Effect of iron(III) salts on steel corrosion in acid solutions. A review

Data on the specific features of the corrosion of steels in acid solutions containing Fe(III) salts are summarized.The current state of studies in the field of their protection in these media using corrosion inhibitors (CI) is analyzed.The main routes by which Fe(III) salts enter acid solutions are considered.Most often, they enter a corrosive environment upon dissolution of oxide scale in the acids.The corrosion of low-carbon steels in acid solutions containing Fe(III) salts results from three partial reactions: anodic ionization of iron; cathodic reduction of protons and Fe(III) cations.The first two reactions occur under kinetic control, and the last one, under diffusion control.The presence of Fe(III) salts in acid solutions often significantly accelerates the corrosion of steels due to predominant hindrance of the cathodic reaction by Fe(III) cations.As a rule, Fe(III) cations significantly reduce the protective effect of CIs of steels added to acid etching solutions.The protective effect of CIs in acid solutions containing Fe(III) salts can be increased by reducing the overall corrosivity of the medium.This can be achieved by addition of H3PO4 that can bind Fe(III) into strong complexes that have lower oxidizing capacity than Fe(III) aqua complexes and mobility in the corrosive environment characterized by the diffusion coefficient.A decrease in the mobility of Fe(III) cations is extremely important for slowing down their reduction, which occurs in the diffusion mode, by the CI.A CI used in a corrosive environment should be efficient not only in slowing down the cathodic reduction of protons and anodic ionization of iron, but should also significantly suppress the reduction of Fe(III) cations that occurs under diffusion control.These requirements are satisfied by H2SO4 + H3PO4 and HCl + H3PO4 solutions containing Fe(III) salts and inhibited by a three-component formulation of a substituted 1,2,4-triazole (IFKhAN-92 corrosion inhibitor) + KNCS + urotropine.It is shown that the efficiency of this formulation as a CI of steels is due to the formation of a polymolecular protective surface layer consisting of IFKhAN-92 molecules, Fe(II) cations and thiocyanate anions, which inhibits the reduction of Fe(III), and due to an additional decrease in the oxidative ability of Fe(III) cations by urotropine.The bibliography includes 122 references.