Effect of Temperature and Composition on Corrosion Behavior of Medium Cr Steel in Salt Solution Containing CO₂

In this paper, the corrosion behavior of three kinds of medium Cr low C steels in the simulated service environment of the transport pipeline was investigated through accelerated corrosion experiments, and the corrosion resistance mechanism of these experimental steels at different temperatures was investigated by electrochemical means. Finally, the reasons for the difference in corrosion behavior were analyzed from the grain boundary and surface Volta potential. The results show that as the temperature rised, the corrosion rate of 5Cr specimens increased sharply, the corrosion type developed from slight general corrosion to severe general corrosion; 7Cr specimen was less sensitive to temperature, and the type of corrosion changed from slight general corrosion to local corrosion; 9Cr specimen was not sensitive to temperature, and the type of corrosion was always local corrosion. 5Cr steel could form a protective product film at 30 °C. As the temperature rised, the protective ability of the product film decreased, and the matrix dissolved easily. The film of 7Cr and 9Cr samples had not yet precipitated and the matrix was difficult to dissolve at 30 °C. However the matrix dissolved easily at 50 °C, and the product film had formed, which played a major role. At 70 °C, the protective effect of the product film decreased, and the gap between the 7Cr and 9Cr samples began to appear. The increase of Cr content helped to refine grains and increased the proportion of low-angle grain boundaries. At the same time, the increase of Cr element helped to increase the maximum Volta potential of the experimental steel and increased the Volta potential difference. As a result the test steel was shown to be resistant to uniform corrosion, but it also increased the risk of pitting corrosion.