Atmospheric Corrosion of Non-Ferrous Metals

Among the non-ferrous metals, zinc, aluminum, and copper are most often employed in outdoor applications such as buildings, installations and transportation systems. Table 8.24 indicates the average corrosion rates of these metals in different types of atmosphere. Compared to steel, these rates are rather small. The values given in parentheses indicate the maximum depth of pits (in jtm) after twenty years of exposure time. 

The atmospheric corrosion of zinc involves the following partial reactions  

Zinc hydroxide Zn(OH)2 precipitates on the surface. In addition, the oxide ZnO is found among the corrosion products, as well as a number of salts of variable stoichiometry, mostly basic and containing hydroxyl groups. Table 8.26 lists the principle corrosion products of zinc together with those of aluminum, and copper. 

The formation of basic salts results from the reaction of zinc hydroxide with certain constituents of the atmosphere. For example, the reaction of zinc hydroxide with carbon dioxide leads to formation of weakly soluble hydroxy-carbonates (or basic carbonates) with the general formula Zn(0H)x(C03)y  

The value of the coefficient y depends on reaction conditions and does not normally correspond to a whole number. The formula Zn5(0H)6(C03)2, for example, designates a compound of stoichiometry with y = 0.4. The different basic carbonates form a whitish patina and the corrosion rate stays relatively low. 

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Symposium on Atmospheric Corrosion of Non-Ferrous Metals, Amer. Soc. Test. Mat., 58th Annual Meeting, June 29 (1955). Spec. Tech. Publn. No. 175, 141-158... 

Anderson, E. A. In "Atmospheric Corrosion of Non-ferrous Metals" ASTM STP 175, American Society for Testing and Materials, Philadelphia, PA, 1955. 

A.R. Mendoza, F. Corvo, Outdoor and indoor atmospheric corrosion of non-ferrous metals, Corros. Sci. 42 (2000) 1123-1147. 

Anderson, E. A. (1947). The corrosion of zinc in the outdoor atmosphere. Symposium on Atmospheric Corrosion of Non-Ferrous Metals, ASTM STP 67. ASTM, Philadelphia, pp. 2-15. 

Atmospheric Corrosion of Non-Ferrous Metals, ASTM STP 175. ASTM, Philadelphia, pp. 126-158. 

STP175, Symposium on Atmospheric Corrosion of Non-ferrous Metals, ASTM, 1956... 

In view of possible or probable variations in mechanical properties among different specimens of the same metal cut from different sheets or other pieces, or even from different sections of the same sheet or piece, it is necessary to pay careful attention to the initial sampling of stock to be used for control, as well as exposure, specimens. An interesting case in which several of these considerations were involved was provided by the long-time atmospheric exposure tests of non-ferrous metals carried out by Subcommittee VI of ASTM Committee B-3 on Corrosion of Non-Ferrous Metals and Alloys in which changes in tensile properties were used as one of the means of measuring the extent of corrosion. 

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. ... 

In 1858 Schonbem noticed that when many substances were exposed to atmospheric oxidation, the oxidisable material appeared to combine with half a molecule of oxygen, leaving the other half in the form of hydrogen peroxide or ozone. This is well exemplified by the corrosion of many non-ferrous metals, such as lead and zinc. When lead, mixed with mercury, is shaken with dilute sulphuric acid in the presence of air or oxygen, lead sulphate is formed, together with some hydrogen peroxide. The amount of the latter is readily ascertained by titration of a portion of the liquid with permanganate, and the quantity of sulphuric acid involved is estimated by titration with alkali. It is then found that the amount of peroxide formed is equivalent to that of the lead dissolved. Thus... 

The impetus for further developments was the recognition of the economic significance of corrosion phenomenon during the 19th century that led the British Association for the Advancement of Science to sponsor corrosion testing projects such as the corrosion of cast and wrought iron in river and seawater atmospheres in 1837. Early academic interest in corrosion phenomenon (up to the First World War) was followed by industrial interest due to the occurrence of equipment failures. An example of this is the corrosion-related failure of condenser tubes as reported by the Institute of Metals and the British Non-ferrous Metals Research Association in 1911. This initiative led to the development of new corrosion-resistant alloys, and the corrosion related failure of condenser tubes in the Second World War was an insignificant problem. 

An extensive study of the corrosion of metals in tropical environments has been carried out by Southwell, etal . Tests have included atmospheric exposure, and exposure in sea-water under mean tide and fully immersed conditions for a range of ferrous and non-ferrous metals and alloys. 

Atmospheric tests on 0.3% copper steel, 7 i -year exposure, from C. Larrabee, Corrosion 9,259 (1953). Atmospheric rates for zinc and copper, 10-year exposure, from Symposium on Atmospheric Exposure Tests on Non-Ferrous Metals, ASTM, 1946. Seawater data from Corrosion Handbook, H. H. Uhlig, editor, Wiley, New York, 1948. Soil data for steel are averaged for 44 soils, 12-year exposure for zinc, 12 soils, 11-year exposure for copper, 29 soils, 8-year exposure from Underground Corrosion, M. Romanoff, Circ. 579, National Bureau of Standards, Washington, D.C. 1957. 

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