Cadmium, atmospheric corrosion

The relative susceptibility of metals to atmospheric corrosion varies widely with the type of contaminant, e.g. zinc and cadmium, two metals that are used for the protection of steel in exposed environments, are both rapidly attacked by organic acidson the other hand, aluminium alloys resist attack by organic acids but may be rapidly corroded by chlorides, especially at crevices or areas of contact. 

One-year test results on zinc and cadmium are in Table 2.22. The results of long-term atmospheric corrosion tests on zinc and cadmium coatings (Fig. 2.35) are compared with those of accelerated corrosion tests (Fig. 2.36) on the same coatings in three cabinets warm and humid, SO2 gas, and sea mist (Strekalov and Berukshtis, 1965). 

Additional results from Soviet investigators (Berukshtis and Klark, 1971) produced the plots shown in Fig. 2.38 for average thickness loss of zinc and cadmium and coatings of Zn and Cd, with and without chromating. These tests at different dates give some indication of the changes in atmospheric corrosivity over the years. 

Berukshtis, G., and Klark, G. (1965). Atmospheric corrosion of steel, zinc, cadmium, copper and aluminium in different coastal and continental regions. Korroz. Met. Splavov Sb., (2), 332-350 (in Russian). 

Hippensteel, C. L., and Borgmann, C. W. (1930). Outdoor atmosphere corrosion of zinc and cadmium electrodeposited coatings on iron and steel. Trans. Am. Electro-chem. Soc., 58, 23-41. 

Colour Retention. The colour retention of red, orange and yellow vitreous enamels containing cadmium sulphide, or sulphoselenide, when exposed to atmospheric corrosion is measured by treating the enamel surface with cupric sulphate solution details of the test are given in ASTM - C538. 

For some non-ferrous metals (copper, lead, nickel) the attack by sulphuric acid is probably direct with the formation of sulphates. Lead sulphate is barely soluble and gives good protection. Nickel and copper sulphates are deliquescent but are gradually converted (if not leached away) into insoluble basic sulphates, e.g. Cu Cu(OH)2)3SO4, and the metals are thus protected after a period of active corrosion. For zinc and cadmium the sulphur acids probably act by dissolution of the protective basic carbonate film. This reforms, consuming metal in the process, redissolves, and so on. Zinc and cadmium sulphates are formed in polluted winter conditions whereas in the purer atmospheres of the summer the corrosion products include considerable amounts of oxide and basic carbonate. ... 

The excellent resistance of zinc to corrosion under natural conditions is largely responsible for the many and varied applications of the metal. In fact nearly half the world consumption of zinc is in the form of coatings for the prevention of corrosion of steel fabrications exposed to the atmosphere and to water. For its varied applications zinc is obtainable in a number of grades. Ordinary commercial (G.O.B.) zinc contains up to about I -5% total of lead, cadmium and iron. Electrolytic zinc has a minimum zinc content of 99-95% and contains small amounts of the same impurities. Special high-purity zinc has a minimum of 99-99% zinc. Even purer zincs are commercially available. 

Table 13.4 Corrosion rates of zinc and cadmium coatings in various atmospheres...

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. 

The corrosion rates of various metals in industrial, marine and rural atmospheres are given in Table 4.76. Zinc has higher corrosion resistance than cadmium and iron in all the three atmospheres. Zinc has higher corrosion resistance than copper in industrial... 

With respect to corrosion, the conventional classification of climates in marine, inland, industrial, etc. types is not sufficient. It should now be specified with respect to the actual chemical components in the atmosphere, as well as humidity and other factors. Recent research in this field has led to much more precise methods for estimating corrosion rates in polluted atmospheres (38). Economically, perhaps even more important problems are caused by the increased corrosion of water supply pipelines. Not only copper is dissolved, but also cadmium from soldered joints, and larger steel and cement pipelines may also be affected. 

CHEMICAL PROPERTIES combustible tarnishes in moist air slowly oxidized by moist air to form cadmium oxide highly corrosion resistant corrosion resistance poor in industrial atmospheres lowers melting point of certain alloys when used in low percentages resistant to alkalies reacts vigorously with strong oxidizers, elemental sulfur, selenium and tellurium. 

---