Corrosive damage is also favoured by the high ratio of surface to mass in the wire specimens. 

Galvanic current Measurement of the galvanic current between two different metals can be easily measured using a zero resistant ammeter ". This method can have specific application, e.g. to provide a signal indicating failure of a protective coating in a process vessel. Commercial probes are available for industrial monitoring. 

Various strategies are employed to prevent corrosion. The use of paint as a protective coating is described in our chapter introduction. A metal surface can also be protected by coating it with a thin film of a second metal. When the second metal is easier to oxidize than the first, the process is galvanization. Objects made of iron, including automobile bodies and steel girders, are dipped in molten zinc to provide sacrificial coatings. If a scratch penetrates the zinc film, the iron is still protected because zinc oxidizes preferentially ... 

Hot dip coating is the process of coating a metallic workpiece with another metal by immersion in a molten bath to provide a protective him. Galvanizing (hot dip zinc) is the most common hot dip coating. 

Today about one-third of all the zinc metal is used for the process known as galvanization. This process provides a protective coating of zinc on other metals. A thin layer of zinc oxidizes in air, thus providing a galvanic corrosion protection to the iron or steel item that it coats. Several processes are used to galvanize other metals. One is the hot dip method wherein the outer surface of the item to be galvanized is pickled and then immersed into a molten zinc bath. A... 

In this activity, galvanized nails are used as a source of zinc. Galvanization is a process by which metals such as steel are dipped in zinc to protect them from rusting. One type of nail is dipped in a hot bath of molten zinc to form a protective coating. Zinc is more easily corroded than iron, so it oxidizes rather than the steel. 

Galvanizing. The process of galvanizing consists of placing a protective coating of zinc upon the surface of iron. Before application of the zinc, the iron must be cleaned of rust or scale by treatment with dilute sulfuric acid, a process known as pickling, which incidentally produces important quantities of copperas, hydrated ferrous sulfate (FeS04-7H20), as a by-product. 

Zinc and its alloys are widely used because they have low melting points and can be easily cast. Therefore, various objects are made from zinc and its alloys. Zinc is widely applied to iron and steel as a protective coating by the process known as galvanization that consists of coating an iron object with a thin layer of zinc. Relative to iron, zinc is an anode, so it is preferentially oxidized. If the coating is broken, the zinc continues to corrode rather than the iron object. When iron is coated with a less reactive metal such as tin, a break in the coating causes the more easily oxidized iron to be corroded at an accelerated rate. 

Galvanization, coating the metal with an unbroken layer of zinc. Zinc is preferentially oxidized and the zinc oxide that forms provides a protective coating (passivation). 

Protective coatings are used extensively on metal or semiconductor surfaces to isolate them or limit access of an aggressive environment (17,18). Frequently these coatings are multilayered and complex in structure, as for example in automobile paints. In this case, the innermost coating is either hot-dipped or electrodeposited zinc ("galvanizing"), over which a zinc-rich polymer-chromate undercoat is placed. The decorative top coat provides a physical barrier to the transport of water and ionic species. It is important to note, however, that protection is achieved electrochemically by the galvanic action of zinc on steel and by the inhibiting action of chromate toward oxidation. 

Galvanizing is another form of cathodic protection. A galvanized pipe is a steel pipe coated with a thin layer of zinc. Since zinc is anodic to steel (Table 7-4), the zinc will corrode preferentially to the iron and so protect it. Moreover, the corrosion products of zinc, the carbonates and hydroxides, adhere to the galvanized surface and render it passive. 

In Fig. 19(a), a defect in a polymer coating on a metal substrate is shown. The anodic dissolution in the defect leads to a positive current peak. The intact polymer-coated area shows zero current. A second important situation, a cut edge of coil-coated galvanized steel, is shown in Fig. 19(b). The zinc dissolution leads to a positive peak while the area of oxygen reduction on the cathodically protected steel surface is characterized by a broad negative current peak. The activation, distribution, and passivation of these local anodes and cathodes can now be studied by the SVET as a function of coating compositions. 

Cathodic Protection in the Case of Polymer-coated Galvanized Steel In most cases, Zn or Zn-aUoy coatings are used to protect steel sheets because of the barrier properties of zinc and its sacrificial... 

External corrosion of water systems may be caused by general corrosion, stray current corrosion MIC, and/or galvanic corrosion. Corrosion mitigation techniques include the application of protective coatings, wrapping pipe in a plastic cover, and the application of CP. The areas of major external corrosion impact are generally those where localized attack may take place, such as in the proximity of other systems like galvanic corrosion or in areas where stray currents may occur. 

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