STUDY OF THE EFFECT OF 20L STEEL MICROSTRUCTURE ON THE RESISTANCE TO SULFIDE CORROSION CRACKING IN HYDROGEN- SULFIDE MEDIUM K2

The paper presents the results of evaluating the effect of 20L steel structure on the resistance to sulfide corrosion cracking in mediums with different concentrations of hydrogen sulfide and pH of the medium. During the research, two heat treatment modes were carried out for 20L grade steel, namely temper hardening (the first mode) and normalization (the second mode), and solutions were selected to study the resistance of 20L grade steel to sulfide corrosion cracking. The microstructural analysis showed a significant difference in the phase composition of 20L steel material depending on the heat treatment mode. The study also revealed that all samples subjected to tempering have no resistance to sulfide corrosion cracking and are destroyed before the end of testing, even in the medium with an average content of hydrogen sulfide and a partial pressure in the range from 10,000 to 1,000,000 Pa (K2 according to the technical specifications of PJSC NK Rosneft). At the same time, the samples subjected to normalization showed mixed results, as some of the samples successfully withstood the entire test (720 hours), and some of the samples destroyed before the test was completed. A pattern is also found out that an increase in the concentration of hydrogen sulfide significantly accelerates the process of destruction of 20L steel under the same loads. On samples that destroyed before the end of the test, a study of the microstructure in the zone of destruction showed the presence of sores and microcracks (18 – 120 microns), which may indicate the course of hydrogen cracking. The results of micro-X-ray spectral analysis showed that the corrosion products in the fracture zone are iron sulfides of various compositions. When comparing the structures of the destroyed samples and the samples that survived the tests, it was found out that the destruction occurred on samples in which the size of the ferritic grain is larger, which is most likely the cause of the destruction, since all the cracks are located across the ferritic grain.