Corrosion Failure And Research Of 18-8 Series Stainless Steel Pipeline And Flange

During petrochemical production,being in contact with highly corrosive medium, high temperature and high pressure processing conditions; the corrosion of petrochemical equipment is particularly serious,especially for the the damage of stress corrosion cracking. This paper clearly analyzes and explains about the failure and Research of the corrosion of 18-8 series stainless steel under the actual operating conditions or environments using the methods of visual observation,microstructural images via scanning electron microscopy(SEM),chemical composition study of corrosion products through Energy Spectrum analyses and mechanical properties' assessments. The overall outcome of this study is the design of methods and strategies for corrosion reductions in real operating conditions.Experiment and analysis results are presented as following :1. The circulating fluid in PTU Purge Treating Unit pipeline contains chloride(Cl-) ions and the high concentration of this ion causes corrosion related failure in the material. Owing to the higher concentration of Cl- ions in the welding line of pipeline,flow-assisted corrosion is found in that spot. The associated failure phenomena induced by it in the material are damage of the local section,stress concentration,stress corrosion cracking and subsequently the cracking leakage.2. The measure for failure prevention of PTU Purge Treating Unit pipeline is to conduct work out for reduction of concentration of Cl- in the medium corrosion products. Similarly, the cleanliness of the inner surface of the welded pipe should be maintained, blister generation at the weld line should be controlled and measures preventing the occurrence of eddy flow must be adopted for the system. In order to prevent the failure of PTU pipeline,mainly to reduce the concentration of medium corrosion products,namely concentration of Cl-. At the same time,in the welding line,the bright and cleanliness degree of the welded pipe inner surface should paid attention to,avoiding big flash and preventing or reducing the emergence of the disturbed flow.3. The material of flange in the entrance of entry line of residual reactor is austenitic stainless steel which generates brittle phase and reduces the material ductility and toughness under long-term high temperature and high pressure. At the time of starting and ceasing, the cooling rate is too fast thus rendering mismatch between the deformation rates of flange and bolt and this generates stress in these elements. During numerous maintenances work consisting of removal and re-installation of the flange,it was artificially tightened by sledgehammer which would result in the highly non-uniform distribution of force in the flange material. The outer part of flange contracts with rain water cooling,leading to increase of flange sealing surface stress. Under the action of these factors, the bottom of flange seal tank produce a high stress concentration. In absence of thermal insulation,the inside and outside temperature difference is finite for the flange trapezoidal groove,and this ultimately leads to stress corrosion cracking. Similarly, with no thermal insulation,the inside and outside temperatures of the short-interact flange trapezoidal groove are finitely different,which ultimately lead to stress corrosion cracking.4. Through the detailed stress analysis about the flange at the start point of entry line in residual reactor,under the working conditions,the flange stress is close to the allowable limit. In flanges with groove,stress concentration is more serious,which would be beyond the allowable stress of material and cause damage. In order to re-manufacture,the flange size should be re-designed.