Atmospheric corrosion-effect initial exposure

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

Weather conditions at the time of initial exposure of zinc and steel have a large influence on the protective nature of the initial corrosion products This can still be detected some months after initial exposure. Finally, rust on steel contains a proportion of ferrous sulphate which increases with increase in SO2 pollution of the atmosphere. The effect of this on corrosion rate is so strong that mild steel transferred from an industrial atmosphere to a rural one corrodes for some months as though it was still exposed to the industrial environment. ... 

The initial exposure conditions have a very marked influence on the subsequent corrosion rate. During the first days of exposure, wet conditions (caused by high relative humidity or rainfall) cause higher corrosion rates than dry conditions. These effects will vary from one material to another. For example, zinc is more sensitive than steel. Differences are explained by the fact that different materials form different corrosion products with different protective properties. A wide variety of structurally related corrosion products can be formed on zinc, the nature of which depends on initial exposure conditions. The seasonal dependence on the concentrations of peroxide and ozone in the atmosphere might also be a contributing factor. 

Nitrogen and oxygen gas purging tests were performed to determine the effect of oxygen concentration in electrolytes on corrosion behavior. The partial pressure of dissolved oxygen did affect the corrosion rate compared to those under ambient experimental conditions. The corrosion rate under a pure oxygen atmosphere for zVJ s and dp/s samples in 0.1 M HCl increased in the initial exposure period by a small amount, then remained at a rate comparable to that of samples exposed only to a... 

However, the formation and maintenance of the protective layers is governed largely by the pH of the environment, especially the acidic pollutants but since zinc forms an amphoteric oxide, strong alkaline conditions also adversely affect its corrosion behavior by interfering with the formation of the protective layers. Figure 1.11, which shows how the corrosion rate of zinc varies with the pH (Roetheli et al., 1932 Belisle and Du Fresne, 1986), indicates that the attack is most severe at pH values below 6 and above 12.5, while within this range corrosion is very slow. The actual rates of corrosion shown in this study are not of direct relevance in practice, where aqueous solutions are normally much more complex and often contain inhibitors, while the initial rates shown would be modified by corrosion products— particularly in atmospheric exposure, where there is the beneficial effect of periodic drying. 

Initial corrosion rates are modified by the subsequent formation of corrosion products, particularly in atmospheric exposure, where there is the beneficial effect of periodic drying periodic drying may, however, have a detrimental effect when dissolved salts become more concentrated in areas that remain moist after most of the water has evaporated, especially when evaporation results in increased acidity of the residual solution. 

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