Features of corrosion and inhibitory protection of steels in acid solutions containing iron(III) salts. Review

Abstract

A critical analysis of literature data on corrosion of steels in solutions of mineral acids containing Fe(III) salts and the possibility of inhibitory protection of steels in these environments has been carried out. The accumulation of Fe(III) salts in acid solutions during their operation at metallurgical, oil and gas enterprises, objects of housing and communal services is a result of dissolution of mineral deposits, scale, thermal scale, and rust containing Fe (III) compounds in them. In addition, additional oxidation of Fe(II) salts in acid solutions to Fe(III) compounds is possible. The effect of the redox potential, determined by the ratio of the content of Fe(III) and Fe(II) cations in them, as well as mobility of Fe(III) cations, on the corrosivity of the systems under consideration was examined. The reasons for the accelerating and retarding action of Fe(III) salts on corrosion of steels in acidic media were analyzed. Modern concepts of the mechanism of corrosion of steels in these environments are discussed. The low efficiency of the inhibitory protection of steels in mineral acids containing Fe(III) salts is demonstrated. It is noted that it is possible to increase the protective effect of some corrosion inhibitors (CI) in these environments by reducing their overall aggressiveness. It can be achieved by introducing H3PO4 into them, which is capable of binding Fe(III) into strong complexes that have a lower oxidizing capacity than Fe(III) aqua complexes and mobility in an aggressive environment. The CI used in an aggressive environment must be effective in slowing down not only the cathodic reduction of protons and anodic ionization of iron, but also significantly suppress reduction of Fe(III) cations, which is realized with the diffusion control. These requirements are met by composite CI based on triazoles, which in H2SO4 + H3PO4 and HCl + H3PO4 solutions containing Fe(III) salts are effective in slowing down the corrosion of low-carbon steel.