Cadmium coatings protection

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. 

Cadmium, being anodic to steel, behaves quite similarly to zinc in providing corrosion protection when apphea as a coating on steel. Tests of zinc and cadmium coatings should be conducted when it becomes necessaiy to determine the most economical selection for a particular environment. 

Compared with other methods, vacuum evaporation produces coatings that have a most satisfactory corrosion resistance, e.g. 0.005 mm of evaporated Cd gives a degree of protection to steel similar to that afforded by 0.01 mm of electrodeposited Cd. Cadmium coatings on ferrous and other substrates can meet authoritative specifications concerning corrosion... 

The features of instances of such potential reversals have been described and include tin coatings on steel in various foodstuffs, particularly acid fruits , cadmium coatings on steel in hard waters , and zinc and aluminium for cathodic protection of steel in natural ground-waters . 

Cadmium cyanide is used as an electrolyte for electrodeposition of thin metalhc cadmium coatings on metals to protect against corrosion. 

While at the end of this test zinc was completely corroded, galvanoaluminum and cadmium showed no sign of red rust. The corrosion-protective effect of the anodized layer is less than that of the nonanodized galvanoaluminum layer, because of the missing eathodic corrosion protection. Yellow chromated 12 pm thick galvanoaluminum and cadmium coatings exhibited no trace of red rust after 1000 h. 

Zinc and cadmium coatings are mainly used for corrosion protection of iron and steel, where they show cathodic protection. 

With the use of cadmium coatings, a variety of environmental protection measures must be connected. A removal of the cadmium layers at the end of the lifetime of the coated part should he guaranteed. 

David, G. (1969). Comparative study of the protection afforded by zinc and cadmium coatings. Corrosion, 17(1), 343-348 (in French). 

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 coatings are produced almost exclusively by electrodeposition. A cadmium coating on steel does not provide as much cathodic protection to the steel as does a zinc coating because the potential between cadmium and iron is not as great as between zinc and iron. Therefore, it becomes important to minimize defects in the cadmium coating. 

Electrolytic cadmium coatings [25] have been widely used to protect fasteners, particularly for exposure in chloride environments. The lubricity of cadmium coatings aids in uniformly tightening threaded fasteners. However, use of cadmium is declining because of toxicity and environmental concerns. 

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. 

Cadmium is a soft metal, which forms a protective coating in air, and burns only on strong heating to give the brown oxide CdO. It dissolves in acids with evolution of hydrogen ... 

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