Four kinds of X100 grade pipeline steels containing different chromium content were designed and corrosion behavior were studied in order to confirm the influence of Cr on the properties of high grade pipeline steel. The corrosion experiments demonstrated that Corrosion rates of Four steels in the experiment decrease with the increasing of Cr content, and the scales on the four steels have a two-layer structure under the corrosion of CO2 and H2S. The outer layer is mainly composed of FeS or FeS1-x and the inner layer consisted of FeCO3. Cr enriches in the inner layer and Cr content of the inner layer increases with the Cr content in matrix. The Cr enrichment enhanced the compactness of the scales further hindered the diffusion of ions from liquid to the surface of steel. The corrosion scale with Cr(OH)3 is anion-selective and reduce the amount of anion reaches metal surface, thus reducing corrosion rate.

The influence of Sn on mechanical properties and CO2 corrosion resistance of oil tube was investigated by TEM, EBSD and electrochemical measurement. The results show that the addition of 0.05%Sn can improve the corrosion-resistance of tested steel, but decrease the elongation and impact energy. Sn affected the activity of tested steels, moved corrosion potential to negative potential and decreased corrosion current density. Consequently, the polarization resistance of electrode reaction increased and corrosion reaction was slowed down due to the effect of Sn. In addition, Sn increased the proportion of low boundary, which improved the corrosion resistance of steel, and accordingly decreased the proportion of large angle grain boundary, which contaminated the elongation and impact energy of steel. However, Sn had little influence on the microstructure, precipitates of steel. And thus the strength of steel did not change obviously.