Stress Generation Accompanying Intergranular Corrosion of X70 Steel (2017)

Stress corrosion cracking (SCC) is a primary degradation mode in steel oil and gas pipelines (1). Pre-emptive detection and control of SCC of pipeline steel in high-pH soils is difficult owing to a prolonged incubation stage of precursory intergranular corrosion (IGC). In high-pH environments, pipeline steel exhibits greatest susceptibility to SCC within a narrow potential range between the active dissolution peak potential and the passivation potential, which may suggest an important role for oxide formation (2,3). In the present work, IGC is characterized by electrochemical and microscopic measurements together with in situ stress monitoring. The latter method is well-suited to detect stress generated by internal grain boundary oxidation (4). Low-carbon ferritic steel samples (X70 type) were cut from a pipeline specimen. After polishing, the specimens were mounted in electrochemical cells with the front surface exposed to solution (aqueous 1 M NaHCO 3 at pH 8.0-8.2) and the back surface utilized for in situ stress monitoring by curvature interferometry. Prior to each constant potential corrosion experiment, the potential was held at -0.978 V vs. Ag/AgCl to cathodically reduce the native oxide layer. Corrosion morphology was examined with scanning electron microscopy (SEM) and profilometry. The internal IGC morphology was exposed at various depths by shallow-angle polishing. Fig. 1 shows the current transients at three potentials between the active current peak potential and the passivation potential. At the two highest potentials, the current density increased to maxima after initial transients associated to passive film formation, and thereafter slowly decayed. Stress measurements at the two highest potentials showed steady compressive stress generation. At -0.575 V, compressive stress increased for about 10 min and then diminished. SEM revealed that localized grain boundary attack initiated during the current rise. During the current decays at -0.521 V and -0.478 V, a micron-thick granular corrosion product accumulated on the steel surface (A in Fig. 2). This external corrosion product is likely an iron oxy-carbonate precipitate (5), with the current decay due to its mass transport resistance. Examination of the internal IGC morphology at these potentials revealed interconnected voids along grain boundaries and a primarily oxide corrosion product coating the attacked steel grains (B-D). At the greatest depths within the IGC layer (D), the oxide completely filled the corroded grain boundaries. In contrast, no oxidation on grain boundaries was detected at -0.575 V. Figs. 1 and 2 demonstrate that the potential dependence of compressive stress follows that of grain boundary oxide formation. This correspondence is evidence that the measured stress is due to steel oxidation along grain boundaries. Compressive stress generation would be localized close to the depth of maximum IGC penetration, where oxide completely fills the grain boundary (D in Fig. 2). The constant rate of stress generation associated with grain boundary oxide formation is permits IGC penetration into steel. We suggest that grain boundary oxide advance may be assisted by tensile stress in the grain boundary ahead of the oxide front, which accompanies compressive oxide stress. Such tensile stress could enhance the driving force for oxygen ion migration to the uncorroded grain boundary, so as to promote self-sustaining oxide penetration (6). ACKNOWLEDGMENT Financial support was provided by U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration under Competitive Academic Agreement Program No. DTPH5614HCAP03 and DTPH5614HCAP01. REFERENCES 1. Y. F. Cheng, Stress Corrosion Cracking of Pipelines, Wiley, 2013. 2. R. D. Armstrong and A. C. Coates, Corros. Sci., 16, 423 (1976). 3. R. N. Parkins and S. Zhou, Corros. Sci., 39, 175 (1997). 4. Ö. Ö. Çapraz, P. Shrotriya, K. R. Hebert, J. Electrochem. Soc., 160, D501 (2013). 5. D. H. Davies and G. H. Burstein, Corrosion, 36, 416 (1980). 6. H. Gao, L. Zhang, W. D. Nix, C. V. Thompson, and E. Arzt, Acta Mater., 47, 2865 (1999). Figure 1