Effect of microstructure and passive film on corrosion resistance of 2507 super duplex stainless steel prepared by different cooling methods in simulated marine environment

The effect of microstructure and passive film on the corrosion resistance of 2507 super duplex stainless steel (SDSS) in simulated marine environment was investigated by electrochemical measurements, periodic wet-dry cyclic corrosion test, scanning Kelvin probe force microscopy, atomic force microscopy, and X-ray photoelectron spectrometry. The results show that the occupation ratio of gamma phase increases with the decrease in cooling rate, whereas the content of alpha phase reduces gradually. In addition, the sigma precipitated phase only emerges in the annealed steel. The pitting sensitivity and corrosion rate of 2507 SDSS reduce first and then increase as the cooling rate decreases. The sigma precipitated phase drastically reduces the protective ability of the passive film and facilitates micro-galvanic corrosion of the annealed steel. For various microstructures, the pits are preferentially distributed within the sigma and gamma phases. The corrosion resistance of 2507 SDSS prepared by different cooling methods is closely related to the microstructure and structure (stability and homogeneity) of the passive film. Normalized steel shows an optimal corrosion resistance, followed by the quenched and annealed steels.