The review summarizes and analyzes the current state of research in the field of corrosion of metals in acid solutions and their inhibitory protection. The most important concepts about the metal corrosion mechanism in acidic media were considered. The existing experimental approaches to the study of metal corrosion in acid solutions and the effect of organic corrosion inhibitors on this process were discussed. It was shown that electrochemical and physicochemical methods play an important role in studies of the state of the metal surface. The mechanisms of metal corrosion inhibition in acid media were analyzed. The thermodynamic and kinetic aspects of adsorption of inhibitors on metals were considered. The maximum efficiency in metal protection is shown by organic compounds whose molecules are capable of chemisorption interaction with the metal surface, forming polymolecular protective layers of molecules chemically bonded with one another.

The treatment of low-carbon steel items with hydrochloric acid solutions is used in many industrial technologies. This process is accompanied not only by metal corrosion losses, but also by hydrogen absorption by the metal. In this study, the kinetics of hydrogen cathodic reduction on low-carbon steel in 2 M HCl containing corrosion inhibitors, namely, quaternary ammonium salts and a 3-substituted 1,2,4-triazole, have been studied. Adsorption isotherms of corrosion inhibitors on cathodically polarized steel surface have been obtained. XPS data provide valuable information on the composition and structure of protective layers formed on steel in HCl solutions containing inhibitors. The main rate constants of the stages of gaseous hydrogen evolution and incorporation of hydrogen atoms into the metal have been determined. The addition of quaternary ammonium salts or 3-substituted 1,2,4-triazole inhibits the cathodic reduction of hydrogen and its penetration into steel in the HCl solution. 3-substituted 1,2,4-triazole is the most efficient inhibitor of hydrogen absorption. The inhibitory effect of this compound is caused by a decrease in the ratio of the hydrogen concentration in the metal phase to the degree of surface coverage with hydrogen. The maximum decrease in hydrogen concentration in the metal bulk in the presence of the 3-substituted 1,2,4-triazole is 8.2-fold, which determines the preservation of the plastic properties of steel as it corrodes in HCl solutions. The high efficiency of the 3-substituted 1,2,4-triazole as an inhibitor of hydrogen cathodic reduction and absorption results from strong (chemical) adsorption of this compound on the steel surface and the formation of a polymolecular protective layer.

Etching of steel items in sulfuric acid solution is used in various human activities (oil and gas industry, metal production, utilities, transport, etc.). This operation is associated with significant material costs due to corrosion losses of the metal. It has been found that an efficient way to prevent corrosion of steel in sulfuric acid solution involves the formation of thin protective films consisting of corrosion inhibitor molecules of triazole class on its surface. It has been shown that the protection of steels with a 3-substituted 1,2,4-triazole (3ST) in H₂SO₄ solution is accompanied by the formation of a polymolecular layer up to 4 nm thick. The 3ST layer immediately adjacent to the steel surface is chemisorbed on it. The efficiency of this compound as an inhibitor of corrosion and hydrogen absorption by steel is determined by its ability to form a protective organic layer, as experimentally confirmed by XPS and AFM data. The kinetic constants of the main stages of hydrogen evolution and permeation into steel in the H₂SO₄ solution were determined. A significant decrease in both the reaction rate of cathodic hydrogen evolution and the rate of hydrogen permeation into steel by the triazole in question was noted. It has been shown that the preservation of the metal plasticity in the acid medium containing the triazole under study is due to a decrease in the hydrogen concentration in the metal bulk.

Corrosion of low-carbon steel corrosion in 2.0 M H₂SO₄ and 2.0 M H₃PO₄ containing Fe(III) salts is studied using the mass loss of metal specimens and voltammetric measurements on a rotating disk electrode. It is established that during steel corrosion in mineral acid solutions containing Fe(III) salts, the anodic ionization of metallic iron proceeds in the kinetic region. The cathodic reaction combines the parallel and independent processes of the kinetically controlled hydrogen evolution and the diffusion-controlled reduction of Fe(III) cations to Fe(II). The potentiometry and cyclic voltammetry on a platinum electrode in acid solutions containing Fe(III) salts shows that Fe(III) cations in these media are bound into complexes with acid anions. The stronger the complexes formed by Fe(III) cations, the lower their oxidation potential and diffusion coefficient. The drop in the diffusion coefficient of Fe(III) cations in mineral acid solutions affects the rate of their diffusion-controlled reduction on steel. Introducing FePO₄ into a H₃PO₄ solution accelerates corrosion of low-carbon steel less than an equimolar Fe₂(SO₄)₃ additive in a H₂SO₄ solution. The effect is a result of the lower diffusion coefficient of Fe(III) cations in a H₃PO₄ solution, relative to a H₂SO₄ solution.

Corrosion of low-carbon steel St3 in 2 M H₂SO₄ containing Fe₂(SO₄)₃ was studied by measuring mass loss of the metal samples and using voltammetric measurements on a rotating disk electrode. Corrosion of steel in this medium involves anodic ionization of steel proceeding in the kinetic region and two cathodic partial reactions: hydrogen evolution and reduction of Fe(III) to Fe(II) cations characterized by kinetic and diffusion control, respectively. The potentiometric data measured on a platinum electrode in 2 M H₂SO₄ containing Fe₂(SO₄)₃ and FeSO₄ made it possible to establish that Fe(III) cations are bound in complexes with sulfate anions in these media. The oxidizing ability of a corrosive medium, characterized by the Fe(III)/Fe(II) redox pair potential, depends on the Fe(III) content in it and is well described by the Nernst equation. The diffusion coefficients of Fe(III) cations were determined by cyclic voltammetry on a platinum electrode in 2 M H₂SO₄ in the presence of Fe(III) and by evaluating the response of the cathodic current of a steel electrode to the convective factor in the same medium. The apparent diffusion coefficient of Fe(III) cations measured on a steel electrode in inhibited media may be significantly lower than that in uninhibited media because the penetration of Fe(III) cation through the protective layer of inhibitor to the metal surface is hindered. The weak inhibition of corrosion of low-carbon steel in H₂SO₄ solutions containing Fe₂(SO₄)₃ by the composite inhibitors under study is the result of the accelerating action of Fe(III) cations on three partial electrode reactions on the metal surface: reduction of Fe(III) and H⁺ characterized by diffusion and kinetic control and anodic ionization of iron.