The early corrosion development of ultra-low carbon bainitic (ULCB) low alloy steel in NaCl solution was studied by ex situ imaging of corrosion morphology and in situ monitoring of microarea current density and potential, and the corrosion mechanism from initial localized corrosion to uniform corrosion was interpreted. The results indicate that the corrosion development of ULCB steel from initial localized corrosion around inclusions to the uniform corrosion on the whole steel surface is controlled by the galvanic couple effect between different phases resulting from their electrode potential difference in electrolyte solution. The early localized corrosion of steel matrix is initiated and accelerated by the galvanic couple effect between MnS inclusions and steel matrix to form the initial corrosion gaps and the circular corrosion spots around inclusions. The ohmic drop caused by solution resistance influences the acceleration effect of the galvanic couple. With the separation of inclusion from steel matrix, this galvanic couple effect becomes invalid, which results in the expansion from localized corrosion to uniform corrosion. The microgalvanic couple between martensite/residual austenite (M/A) islands and bainite ferrite also accelerates the anodic dissolution of bainite ferrite phase; however, its acceleration corrosion effect is much weaker than that caused by MnS inclusion.

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