Atmospheric corrosion local nature

Atmospheric corrosion is electrochemical ia nature and depends on the flow of current between anodic and cathodic areas. The resulting attack is generally localized to particular features of the metallurgical stmcture. Features that contribute to differences ia potential iaclude the iatermetaUic particles and the electrode potentials of the matrix. The electrode potentials of some soHd solutions and iatermetaUic particles are shown ia Table 26. Iron and sUicon impurities ia commercially pure aluminum form iatermetaUic coastitueat particles that are cathodic to alumiaum. Because the oxide film over these coastitueats may be weak, they can promote electrochemical attack of the surrounding aluminum matrix. The superior resistance to corrosion of high purity aluminum is attributed to the small number of these constituents. 

Other factors of importance in atmospheric corrosion of zinc are (i) the distance from the ground (ii) orientation of the samples (iii) wind or rain shielding (iv) distance to the local contaminant sources (v) wind, radiation (vi) condensation and drying rate (vii) amount of contaminants and nature of corrosion products and (viii) seasonal variation of factors also should be considered. This shows the complexity of the problem of determining the atmospheric corrosion rates to a high degree of certainty. This uncertainty is exemplified by the observed corrosion rate of 0.6-3.8 pm/yr at 26 sites in rural area in Spain.95 The corrosion rate of 8.5 pm/yr observed on the zinc coating in an under-vehicle situation is comparable to severe marine atmospheric conditions.96... 

The corrosion effect on constructions in the atmosphere due to acidification is in most areas mainly of local nature. It should be stressed, however, that acidification of soil and water can lead to increased corrosion of buried installations and of installations in water including water pipes (3, ). This is on the other hand mainly a regional problem, where the long-range transport of air pollutants plays an important role. The following subdivision may thus be used. 

The forms of corrosion encounter in diverse engineering structures have a common oxidation mechanism lepiesented by an anodic reaction, such as M — M+ + ze, eq. (1.1a). Thus, corrosion may be due to chemical or electrochemical reactions. One common corrosion process is the formation of ferric hydroxide, Fe OH)3, as indicated by the sequence of reactions given by eq. (1.7. Therefore, corrosion is classified as a localized or general oxidation process. It manifests its natural or forced behavior in various forms from atmospheric corrosion of steel stmctures to oral corrosion on dental alloys due to the effect of saliva and food. 

The formation of corrosion products, the solubility of corrosion products in the surface electrolyte, and the formation of passive films affect the overall rate of the anodic metal dissolution process and cause deviations from simple rate equations. Passive films distinguish themselves from corrosion products, in the sense that these films tend to be more tightly adherent, are of lower thickness, and provide a higher degree of protection from corrosive attack. Atmospheric corrosive attack on a surface protected by a passive film tends to be of a localized nature. Surface pitting and stress corrosion cracking in aluminum and stainless alloys are examples of such attack. 

Corrosion of materials is a direct chemical reaction of a metal with its environment or a flow of electricity in an electrochemical reaction in an aggressive medium such as natural media (atmospheric, water or underground) or process media. Local cells (short-circuited) electrochemical cells of the same active metal or between an active metallic surface and that of another more noble conducting material can give rise to corrosion. The following general reaction may be written as ... 

In 1993 epoxy- lysulphide based coatings were used to coat two 250 MW thermal power projects at Dhanu, about 100 km north of Bombay. The site is on the coast, less than 1 km from the sea and as such the nature of the local atmosphere is highly corrosive, with continuous sea breezes, and the soil is saturated with marine salt. The coating has shown satisfactory performance (Figure 10.7) (Morton International, undated c). This project was constructed under the control of Bombay Suburban Electricity Supply Co. Ltd (BSES). 

Due to the various industrial applications and economic importance of alutninium and its alloys, its protection against corrosion has attracted much attention (Aballe et al., 2001 Cheng et al., 2004 Hintze and CaUe, 2006). Most aluminium alloys have good corrosion resistance towards natural atmospheres and other environments, because alutninium alloy surfaces are covered with a natural oxide film of thickness about 5 nm (KUckic et al., 2000). However, in the presence of aggressive ions, like chloride, the protective layer can be locally destroyed and corrosive attack takes place (Kliskic et al., 2000). Yet, if correctly protected, applications of aluminium alloy may be more reliable and have long service life. 

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