Effect of Microstructure on the Morphology of Atmospheric Corrosion Pits in Type 304L Stainless Steel

Abstract

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 MgCl 2 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.

Revealing the growth kinetics of atmospheric corrosion pitting in aluminum via in situ microtomography

“…Both laboratory-based and synchrotron XCT have been used to study pit growth under immersed and atmospheric conditions, for galvanic couples and the evolution of environmentally-assisted cracking 2,[4][5][6][7][8][9][10][11][12][13][14][15] . Several recent studies utilized X-ray microtomography to directly measure the rate and morphology of intergranular corrosion of AA7075 and AA2024 during the initial stages of atmospheric corrosion under millimeter-sized chloride salt droplets 3,16,17 .…”

The effect of relative humidity change on atmospheric pitting corrosion of stainless steel 304L

“…Figure 7(b) is a summary of the size change of the main part of the pit (excluding the deep fissure), during cycling. The deep fissure was related to the presence of ferrite in the studied samples [38]. It can be observed that there was a slight increase in the pit width and pit depth.…”

Toward Understanding the Effects of Strain and Chloride Deposition Density on Atmospheric Chloride-Induced Stress Corrosion Cracking of Type 304 Austenitic Stainless Steel Under MgCl2 and FeCl3:MgCl2 Droplets

“…74 In a more recent work, Mohammed-Ali et al reported atmospheric corrosion pits having a layered morphology, being related to the microstructure processing orientation (favored dissolution ferrite along the rolling direction) as well as ferritic grains, which were band-wise arrayed along the rolling direction and corroded selectively promoting anisotropic dissolution. 75 The pits we observed in the presence of ferric ions, however, cannot be related to the microstructure (only) since the step marks (that which gives the whirlpool appearance) within the pit are as small as ≤1 µm which is by far less than the grain size of the material (~45 µm). It may, however, be related to slip bands which can have different dissolution characteristics.…”

“…21,54,88 Pit initiation, however, can be favored on inclusions or carbide precipitates, and delta ferrite has been reported to dissolve selectively under atmospheric corrosion conditions. 54,67,75,88 Pits can further nucleate preferentially on regions containing high strain and facilitate early breakdown of passive film. 6,[63][64][65][89][90][91] Hydrogen, formed during corrosion reactions, can enhance the pitting susceptibility by lowering the electrochemical nobility in local scale due to trapped hydrogen and perturbed the passive surface oxide film, facilitating early breakdown.…”