Effect of Small Amounts of Cr and Mo on Aqueous CO₂ and H₂S Corrosion of Low-Alloyed Steel

One of the important industrial issues is the corrosion of carbon and low-alloy steels exposed to a wet environment where CO₂ and/or H₂S are dissolved. In the 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 the 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 a 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) is 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 measurements. The corrosion product formed on the steel surface was also analyzed by scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy. 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.