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• The under-deposit corrosion mechanism of tubing steel is clarified in detail. • The synergistic effect of CO 2 and SRB causes under-deposit corrosion. • The enrichment of Cl - accelerates the localized corrosion process beneath the corrosion products. The tubing steel served for the injection and production wells of CO 2 flooding was investigated using various characterization methods. The results showed the corrosion failure of the tube was caused by the jointed action of sulfate-reducing bacteria (SRB), CO 2 and chloride ions ( Cl -) during service. The corrosion product film formed on the tubing steel exhibited a bilayer structure. The outer layer was mainly composed of FeS and FeCO 3 , which was caused by SRB corrosion and CO 2 corrosion respectively, and the inner layer was primarily iron oxide and oxyhydroxide caused by Cl - enrichment and CO 2 corrosion. FeCO 3 was found in both layers. Accordingly, the synergistic effect of SRB and CO 2 resulted in the under-deposit corrosion, and the enrichment of Cl - accelerated the localized corrosion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Branching cracks reveal the failure was attributed to stress corrosion cracking. • Initiation of SCC cracks mainly results from intergranular corrosion. • The SCC cracks propagated in a mixed mode of TGSCC and IGSCC. • Sensitized material, chloride and residual stress lead to the SCC failure synthetically. The investigation of this paper aimed at finding the failure cause of a heavily cracked geothermal water convection tube, utilizing electron backscattered diffraction, double loop potentiodynamic polarization test, and energy-dispersive X-ray spectroscopy. Results show the failure was attributed to stress corrosion cracking (SCC). Chloride in the service environment has induced the initiation of SCC cracks. The sensitization of matrix increased the susceptibility to intergranular cracking, which may result from improper heat treatment before service. The residual stress arising from the manufacturing process promoted the failure process. The SCC cracks originated from the internal tube. In the early stage, the cracks propagated along austenite grain boundaries. Thereafter, they transformed into a coexisting mode of intergranular cracking and transgranular cracking. The failure was attributed to the synthetical effect of sensitization, chloride, and residual stress. [ABSTRACT FROM AUTHOR]