Effect of microstructure on the morphology of atmospheric corrosion pits in type 304L stainless steel

Atmospheric corrosion of stainless steel is of concern for intermediate level nuclear waste (ILW) containers. The effect of microstructure on the morphology of atmospheric corrosion pits in Type 304L stainless steel plate was investigated on three orthogonal planes under MgCl2 droplets. Pits on the top surface of the plate show ring-like structures, whereas pits on the plate sides show a striped morphology. Synchrotron x-ray tomography of Type 304L stainless steel pins shows the presence of similar striped attack. Scanning electron microscopy on plate samples revealed the presence of parallel bands along the rolling direction. Energy dispersive spectroscopy maps and line scans across these bands indicated a local increase in the Cr/Ni ratio consistent with a ferrite phase, likely residual delta-ferrite formed during solidification. Vibrating sample magnetometer (VSM) detected the presence of ferrite on the base alloy. X-ray diffraction and electron backscatter diffraction quantified the volume fractions of ferrite and austenite phases. Ferrite phases affect the morphology of pits and promote pit propagation along the rolling direction.