S.C. Hayden, C. Chisholm, T.J. Kucharski, W. Mook, R.O. Grudt, A. Ilgen, D. Bufford, K. Hattar, K. Jungjohann, M.L. Ostraat
Aramco Services Company,
Keywords: corrosion, pipeline, steel, nanoscale
Summary:Corrosion of steel pipelines continues to present major challenges to the oil and gas industry, with billions of dollars spent annually on technologies to mitigate corrosion-related losses. The processes that govern steel pipeline corrosion are still not well understood, due in part to the complexity and changing nature of the environments in which corrosion occurs and to the daunting number of factors (e.g., chemical species, pressure, temperature, scales, etc.) that determine the nature and mechanics of the process once corrosion has begun. This collaboration seeks to tackle this problem at its source by elucidating the nanoscale processes involved in the onset of steel corrosion. We have applied a nanoscale approach to investigate the mechanisms responsible for the advent and progression of corrosion in carbon steel low-alloy carbon steel in wet hydrogen sulfide (H2S) and carbon dioxide environments (CO2). The goal of this project is to develop a mechanistic understanding of the advent of corrosion and to identify the nanoscale features and compositional fluctuations that render steel surfaces susceptible to degradation in wet environments. These key factors will be essential to the development of future strategies to reduce corrosion-related losses. Coupons of pipeline steel (1018 low-carbon steel) were sectioned and polished for initial characterization, after which lift-out TEM samples were fashioned by cutting out thin foils with a focused ion-beam (FIB). Grain orientation mapping and EDS were employed to determine the grain texture and generate elemental compositional maps. Comparison to bulk surface characterization identified direct correlation in chemical composition with the addition of a thin (1-2 nm) amorphous surface layer in the FIB foil samples. Micromanipulation was employed to place the foil samples onto the edge of a SiN membrane window, and the thin-film sections were initially mapped in vacuum to identify the locations of defect regions or interesting nanoscale features. Samples were then exposed to a flow of aqueous H2S and CO2 using a microfluidic liquid-cell holder. Pre-selected regions of interest were monitored throughout the in situ exposure of the low-carbon steel samples, and images were acquired periodically to generate video sequences of nanoscale corrosion in real time. These in situ TEM results will be presented along with mirrored ex situ bulk-scale corrosion results in order to best capture the ways in which corrosion at the nanoscale is predicted to affect corrosion observed in larger size scale regimes.