Coatings continued galvanic

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)... 

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

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. 

Metallic coatings are either more noble than the substrate, as in chromium plate on mild steel, or are base metals which corrode more easily than the substrate. In the former case, the underlying metal is protected by a continuous impervious film of the noble metal which is itself resistant to attack. This method is adequate provided the surface coating contains no holes or flaws and remains intact. Penetration of this layer by the corrosive agent leads to galvanic corrosion at the interface of the two metals. If the object is coated with a more base metal, then protection is by both the physical barrier of the metal film and by cathodic protection at any subsequent defects (Fig. 19). 

In the winter of 1984-1985, test samples on Plexiglass plates were exposed in each of the test cities. These included samples of bare weighed auto-body steel, weighed galvanized steel, and painted auto-body steel with stainless steel trim. Where painted panels were involved, cut edges were coated in the stainless steel trimmed panels, the ends of the stainless steel bolts used for electrical continuity were coated as well. Nylon bolts were used to attach the other test specimens to the Plexiglass plate which was mounted on the front bumper of an automobile. 

Hypothesize Suppose in galvanization, copper was plated on iron instead of zinc. Would copper continue to protect the iron from corrosion, as zinc does, if the copper coating became broken or cracked Explain. 

Galvanized steel is a common example of galvanic coupling where steel (Fe) with a standard electrode potential of—0.440 Vvs. SHE is cathodicaUy protected by a coating of zinc with 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 the steel from corrosion because of its sacrificial properties. Because zinc is less noble than steel, it acts as the anode. The sacrificial anode is continuously consumed by anodic dissolution and protects the more positive metal from corrosion. In practice, sacrificial anodes are... 

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