Atmospheric Corrosion of Steel

From the corrosion point of view, sulfur dioxide is the most critical pollutant. It originates primarily from the combustion of fossil fuels such as oil and coal in domestic heating systems and industrial power plants. 

The formula NO refers to a whole family of nitrogen oxides. They are formed in high-temperature combustion, particularly in automobile motors. In terms of corrosion, NOx plays a lesser role than either SO2 or chloride ions. 

Chlorides are found as air pollutants mostly in maritime regions, where they are carried by wind-bome water droplets (aerosols). The salting of roads in winter represents another source of chloride that is of particular importance for the corrosion of vehicles and highway bridges. 

Airborne particles can have both natural and man-made origins. Soot particles from domestic heating, power plants, waste incineration or diesel engines are the result of an incomplete combustion. They can adsorb sulfur species that make them particularly corrosive. Natural sources of airborne particles include volcanoes, trees, 

The corrosivity of an atmosphere depends mostly on three parameters  

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It confirms that the acceleration rate caused by chloride ions on atmospheric corrosion of steel and copper depends on the characteristics of rain regime. For a place having high amount and time of rain, a lower acceleration on corrosion rate should be expected for a given chloride deposition rate... 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

At exposure of steel in heavily polluted industrial atmosphere the corrosion rate on the upper side of steel panels exposed at 45° inclination was only 37 per cent of the total corrosion. In clean air, by contrast, the corrosion effect of rain was predominant and the upper sides of the test panels corroded faster than the undersides ( 6). The atmospheric corrosion of steel proceeds in local cells, where the sulphate nests acts as anodes. This may be the explanation why the washing effect of rain prevails in polluted atmospheres, as rain water may wash away sulphates from the nests. 

ATMOSPHERIC CORROSION OF STEELS 22.13.1 Chemistry of Anticorrosive Rust... 

Atmospheric corrosion of steel in the presence of chloride is summarized in Fig. 10.6 [32]. 

T. Kamimura, K. Kashima, K. Sugae, H. Miyuki, T. Kudo, The role of chloride ion on the atmospheric corrosion of steel and corrosion resistance of Sn-bearing steel, Corros. Sci. 62 (2012) 34—41. 

Figure 9.1. Atmospheric corrosion of steels as a function of time in an industrial environment (C. Larrabee, in Corrosion Handbook, H. H. Uhlig, editor, Wiley, New York, 1948, p. 124)...

Atmospheric corrosion of steels is very complex and is a function of interplay of various factors like electrolytes pH and potentiodynamic scan rates and so on. In order to emulate these factors to form protective rust a test plan is given in Table 4.10. 

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). 

Mansfield, F., and Tsai, S. (1979). Laboratory Studies of Atmospheric Corrosion of Steel and Zinc. Corrosion 79, Atlanta, NACE, Katy, TX, Paper 208, 11 pp. 

Reactions (8.29) and (8.30) are responsible for the phenomenon of acid rain in polluted areas, the pH of rainwater and of fog can reach relatively low values, typically between 4 and 5. At the surface of exposed steel, the pH remains higher (5 to 7), because the reaction (8.31) consumes sulfuric acid. This explains the fact that proton reduction (2 H" + 2e H2) is less important for the atmospheric corrosion of steel than oxygen reduction. 

The atmospheric corrosion of steel, commonly referred to as rusting of steel, can be represented in a simplified way by the following stoichiometric equation ... 

The term green rust designates a variety of intermediate reaction products, generally amorphous, that are found during the atmospheric corrosion of steel in the presence of chlorides and/or sulfates. More precisely, these products form during the transformation of Fe(OH)2 into y-FeOOH. Green rust consists of mixed ferric and... 

The presence of rust influences the rate of atmospheric corrosion of steel in two ways. One hand, a porous rust layer increases the surface area exposed to the atmosphere and thus the number of adsorption sites. More pollutant can adsorb onto the surface, and water condenses more easily. On the other hand, rust layers slow down the rate of corrosion by providing a barrier (although imperfect) between the reactive metal surface and the atmosphere and thus reduces access of oxygen. 

Figure 8.19 A schematic illustrating the mechanism of atmospheric corrosion of steel (a) shows the inital phase of the corrosion of a metallic surface covered by a thin water film (b) shows the reactions that take place on a wet surface in the presence of a rust layer and (c) shows the oxidized state of a dry rusted surface with precipitated iron sulfates.

The results of Figure 8.21 provide further evidence for the participation of two cathodic partial reactions in the atmospheric corrosion of steel. A thin sheet of rusted iron is periodically exposed to humidity in a closed ehamber eontaining oxygen and water vapor. Simultaneously, the rate of iron oxidation (using a magnetic method) and the rate of oxygen reduction (determined volumetrically) are measured. The sample is... 

In conclusion, the atmospheric corrosion of steel involves two distinct surface states, wet and dry, which differ in terms of anodic and cathodic partial reactions taking place. The average rate of corrosion depends above all on the action of humidity cycles and it is accelerated by the presence of sulfates and other anionic contaminants. By forming an electrolyte these permit the creation of corrosion cells between the cathodic and anodic sites in the porous rust that covers the surface. 

The main environmental factors that govern the corrosion rate of steel in the atmosphere are temperature, time-of-wetness, and type and amount of chemical contamination. Time-of-wetness includes not only time when the steel is wet from precipitation or dew, but also time above a certain critical humidity," which can vary from about 50-70 % relative humidity, depending upon the contaminants present in the air [3]. For most inland sites, the most important chemical contaminant affecting atmospheric corrosion of steel is sulfurdioxide. At coastal sites, and locations where de-icing salts are used, chloride content is most important. 

Included in this estimate was corrosion attributed to chemical processes to corrosion of highways and bridges from deicing chemicals to atmospheric corrosion of steel fences to atmospheric corrosion of various outdoor structures such as buildings, bridges, towers, automobiles, and ships and innumerable other applications exposed to the atmospheric environment. It has been further estimated that the cost of protection against atmospheric corrosion is approximately 50% of the total cost of all corrosion protection methods. 

It is logical to start with carbon steel because this is the most widely used structured material. Up to the last 20 years or so, atmospheric corrosion of steel was believed to be a process of general corrosion proceeding in cells with microscopic anodes and cathodes. More recent studies have determined that the corrosion process, which is electrochemical in nature, takes place in cells of microscopic dimensions with very distinct anodic and cathodic areas. 

Atmospheric corrosion of steel is a function of location. In country air the products of corrosion are either oxides or carbonates. In industrial atmosphere sulfuric acid is present, and near the ocean some salt is in the air. Corrosion is more rapid in industrial areas because of the presence of the acid, and it is higher both near cities and near the ocean because of the higher electrical conductivity of the rain and the tendency to form soluble chlorides or sulfates, which cause the removal of protective scale. 

In the theory of atmospheric corrosion of steel based on the acid regeneration cycle, sulfuric acid formed by oxidation of SO absorbed in the rust layer attacks the steel according to the overall equation ... 

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. 

Sulfur dioxide (SO2) is an important pollutant with a detrimental effect on the atmospheric corrosion of steel. Fig. 1-27. 

Figure 1-28. Atmospheric corrosion of steel under different environmental conditions (Barton 1973).

State the mechanism of atmospheric corrosion of steel in an industrial atmosphere containing SO2, C and chloride particles. 

Chloride contaminant influences the atmospheric corrosion of steel principally by... 

Which of the following methods cannot be used to combat atmospheric corrosion of steel ... 

What is the role played by temperature in the atmospheric corrosion of steel What is the effect of wind velocity, thermal capacity of the metal and its insulation properties on temperature ... 

Explain briefly the atmospheric corrosion of steel based on an electrochemical cycle. How is Fe(OH)2 converted to SO " and what is its effect on atmospheric corrosion ... 

Ericsson, R. (1978). The influence of sodium chloride on the atmospheric corrosion of steels. Werkstoffe und Korrosion, Germany Frankftnt, 29,400. 

Nishikata A, Yamashita Y, Isatayama H, Tsuru T, Usami A, Tanabe K and Mabuchi H (1995), An electrochemical impedance study on atmospheric corrosion of steels in a cyclic wet-dry condition , Corros. Sci. 37, 2059-2069. 

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