Iron oxide and oxyhydroxide phases formed on steel surfaces and their dissolution in acidic media. Review

Literature data on the formation of iron oxides and oxyhydroxides phases on steels and the features of their removal from the metal surface by acid solutions are summarized. In the course of thermal and thermomechanical treatment of steels, a dense scale layer with a complex structure is formed on their surfaces, which includes FeO, Fe3O4 and Fe2O3. During corrosion of steels in a humid atmosphere and in aqueous media, rust is formed on their surfaces, which is heterogeneous in structure and consists of FeOOH, Fe3O4 and Fe2O3. Cleaning of steel from scale and rust is carried out with solutions of various acids. These processes encounter problems with fast and high-quality removal of thermal scale. The kinetic laws of dissolution of such scale and individual phases of iron oxides in acid solutions are considered. Physicochemical methods for determining the composition and structure of scale and rust and experimental approaches to the study of the processes of dissolution of these substances in acid solutions are discussed. Important information about the removal of scale from steels is provided by electrochemical methods (potentiometry, electrochemical impedance spectroscopy, coulometry, voltammetry). Acid solutions with addition of corrosion inhibitors (CIs) are used to minimize the corrosion losses of steels during acid pickling. In this regard, an important requirement for such CIs is the absence of a retarding effect on the removal of the surface iron oxide and oxyhydroxide phases. At the same time, CIs should effectively slow down the corrosion of steel covered with scale or rust. The bibliography includes 154 references.