Galvanic protection, zinc

Aluminum coatings on steel will perform in a manner similar to zinc coatings. Aluminum has good resistance to many atmospheres in addition, being anodic to steel, it will galvanically protect exposed areas. Aluminum-coated steel products are quite serviceable under high-temperature conditions, for which gooa oxidation resistance is required. 

Thus, in the case of iron coated with zinc (galvanized sheet), zinc would protect iron by sacrificing itself, i.e., by anodically dissolving in the corroding media. However, in the case of iron coated with tin (tinned sheet), tin would protect iron against corrosion by virtue of its own corrosion-resistance properties however, any flaw in the coating would enhance the corrosion of iron since it is anodically disposed to tin according to their placements in the electrochemical series. 

Galvanizing is a process in which iron is covered with a protective layer of zinc. Galvanized iron is often used to make metal buckets and chain-link fences. Galvanizing protects iron in two ways. First, the zinc acts as a protective layer. If this layer is broken, the iron is exposed to air and water. When this happens, however, the iron is still protected. 

Steel objects, when exposed to humid atmospheres or when immersed in electrolytes, corrode at a rapid rate. For example, abrasively polished, cold-rolled steel panels will show signs of rust within 15 minutes when immersed in dilute chloride solutions with pH in the range of 7-10. One of the methods used to control this rapid corrosion is to coat the metal with a polymeric formulation such as a paint. The role of the paint is to serve primarily as a barrier to environmental constituents such as water, oxygen, sulfur dioxide, and ions and secondarily as a reservoir for corrosion inhibitors. Some formulations contain very high concentrations of metallic zinc or metallic aluminum such that the coating provides galvanic protection as well as barrier protection, but such formulations are not discussed in this paper. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

Tin—zinc coatings (75 wt % tin) have application as a solderable coating for radio, television, and electronic components. They also provide galvanic protection for steel in contact with aluminum. 

The protective process is different for zinc-plated, or galvanized, iron. Zinc is more easily oxidized than iron (see Table 19.1) ... 

Galvanization protects iron in two ways. As long as the zinc layer is intact, water and oxygen cannot reach the irons surface. Inevitably, the zinc coating cracks. When this happens, zinc protects iron from rapid corrosion by becoming the anode of the voltaic cell set up when water and oxygen contact iron and zinc at the same time. Figure 20.18 illustrates how these two forms of corrosion protection work. 

Uses Corrosion protective coating for plated and unplated metals base coat for painting of nonferrous metals Features Approved for governmental and industrial spec, finishing of zinc plate, cadmium plate, hot-dipped galvanized steel, zinc die-cast... 

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