Galvanic corrosion cadmium coating

Corrosion within the tower itself is due mainly to the particular conditions existing therein (air, humidity and temperature) and also to the chemical treatment of the water. All construction materials exposed to these conditions must be selected carefully. Hardware and piping for distribution headers have been successfully made with hot-dipped galvanized steel, cadmium-coated steel, stainless steel and silicon bronze. 

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... 

Cadmium and zinc electroplating provides galvanic corrosion protection when coated on steel. Deposit thickness can vary between 5 and 25 p,m (0.2 and 1 mil), and typical applications for both coatings are found in Table 10. Cadmium is preferred for the protection of steel in marine environments, whereas zinc is preferred in industrial environments. Cadmium is also preferred for fastening hardware and connectors because its coefficient of friction is less than zinc. Cadmium is toxic and should not be used in parts that will have contact with food. Precautions for minimizing hydrogen embrittlement should be taken because cadmium plating is more susceptible to such embrittlement than any other plated metal. 

On the scale of potentials, zinc is more electronegative than aluminium, while cadmium has a potential very close to that of aluminium. Galvanised or cadmium-coated steel fasteners can, therefore, be used to join and assemble stmctures made from aluminium alloys. It should just be remembered that when these coatings become too worn to protect the steel and the aluminium, the previous scenario applies in which there is contact between the aluminium alloy and bare steel. Chromium-plated steel does not lead to galvanic corrosion with aluminium, as long as chromium covers the nickel underlayer however, the contact between aluminium and the nickel underlayer would lead to galvanic corrosion of aluminium. 

In practice, contacts with stainless steel and zinc or cadmium-coated steel are the most common ones in constructions, and especially in metallic fittings. Experience throughout the world demonstrates that even without insulation between the two metals, galvanic corrosion does not lead to problems in these assemblies, if the design is such that any... 

Galvanic corrosion of aluminium in contact with galvanised or cadmium-coated steel is not observed, as long as the zinc or cadmium coating protects the steel. 

Distribution Systems. A substantial amount of contamination of drinking water can occur while the water is in transit to the consumer after treatment. Pipes are made of copper, galvanized iron, asbestos-cement, lead, or plastic, and often polymeric or coal tar coatings are used. All of these are capable of contributing contaminants to the water, especially if the water is corrosive. Lead, copper, cadmium, and polynuclear aromatic hydrocarbons in finished water are primarily problems of water distribution and not source water contamination. Physical deterioration of the distribution system can also permit biological contamination to occur during transit. 

In hoi galvanizing, zinc is applied to iron and steel parts by immersing the parts into u butli of triollen zinc Whereas in principle almost any metal could be coaled with molten zinc, this coating serves no worthwhile purpose on most metals. The combination of zinc and ferrous materials are almost uniquely suited to each other. Aluminum and cadmium are the only other similar combinations, Zinc provides iron parts with better corrosion protection by developing a coating of zinc and zinc compounds on the base metal surface. 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

For sacrificial coatings (e.g., zinc, cadmium) and, in certain environments, also aluminum and tin on steel, the direction of galvanic current through the electrolyte is from coating to base metal as a result, the base metal is cathodically protected (Fig. 14.1). As long as adequate current flows and the coating remains in electrical contact, corrosion of the base metal does not occur. The degree of... 

By providing a barrier between the substfate and the environment, or by cathodically protecting the substrate, metallic coatings protect the substrate from corrosion. Coatings of chromium, copper, and nickel provide increased wear resistance and good corrosion resistance. However, these noble metals make the combination of the substrate (mostly steel or an aluminum alloy) with the protective layer sensitive to galvanically induced local corrosion. Nonnoble metallic layers such as zinc or cadmium provide good cathodic protection but show poor wear resistance. 

Cadmium occurs mostly in association with zinc and gets into water from corrosion of zinc-coated ( galvanized ) pipes and fittings. 

Ion vapor deposition of aluminum is attractive because it avoids the environmental and toxicological problems associated with cadmium. It does not cause hydrogen embrittlement of steel or solid metal embrittlement of steel or titanium, and it should be more galvanically compatible with aluminum alloy structure and avoid the exfoliation corrosion of sensitive alumimun alloy structme. However, views on the ability of alumimun to protect steel fasteners appear to vary. A view is that the presently available pure aluminum coatings are not able to provide adequate sacrificial protection to steel in a chloride ion environment, and for this reason a recommendation is made for the development of alruninrun coatings containing small amounts of zinc or other elements for improved protection. 

Nickel, cadmium and zinc have all been shown to be capable of delapng and, in some cases, preventing the corrosion of reinforcing steel in concrete, but only zinc-coated (galvanized) bars are commonly available. Field studies of galvanized bars in service for many years in either a marine environment or exposed to deicing salts have failed to show any deficiencies in the concrete. ... 

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