One of the important industrial issues is corrosion of carbon and low alloy steels exposed to a wet environment where CO2 and/or H2S are dissolved. In case of carbon and low alloy steels, mainly iron-based corrosion products such as iron carbonate and iron sulfides can form depending on the condition. Microalloys added in small amounts to carbon steel can change the water chemistry near the steel surface due to dissolution of the microalloys, which may also alter the corrosion resistance of the steel and corrosion morphology. In this study, the individual microalloying effects of small amount (1 mass%) of Cr and Mo added in carbon steel were investigated in a wet CO2 and H2S environment, where the formation of mackinawite (FeS) are considered to be the dominant iron-based corrosion product. The variation of corrosion rate and corrosion potential of the microalloyed steels was monitored in the NaCl solution aerated with CO2 - 10% H2S gas by linear polarization resistance (LPR) measurements. The corrosion product formed on the steel surface was also analyzed by SEM, EDS, XRD, TEM and XPS. The experimental results show that the addition of a small amount of Cr improves corrosion resistance by forming a dense film of FeS on the surface, while the addition of a small amount of Mo delays the formation of FeS on the surface and the corrosion resistance behavior is similar to that of steel with no additions.

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