Role of the Convective Factor in the Corrosion of Low-Carbon Steel in Solutions of Sulfuric Acid Containing Iron(III) Sulfate

The authors study the corrosion of low-carbon steel in solutions of H2SO4 containing Fe-2(SO4)(3) with and without additives of individual and mixed corrosion inhibitors. It is established that the oxidizing capacity of the given system (in which the reactions between iron, an acid solution, and Fe(III) salts are thermodynamically allowed), characterized by the redox potential of an Fe(III)/Fe(II) pair, is largely determined by its anionic composition: sulfate anions of a corrosive medium bind Fe(III) cations into complex compounds, reducing their oxidizing ability. Partial reactions of the anodic ionization of iron and the cathodic reduction of H+ and Fe(III) cations are revealed in analyzing the effect convection of the medium has on the electrode reactions of low-carbon steel. The first two reactions are characterized by kinetic control; the third, by diffusion. It is shown that the accelerating effect Fe-2(SO4)(3) has on the corrosion of steel in a solution with H2SO4 is mainly due to the reduction of Fe(III). In contrast, the accelerating action of Fe(III) cations affects all partial reactions of steel in an inhibited acid. There is a large drop in the apparent coefficient of diffusion of Fe(III) cations (D-Fe(III)) in inhibited solutions, relative to an uninhibited medium. Data on the corrosion of low-carbon steel in the given media, obtained from the mass loss of metal samples, are in full agreement with results from studying partial electrode reactions. Consideration is given to the accelerating effect Fe-2(SO4)(3) has on the corrosion of steel in solutions of H2SO4 with and without inhibitors. In these environments, the corrosion of steel is determined by the convective factor, which is typical of processes with diffusion control. The empirical dependence of the rate of steel corrosion on the intensity of the medium's flow is described by linear dependence k = k(st) + lambda w(1/2), where k(st) is the rate of the corrosion of steel in a static environment, w is the rotational speed of the propeller agitator creating the flow of the medium, and lambda is an empirical coefficient.