The rate of formation of iron carbonate on the metal surface depends on the precipitation rate of Fe2+ and CO32-, which is governed by precipitation kinetics. When an iron carbonate layer forms on the surface it acts as a diffusion barrier and prevents the metal from further corrosion. Precipitation of iron carbonate occurs when the concentrations of Fe2+ and CO32- exceeds the solubility limit, or in other words the degree of super saturation has to exceed unity to achieve precipitation. For iron carbonate, it is given as
Super saturation = CFe2++ CCO32-/ Ksp ----------------------------- (4)
where, Ksp is the solubility limit which allows for the determination of the activity of ions in solution using the equilibrium reaction equation. Therefore, increasing the concentration of Fe2+ ions in solution either from external source or from anodic reaction helps to achieve a protective film by increasing super saturation. The rate of precipitation can be varied by changing the temperature of the bulk solution as explained earlier and also by changing the degree of super saturation.
See Also:
Effect of hydrodynamics on Carbon Dioxide CO2 corrosion on carbon steel pipe lines
Effect of CO2 partial pressure on CO2 corrosion on carbon steel pipe lines
Effect of Fe2+ concentration on Carbon Dioxide CO2 corrosion on carbon steel pipe lines
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