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The thorough mechanical, corrosion-mechanical, hydrogen permeation studies of X52 steel in as-received condition and after 30 years of service in gas pipeline have shown the degradation of the bulk metal properties due to the exploitation. The structure degradation of exploited steels decreased the resistance to crack propagation, to stress corrosion cracking (SCC), HE and HIBC. The exploited steel exhibited specific anomalies of mechanical behaviour: (1) decrease in resistance to brittle fracture with simultaneous decrease in hardness; and (2) the opposite change of the plasticity parameters: decrease in the RA accompanied by the increase in elongation. Deformation ageing and formation of micro defects, assisted by entering and accumulated hydrogen evolved in corrosion processes which takes place in brine deposited inside the pipes, have been proposed to be responsible for the metal degradation and specific modification of the properties during the long-term service of pipes. The extent of the steel degradation not being able to be detected by the standardized mechanical properties and routine microstructure observation can be efficiently determined by the specially modified methods and conditions. The information may be used in assessment of the conditions and of the life-time of operated and newly constructed gas pipelines.