Effect of Pressure on Metal Corrosion Rate

Due to the unique properties of high subcritical and supercritical aqueous systems (SCAS), two corrosion mechanisms, i.e., electrochemical oxidation (EO) and chemical oxidation CO), have been postulated to describe the corrosion of metals and alloys in high temperature media,as outlined above. EO usually involves two or more coupled partial redox reactions at different sites on the corroding metal surfaces in relatively high-density SCAS. On the other hand, CO is postulated to occur through direct reaction of aggressive species with the metal in one act (but possibly in several elemental steps) on one site in low-density supercritical aqueous 

By taking the natural logarithm of Eq. (71) and dilferentiating with respect to pressure, the pressure dependence of the corrosion reaction rate in high temperature aqueous systems can be expressed as, 

As illustrated by Eq. (80), the pressure effects on corrosion reaction rate can be attributed to the impact of pressure on the activation process and on volume concentration of the aggressive species. The volumetric concentrations (mol/1 of the solution) of the aggressive species, Ch+, in Eq. (80) is density-dependent and can be expressed in terms of the molal (mol/kg of solvent) concentration by 

The pressure dependence of the corrosion reaction rate (Eq. 80) can be further modified by applying the above equations, such that 

As illustrated in Eq. (80), the effect of pressure on the rate of corrosion of metals in high temperature aqueous systems can be attributed to the activation process, to clmges in the degree of dissociation of aggressive species, and to the system isothermal compressibility. 

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