Effect of Ti Microalloying on the Corrosion Behavior of Low-Carbon Steel in H2S/CO2 Environment

The corrosion behavior of two types of low-carbon steel (denoted steel A and steel B) in H2S/CO2 was investigated through immersion tests, and the effect of Ti microalloying on the corrosion mechanism was analyzed. The microstructures, corrosion kinetics, corrosion surface morphologies, corrosion phases, cross-sectional morphologies and elemental distributions of the materials were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), energy-dispersive spectroscopy (EDS), x-ray diffraction (XRD) and electron probe microanalysis (EPMA). The results showed that the corrosion rate of steel B with Ti microalloying was lower than that of steel A; the amount and formation rate of corrosion products in these two materials were different. As the immersion time increased, the corrosion products changed from iron-rich mackinawite to sulfur-rich pyrrhotite. In the early stage of corrosion, the corrosion products were primarily mackinawite, which controlled the corrosion phase through the entire process. After 384 h of immersion, pyrrhotite was observed, which can effectively protect the steel substrate, providing excellent corrosion resistance. The corrosion products in steel B with Ti microalloying were more compact than those in steel A. Moreover, Ti enrichment in the oxide film was observed in steel B, which can further improve its corrosion resistance. The results from this study can provide an important reference for the oil and gas industry.