Corrosion of Low-Carbon Steel in a Flow of Solutions of Mixture of Sulfuric and Phosphoric Acids Containing Iron(III) Salts
摘要
The corrosion of low-carbon steel in solutions of 1 M H2SO4 + 1 M H3PO4 containing Fe(III) salts is studied relative to similar solutions of 2 M H2SO4 and 2 M H3PO4. Steel corrodes in these systems by reacting with the acid solution and salts of Fe(III). Partial reactions of the anodic ionization of iron and the cathodic reduction of H+ and Fe(III) cations occur on steel. The first two are characterized by kinetic control; the last, by diffusion control. The accelerating effect Fe(III) cations have on the corrosion of steel in the studied environments is mainly due to the reduction of Fe(III). A rotating steel disk electrode is used to study the effect the nature of solution convection has on the kinetics of Fe(III) reduction in the considered corrosion systems. Coefficients of diffusion are determined for Fe(III) cations in solutions of H2SO4, H2SO4 + H3PO4, and H3PO4 via the cyclic voltammetry method with using a Pt electrode, allowing the construction of model dependences of the cathode current density of a steel disk on its frequency of rotation. The dependences are then compared to experimental data in order to establish the reasons for the observed differences. It is assumed that the differences are determined by the release of hydrogen gas on steel, which both shields the steel’s surface and changes the flow of the aggressive environment near the electrode from laminar to turbulent. Empirical dependences of the rate of steel corrosion on the intensity of the medium’s flow in the considered environments, obtained using data on the mass loss of metal samples, are described by linear equation k = kst + λw1/2, where kst is the rate of steel corrosion in a static environment, w is the speed of rotation of the propeller mixer that creates the flow of the medium, and λ is an empirical coefficient.