Cadmium, galvanic corrosion

The conductivity and the composition of the medium are controlling factors in the extent of galvanic corrosion, as evidenced by the corrosion rates in 3% NaCI and tapwater (see Table 4.71). Cadmium-plated steel gives a lower corrosion rate than steel. 

The zinc, cadmium or tin plating reduces the galvanic corrosion of magnesium, as seen in Figure 4.6. The compatibility of various fasteners shown in the figure leads to the order of effectiveness tin > cadmium > zinc. 

E6.1. Predict the possibihty of galvanic corrosion in sea water for the following coupled pairs of alloys and metals (i) aluminum alloys and aluminum brass, (ii) cadmium and manganese bronze, (iii) zinc and tin, (iv) low alloy steel and stainless steel 410, (v) low alloy steel and stainless steel 430, (vi) nickel 200 and Ni-Cr-Mo-Cu-Si... 

Table 7.3 shows an example of galvanic corrosion rates of aluminium alloys in 3.5% NaCl solution when coupled to different materials. For instance, it is seen that the contact with low-alloy steel gives considerably higher galvanic corrosion rates on aluminium than does contact with the - from a practical point of view - more noble stainless steels as well as Ni- and Ti-based alloys (regarding material descriptions, see Section 10.1). The table reflects the cathodic efficiency of the various materials coupled to aluminium (with the exception of cadmium, zinc and aluminium alloys) in the actual environment. 

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. 

Where electrical cables penetrate a dissimilar metal partition or bulkhead, precautions against galvanic corrosion should be taken. Cadmium- or zinc-plated parts or zinc-based alloy parts should not be used within or in the proximity of electrical equipment subject to phenolic vapors emanating from... 

Cadmium 0.005 0.005 Kidney damage Corrosion of galvanized pipes erosion of natural deposits discharge from metal refineries runoff fiom waste batteries and paints... 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

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. 

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. 

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. 

World mine production of zinc is 9 Mt, with almost all of it derived from sphalerite [(Zn,Fe)S], which is also the principal primary source of cadmium and several other metals, such as germanium and indium. China and Australia are the largest producers, but several other countries mine significant amounts. About half of the annual consumption is for the manufacture of galvanized products to resist corrosion, primarily in the automotive and construction industries. 

In contrast to inorganics such as mercury, copper, arsenic, lead, and cadmium, there have been fewer studies that focus only on zinc removal by carbonaceous adsorbents [199-203]. The presence of zinc in water appears to be due primarily to corrosion of galvanized metals [33]. The dominant oxidation state in aqueous solution is Zn(II), while the dominant species (see Fig. Al) are Zn at pH < 9 and Zn(OH), at pH > 9 [202]. 

FERROUS SULFATE or FERROUS (II) SULFATE or FERROUS SULFATE, ANHYDROUS (7720-78-7 7782-63-0, heptahydrate) Fei(S04)j Fe2(S04)j 7Hi0 (heptahydrate) Aqueous solution is strongly acidic. Reacts violently with strong bases, amines, amides, and inorganic hydroxides. Inconpatible with strong oxidizers. Aqueous solution is incompatible with sulfuric acid, caustics, alkylene oxides, ammonia, aliphatic amines, alkanolamines, amides, epichlorohydrin, organic anhydrides, isocyanates, methyl isocyanoacetate, vinyl acetate. Corrosive to copper and its copper alloys and to mild and galvanized steels. FERRO YELLOW (1306-23-6) see cadmium sulfide. 

Galvanized steel General corrosion Excessive zinc, lead, cadmium, and iron release causing blockage of pipe. 

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... 

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. 

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