OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 cases the equipment works with aggressive media that accelerate the corrosion process, the presence of internal stresses affecting this process becomes a significant factor. However, it should not be forgotten that various mechanisms [8–11] related to the presence of inclusions, the magnitude of internal stresses, the dispersion of the material, etc. take part in the process of corrosion damage. The influence of these mechanisms on the corrosion process is ambiguous, that is why it is necessary to clearly differentiate the effect of second-order stresses on corrosion processes. There are thermal methods for treating products to reduce internal stresses such as: annealing, tempering and cold working [7]. The use of thermal methods can reduce the strength of the material or even lead to increased corrosion susceptibility. Mechanical methods can also be used to reduce internal residual stresses. The most widespread method is based on material stretching at room temperature. The essence of the method is the plastic deformation of the material not exceeding 0.5–2% [4]. It should be clarified that plastic deformation is understood as the change in geometric dimensions remaining after the removal of the load [5]. The decrease in the magnitude of internal stresses during this kind of plastic deformation is associated with a slight distortion of the metal crystal lattice under the action of tangential stresses, resulting in irreversible displacement of atoms. After removal of external tensile stresses, the elastic component of deformation is eliminated [17, 18]. A small part of the strain remains, and the material is almost completely free from residual stresses [6]. Plastic deformation occurs due to slip and twinning processes, resulting in an increase in the number of linear defects in the form of dislocations [3, 7]. The literature review conducted shows that the influence of the residual stress state of the material on the corrosion rate is not fully studied [1–3]. Literature sources mainly consider the process of electrochemical corrosion of metal depending on the magnitude of tensile stress applied to the object [3], but there is no data reflecting the initial state of the material and its influence on the rate of corrosion process. Based on the above, this paper examines the effect exerted by the plastic deformation of the material on the corrosion rate of low alloy carbon steel St3. Research methodology The results given in this paper are obtained on specimens made of St3 steel sheets as received. This steel is widely used to manufacture various steel structures, pipes and equipment. The specimens 4×70×25 mm in size were cut across the rolling direction. Determination of internal stresses was carried out on X-ray diffractometer DRON-7, according to the method of S.S. Gorelik [3]. The method is based on the comparison of data obtained on the specimens under study with the data obtained on the reference specimens, which is an annealed material with the minimum magnitude of internal residual stresses. Corrosion tests of the specimens were carried out in laboratory conditions for 72 hours at temperature 20 °C. A 5 % hydrochloric acid solution was used as an aggressive medium. The container with the specimens under study and aggressive medium was placed in the thermostat; there was no direct contact between the specimens under study. The mass of specimens was determined using laboratory scales SHIMADZU UW620h as an average value of three measurements. Geometric dimensions of the specimens were determined using a caliper. Corrosion tests were carried out according to the method [6]. The criterion for assessing the corrosive effect was the corrosion rate, which was calculated using the formula: , m v St D = (1) where Dm is a relative weight loss (g); S is a surface area of the specimen in contact with an aggressive medium (m2); T is a time of contact of the specimen with an aggressive medium (days). The specimens were stretched using a universal testing machine I1185M (100 kN). The measurement accuracy was not more than ±1 %.
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