Influence of internal stresses on the intensity of corrosion processes in structural steel

Introduction. The behavior of metal in a corrosive environment can be ambiguous, which is due to the peculiarities of the corrosion process. Both external and internal factors influence the corrosion process. External factors are determined by temperature, humidity, type of corrosive medium, etc., while internal factors depend on the parameters of the system (material): the presence of inclusions, phase composition, structure, and the magnitude of internal residual stresses. Internal factors ambiguously affect the behavior of the material in a certain aggressive medium, which ultimately affects the time of corrosion damage of the material and, as a consequence, the time of operation of objects made of this material. Therefore, differentiation of the influence of various internal factors on the rate of corrosion process in an aggressive environment is a priority area of research. The purpose of the present work is to consider the influence of the magnitude of internal residual stresses on the rate of corrosion process in an aggressive medium — 5% sulfuric acid solution. The object of research conducted in the work is sheet rolled steel St3 as received after different magnitude of plastic deformation, from which the specimens under study were made. The methods of investigation: microstructural study of deformed specimens was carried out on optical microscope Olympus GX53; software SIAMS 800 was used to compare the structure of the obtained material with the atlas of microstructures, determine the score of grain structure, determine the anisotropy of the structure after deformation of the material; X-ray diffractometer DRON-7 was used to register diffraction patterns and determine internal stresses; laboratory scales SHIMADZU UW620h was used to measure the mass of the specimens under study; tensile strength of the material’s specimens was measured. Results and Discussion. The obtained results show that the plastic deformation of the material in the rolling direction has an ambiguous effect on the structure anisotropy. When the degree of plastic deformation increases, there is an ambiguous change in the grain anisotropy value, which is associated with the internal effects of the processes occurring in the material structure during plastic deformation, such as: sliding of the crystal lattice in the {111} <110> directions; the occurrence of reverse residual internal stresses due to the presence of inclusions in the steel structure. However, the degree of plastic deformation correlates quite well with the magnitude of internal residual stresses. The increase in the magnitude of internal residual stresses leads to an increase in the corrosion rate of structural steel St3 in 5 % hydrochloric acid solution. The obtained dependence is described by a linear equation with a high coefficient of determination, which indicates that there is a strong relationship between the magnitude of internal residual stresses and the rate of corrosion of the material. At the same time, the coefficient of influence of internal stresses on the corrosion rate is equal to 0.72, which additionally proves the existence of interrelation between the considered parameters.