Voloshyn V. Regularities of corrosion-mechanical fracture of steels of pipeline systems under static, cyclic and cavitation loads

An actual scientific and technical task has been resolved in this work devoted to establishment of the regularities of influence of chemical composition of aqueous environments and type of mechanical loading on the resistance of steels of pipeline systems and their welded joints to corrosion-mechanical fracture and development of methods for predicting their corrosion fatigue durability, as well as application of inhibitor protection during cavitation in aqueous environments. The object of the study is corrosion-mechanical fracture of carbon and low-alloyed steels in the as-received state and after long-term operation. The purpose of the research was to determine the electrochemical features and regularities of corrosion-mechanical fracture of steels of pipeline systems and welded joints, depending on chemical composition of aqueous environments and type of operating loading. Research methods: electrochemical methods (polarization method, disk rotating electrode method); gravimetric method; impedance method; determination of microhardness, resistance to stress corrosion cracking, localized corrosion, cavitation corrosion, fatigue and corrosion-fatigue crack growth; metallographic analysis; methods of mathematical statistics. The main factors reducing corrosion fatigue durability of the carbon steel in tap water under cavitation, cyclic loading and their combined action is established. For the first time, the effectiveness of inhibitor protection of the low-alloyed steel against cavitation-corrosion damage in tap water using triethylammonium iodide is revealed. It has been revealed that the main mechanism of corrosion-mechanical fracture of low-alloyed steels in bicarbonate aqueous solutions is hydrogen embrittlement caused by additional reduction of bicarbonate ions with hydrogen evolution, and not local anodic dissolution of metal. An electrochemical express method for predicting corrosion-fatigue durability of ferritic-pearlite low-alloyed pipeline steels in bicarbonate aqueous environments depending on their composition is developed. It is revealed that the threshold stress intensity factor for corrosion fatigue crack growth in the welded joint of low-alloyed pipeline steel in NS4 solution, which simulates the soil environment, is most intensively reduced by a combination of a number of factors: long-term operation, high loading-cycle asymmetry and cathodic protection potential. The obtained results and developed methodologies were used in Affiliate “Prykarpattransgaz” and LLC “Porttehekspert” (Odesa).