Influence of Cr content on microstructures and acid corrosion properties of x120 grade pipeline steel

Effects of Cr on microstructures and acid corrosion properties of X120 grade pipeline steel were studied with SEM and TEM and mass loss. The mechanical properties results show that the microstructures of four tested steels are all tempered martensites, and the mechanical properties are all up to X120 grade. Yield strength and tensile strength of the four tested steels are improved with the increasing of Cr content. The precipitated phases of tested steels are (Nb, Ti) (C, N). Precipitation of Cr is little, and was only discovered in the 3Cr steel through the experiment. The corrosion experiment results indicate that the corrosion scales on the four tested steels have two layers structure under the condition of CO2/H2S. The outer layer is mainly containing FeS, the inner layer FeCO3. Cr enriches in the inner layer, and the content increases with the increasing of Cr element in matrix. The Cr-rich enhances the compactness of the corrosion scales, and the scales have ion-passing-selectivity, which hinders the moving of anion to matrix, and finally the resistance to CO2/H2S corrosion increases.