Estimation of Air Oxidation Rate of Fe2+ in Concentrated Chloride Solution

摘要

Atmospheric corrosion of steel typically proceeds during wet and dry cycles, especially during the drying. During the drying, the electrolyte becomes thinner, and the corrosion products (rust) forms. The interface environment between the surface and rust strongly affects corrosion rate. The chloride ion, which is one of the major factors of corrosion, concentrates at the interface due to drying and electrophoresis. However, it is difficult to measure the interface environment (i.e. ion concentration) directly, because of the limitation of electrolyte volume and area. In order to clarify the interface environment, we have applied a numerical chemical reaction model. The chemical reactions such as hydration or oxidation occur complexly in minute area. Dissolved Fe ion (Fe~(2+)) during corrosion is oxidized to Fe~(3+)-species, and the hydration of Fe~(3+)-species determine pH. Since the air oxidation is slower than the other reactions, it controls total reaction rate and pH of the aqueous solution. We have studied the role of highly concentrated chloride ion at local anode sites. The variation of pH was measured using the minute anode chamber, where anion selective membrane was set to the bottom in order to simulate the ion selective permeability of rust layer. The pH change measured in the case of high chloride concentration was faster than that of low chloride concentration. The result suggests that the chloride ion accelerates the air oxidation. To determine the reaction rate formula of the air oxidation of Fe~(2+), the amount of Fe~(3+)-species was calculated from the pH using a model based on the chemical equilibrium in the concentrated solution. The reaction rate formula was calculated from the variation of Fe~(3+)-species. The formula was expressed by following two terms without chloride ion activity. One is the reaction rate in neutral solution. The other is in acid solution. Proposed formula demonstrates that chloride ion affects the activity coefficient of the ion, and consequently accelerates the air oxidation.