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The paper focuses on the analysis of initiation and propagation of CO2 corrosion in several samples of low-alloy steel with different microstructures using scanning electrochemical microscopy (SECM) and other microscopy techniques. It is found that the corrosion rate and the mode of corrosion are highly sensitive to the microstructure. The overall current density is much higher and more uniformly distributed for the tempered martensite structure than for samples having either a ferritic-pearlitic microstructure or a microstructure combining ferritic, bainitic and martensitic-austenitic regions. As a result, the sample with the tempered martensite structure undergoes uniform corrosion, while the other two samples undergo selective corrosion. The SECM maps show that regions of polygonal ferrite generate larger anodic currents than the pearlitic structure in the early stages of corrosion. The residual cementite lamellae provide greater cathodic surface areas after the initial dissolution of ferritic lamellae within pearlite, promoting galvanic corrosion and subsequently enhanced dissolution of ferritic lamellae. According to SECM data, the dissolution of iron in polygonal ferritic grains is 2.4 times faster than that of ferritic lamellae in pearlitic regions.

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