Abstract: Corrosion behavior of 7%Cr steel with three different types of microstructure in CO₂ environment was tested by mass loss method and electrochemical method.The reasons for the difference of corrosion resistance caused by microstructure type were analyzed from grain boundary and surface potentialby by means of electron back-scattered diffraction and scanning Kelvin probe microcope. The results show that the microstructure of the rolled 7%Cr steel is composed of bainite+ferrite, the quenched sample is composed of quenched martensite, and the normalized sample is composed of ferrite+a small amount of pearlite. The order of corrosion resistance of the tested steel is quenched>normalized>rolled, and the enrichment degree of Cr element in the product film of the tested steel and the self-corrosion potential of its polarization curve are consistent with this law, while the maximum potential difference on the surface of the sample is opposite. The existing potential difference between the two phases in the rolled sample is easy to form a galvanic cell; the Cr element in the quenched sample can quickly diffuse through the grain boundary to the corrosion position to form a product film; the free Cr in the normalized sample is less and the high proportion of high-angle grain boundary is not conducive to its diffusion to the corrosion interface.

Key words: medium Cr steel, CO₂ corrosion, grain boundary, scanning Kelvin probe microscope