A systematic investigation of pitting failure of mild steel in marginally sour environments was performed with the objective of understanding and predicting the occurrence of localized corrosion. While localized corrosion can happen due to a variety of reasons, recent work has shown that mild steel was particularly susceptible to pitting in environments containing traces of H2S (ppm level in the gas phase, which equates to ppb level of dissolved O2 in the liquid phase) of H2S. Relevant research works related to localized corrosion of mild steel exposed to O2, CO2 and H2S containing aqueous environments were carefully reviewed and a critical comparison was performed, identifying experimental methodologies, common mechanisms and gaps in understanding. A comprehensive parametric study was conducted to identify the operating parameters controlling the occurrence of localized corrosion in marginally sour environments. As a result, pitting was found to occur under the following conditions: 0 mbar < pH2S < 0.15 mbar, pCO2 > 0 bar, temperature < 60C, bulk pH < 6, on X65 mild steel (not on pure iron), in NaCl concentrations of 0, 1, and 10 wt.%, with 3 ppb(w) < [O2]aq < 40 ppb(w). Surface analysis (FIB-TEM-SAED-PED) identified a typically 200 nm thick, porous, detached, and partially oxidized amorphous mackinawite layer precipitated within a Fe3C network. The role of O2 was further investigated to explain the unexpected presence of oxides in the corrosion product layer. Initially, FeS was thought to have been oxidized during the post processing analysis. However, in situ Raman microscopy later showed that oxygen ingress during the experiment was the origin of iron oxide formation. In addition, when [O2]aq < 3 ppb(w), neither corrosion product precipitation nor pitting was observed on the steel surface in any conditions tested, while the uniform corrosion rate remained low. In this case, the protectiveness was due to the presence of a very thin FeS chemisorbed layer. In the presence of oxygen ([O2]aq > 3 ppb(w) at 1 bar total pressure), this FeS chemisorbed layer partially oxidized, leading to the formation of iron oxides. The volume change caused by the phase change (FeS chemisorption layer to iron oxides) exposed the underlying steel surface to the corrosive environment. These exposed local spots corroded due to the presence of CO2, which can typically lead to corrosion rates as high as 3 ~ 4 mm/y, initiating pitting. A phase equilibrium diagram was developed for the Fe/H2S/H2O/O2 system based on the minimization of Gibbs free energy. The diagram includes redox reactions involved in the transformation of several FeS polymorphs in aqueous solution driven by various concentrations of dissolved oxygen over a range of pH values. Pathways for the transformation of mackinawite into greigite, magnetite or hematite were identified depending on the concentration of dissolved oxygen. This study shows that the chemisorbed FeS layer can be partially oxidized in the presence of oxygen, leading to pit initiation in marginally sour environments. The phase equilibrium diagram of the H2S/H2O/O2 system indicated that in aqueous solution, H2S could also be catalytically oxidized into SO42- at pH2S = 4x10-4 bar, releasing H+ as a coproduct of this reaction. This was verified by monitoring the pH during the corrosion process. Water chemistry analysis revealed that a small portion of H2S could have been oxidized into H2SO4 in the electrolyte, with Fe2+ and Ni2+ serving as the catalysts. At the active corrosion sites, the higher [Fe2+] may have further enhanced this process, leading to lower local pH and lower saturation degree of FeS to prevent regeneration of the product layer inside the pit. This mechanism, together with the galvanic coupling effect between the actively corroding pit (anode) and the mackinawite covered cathode, is thought to govern the pit propagation. To summarize, the investigations reported herein have fully revealed the mechanism of localized corrosion of mild steel in marginally sour environments. In industrial processes, once the crude oil leaves the deep underground anoxic environments, it is not uncommon to measure oxygen content as high as 20 ppbw (6x10-7 mol/L), which may not be low enough to prevent pitting in marginally sour environments.