RESEARCH OF MECHANOCHEMICAL CORROSION OF STEELS 20 AND 09G2S

Abstract

Background During operation, many metal structures are under conditions of joint exposure to aggressive environments (natural and / or technological) and mechanical loads. Moreover, the damage caused by combined action of corrosion and stress is often more significant than by a simple «application» of damage caused by mechanical stress and the influence of an aggressive environment acting separately. Carrying out destructive tensile testing of equipment materials, there is a possibility of inaccurate results on the determination of the mechanical characteristics of this material in connection with the actual operation of the material in an aggressive environment. This, in turn, can lead to erroneous calculations of the tensile strength and resource of the equipment. In this regard, the study of the behavior of materials under the influence of various mechanical loads in aggressive environments, as well as the determination of the limiting state of a metal by measuring its electrode potential, are of particular relevance. Aims and Objectives Determination of the influence of a corrosive medium on the change in the mechanical properties of structural steels 09G2S and 20 using the tensile test as an example. Identification of the dependence of changes in the electrode potential of 09G2S and 20 steels on mechanical loads of various magnitudes. Methods For testing, the most common structural steels used at oil refineries and petrochemical plants - steel 20 and 09G2S - were selected. Tensile tests were carried out in accordance with State Standard 1497-84 «Metals. Tensile test methods». The aggressive environment was selected according to State Standard R 9.905-2007 (ISO 7384: 2001, ISO 11845: 1995) «Unified system of protection against corrosion and aging. Corrosion test methods. General requirements». Tensile tests of the samples were carried out on a tensile testing machine IR 5113-100 in accordance with the Equipment Operation Manual and using a corrosion-proof cell made of organic glass. Aggressive experimental medium was a 3 % aqueous solution of NaCl. Results As a result of tests, it was found that the mechanical properties of low-alloy steel 09G2S and carbon steel 20 deteriorate when exposed to an aggressive environment. So, for steel 20 samples, the tensile strength, on average, decreased by 8.8 %, and the time to rupture - by 4.24 %. For specimens made of 09G2S steel, the decrease in tensile strength in a corrosive medium was 23.1 %, and the time to rupture was reduced by 4.3 %. Measurement of electrode potentials during tensile tests showed that stress state can be predicted from changes in metal potential values.