Aqueous layer atmospheric corrosion conditions

Under most atmospheric corrosion conditions, the anode reaction rather than the cathode reaction is observed to be the rate-limiting step [2]. Upon evaporation of the aqueous layer, a film of corrosion products—consisting of metal hydroxides or metal oxyhydroxides—may precipitate. With repeated condensation-evaporation cycles, this film usually hinders the transport of ions through the corrosion product or the transport of Me from the anodic site. Hence, the anodic reaction rate is lowered and, thereby, the atmospheric corrosion rate. 

When the aqueous layer is thin enough (less than 10 pm or so, see below) to permit ample access of oxygen to the metal surface, the anode reaction rather than the cathode reaction is rate limiting. This is the most common situation in atmospheric corrosion [5]. However, the surface and exposure conditions alter over a dry-wet-dry cycle or with extended exposure time and may eventually reach a situation in which the cathode reaction becomes the rate-limiting part (Sect. 3.1.2.3). 

In outdoor exposure conditions subject to wet-dry cycles, the actual concentration of most corrosion-stimulating gases under many conditions is not at equilibrium between the gas in the atmosphere and the same gas in the aqueous layer. Even so, thermodynamic considerations have been used for predicting the formation of different corrosion end-products and their stability. Figure 3.1 is a schematic illustration of processes occurring at the aqueous layer. 

Atomospheric corrosion is the result of interaction between a material—mostly a metal—and its atmospheric environment. When exposed to atmospheres at room temperature with virtually no humidity present, most metals spontaneously form a solid oxide film. If the oxide is stable, the growth rate ceases and the oxide reaches a maximum thickness of 1 to 5 nm (1 nm = 1(T m). Atmospheric corrosion frequently occurs in the presence of a thin aqueous layer that forms on the oxidized metal under ambient exposure conditions the layer may vary from monomolecular thickness to clearly visible water films. Hence, atmospheric corrosion can be said to fall into two categories damp atmospheric corrosion, which requires the presence of water vapor and traces of pollutants, and wet atmospheric corrosion, which requires rain or other forms of bulk water together with pollutants [3]. 

The atmospheric corrosion of Mg alloys is a complex process which results from the interaction between a metal and its atmospheric environment. A prerequisite for atmospheric corrosion is the presence of a water layer on the surface. The thickness of the water layer varies considerably with the climatic conditions and may range from monomolecular thickness to clearly visible water hlms. The formation of an aqueous layer occurs in humid air by adsorption on the hydroxylated oxide present on most metal surfaces exposed to ambient conditions. The thickness of the reversible adsorbed water him varies with the relative humidity (RH). Table 7.1 shows the approximate number of water monolayers on a metal surface at 25 and steady state conditions (1). Thicker aqueous hlms can also form in the atmospheric environment by condensation, precipitation or water absorption by hygroscopic substances on the surface. 

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. 

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