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|Stress corrosion crackling of pipeline steels in contaminated aqueous CO₂ environments
|This work addresses the risk of Stress Corrosion Cracking (SCC) in CO₂transport pipelines. The susceptibility of X80 pipeline steels in aqueous CO₂environments in the presence of nitrates and sulphites is investigated using electrochemical potentiodynamic tests and Slow Strain Rate Tests (SSRT) at 23 and 75°C. The electrochemical measurements showed that in CO2-free and CO₂-saturated systems, the material presents an active-passive transition in bicarbonate / carbonate solutions with nitrate and sulphite. This indicated that SCC is possible in all the test environments. SCC occurred in bicarbonate / carbonate solutions with nitrates and sulphites at 75°C, both under CO₂-free and CO₂-saturated conditions. SCC severity declined as the potential moved towards the free corrosion potential. Cracking was still observed at +50 mV from Ecorr. The cracking mode in the active domain was transgranular for all the systems. In CO2-free systems, the severity and cracking mode in the HCO₃--CO₃² --H₂O and NaNO₃-HCO₃--CO₃² --H₂O systems was similar in the active-passive transition domain at 75°C. Crack growth was controlled by anodic dissolution and the crack mode was intergranular for both systems. At high pH (>9), the overall cracking mechanism remains dominated by the HCO₃--CO₃² --H₂O system even in the presence of nitrates. The addition of sulphites to bicarbonate / carbonate solutions however decreased the severity of cracking and shifted the cracking mode to transgranular. In CO₂-saturated systems, the SCC susceptibility in all test environments decreased with lower pH. Yet the highest susceptibility to cracking in the active-passive domain was identified in the nitrate-containing systems. With the drop in pH, nitrate SCC becomes the dominant mechanism when nitrates are present in the HCO₃--CO₃² --H₂O system. The addition of CO2 shifted the mode of cracking to transgranular in the active-passive domain in the pure bicarbonate / carbonate solution.
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