Study On Pitting Behavior Of Domestic Ultra - Pure Ferritic Stainless Steel In Cl ~ - Environment

Based on the fact that the stress corrosion on oil casing steel is serious in the development environment of acidic oil and gas filed with high concentration of H2S and Cl-and the theory, also the engineering status quo, that pitting corrosion mainly causes the stress corrosion, the two kind of homemade ultrapure ferrite stainless steel Haiku-1and Haiku-2with better mechanical properties and corrosion resistance are studied in order to replace the G-3nickel base alloy that is currently used. With the use of electrochemical workstation, SSRT, SEM, EDS, XRD and other modern means of physical and chemical testing, the pitting behavior of the two steel in Cl-environment was studied and the pitting mechanism, pitting susceptibility factors, the impact of pitting on the susceptibility to stress corrosion were discussed theoretically to provide theoretical guidance for the homemade ultrapure ferrite stainless steel in security service in the oil and gas industry.For chemical immersion testing:SEM, EDS and XRD analysis were conducted to study the samples after different soaking time in order to determine the growth of pitting in the two materials including the incubation period, the development and growth of the period. The test results after further discussion showed that:in the pitting incubation period, due to the enrichment of Cl-, depleted Cr zone may occur and the uneven, easily dissolved passive film was formed, resulting in the activation of a new center at the edge of the pits and a new nucleation which would accelerate the shedding process of the iron base. This was the major factor for high corrosion rate in the early corrosion period. In the pitting development period, the formation of uneven passive was because of promotion of the self-acidification catalytic effect for the development of pitting. In the pitting growth period, a uniform and stable passive film was formed on the pits edge and the corrosion had reached a balanced state.Based on knowledge of the pitting mechanism of the two materials, this paper studied the pitting susceptibility factors through electrochemical techniques. Polarization curves, cyclic polarization curves and Mott-Schottky curves were tested at different Cl" concentration and different temperatures. The results showed that:the pitting corrosions resistance decreased as the Cl-concentration increased and in the same environment the passive film self repair ability of Haiku-1was better than that of Haiku-2, however the pitting corrosion resistance of Haiku-2was better that that of Haiku-1. As the temperature increased, the pitting corrosion resistance of the steel was also reduced. And below40℃, the pitting corrosion resistance of Haiku-2was quite the same as that of Haiku-1, it can replace the Ni contained Haiku-1in pitting corrosion-prone environment. In addition, electrochemical corrosion tests were conducted in a simulated environment of Luojiazhai, the results showed that the two materials in the environment could form a more stable and dense passive film, which can prevent aggressive ions.To study the impact of pitting on the stress corrosion sensitivity, three tests were designed, which include SSRT test in air, in corrosion solutions and samples with pre pitting holes in corrosion solutions. SSRT curves, stress corrosion sensitivity coefficients and fracture SEM examination under different conditions were analyzed. The results showed that both Haiku-1and Haiku-2stainless steel samples in the air and in the simulation solution of Luojiazhai had the gliding fracture and the stress corrosion sensitivity coefficients were in a safe area, which meant the stress corrosion didn’t occur and the materials had meet the environmental requirements of resistance to stress corrosion. The specimen with pre pitting holesn the simulation Luojiazhai present a tendency from gliding fracture to brittle fracture and the stress corrosion sensitivity coefficients were in a danger zone. Therefore, pitting corrosion may promote or accelerate stress corrosion occurring in the actual project and the pitting corrosion protection work should be focused.