Atmospheric corrosion behavior of ferritic-pearlitic, spheroidized, and bainitic microstructures of a mild steel

The present study investigates the comparative atmospheric corrosion behavior of three microstructural variants of mild steel (MS), namely, ferritic-pearlitic, spheroidized, and bainitic, made by air cooling, air cooling followed by spheroidizing annealing, and austempering, respectively. The samples were kept outdoors for three years for analyzing the extent of atmospheric corrosion. The samples were intermittently removed at 1, 2, 3, 6, 18, 24, and 36 months and corrosion rates were calculated through the weight loss method. The corrosion rates of the steels irrespective of the microstructure were high in the beginning, and later it slowed down due to the formation of stable rust phase, i.e., goethite phase (alpha-FeOOH). In addition, after three years of exposure, the bainite variant exhibited the lowest corrosion rate. Overall, the initial corrosion rates were found to predominantly depend upon the corrosion damage due to the formation of micro-galvanic couples. However, at the later stage of exposure, the corrosion rates were influenced by the variation of rust phase fractions and their compactness.