Microelectrochemical Investigation of Atmospheric Aging Effect on Pitting Corrosion Resistance at Manganese Sulfide Inclusion in Type 304 Stainless Steel
Authors: Chiba Aya , Muto Izumi , Sugawara Yu , Hara Nobuyoshi 
Published: February 2020

Stainless steels are widely used in architectural applications, such as roofing and walls. In atmospheric environments, pitting causes discoloration (rust staining) on the steel surfaces, which compromises the esthetic appearance of architecture considerably. The pitting behavior of stainless steels in outdoor environments has been extensively investigated through long-term exposure tests 1. It is generally accepted that most stainless steels experience severe corrosion within the first two years in marine environments, and there is a correlation between rust staining resistance and pitting resistance 2. Even in atmospheric environments, sulfide inclusions, such as MnS, act as initiation sites for pitting. Considering these points, it is reasonable to consider the possibility that atmospheric aging improves pitting corrosion resistance at MnS inclusions in stainless steels. In our previous works 3-5, we studied the pit initiation behavior at the MnS inclusions in Type 304 stainless steels. The pit initiation mechanism was found to be as follows: 1) the dissolution of the MnS inclusions leads to the deposition of elemental sulfur on and around the inclusions; 2) the synergistic effect of the elemental sulfur and chloride ions causes the dissolution of the stainless steel side at the MnS/steel boundaries, resulting in the formation of trenches; 3) the hydrolysis reaction of Cr3+ released from the steel matrix dissolution decreases the pH in the trenches, and at the same time, the electrode potential at the bottom of the trenches decreases due to the IR-drop. Finally, the pit initiation is defined as the local transition from the passive to active state at the bottom of the trenches. Atmospheric aging is an important subject for the corrosion engineering of stainless steels, and a thorough understanding how the aging affects the described pit initiation process at the MnS inclusions is expected to help in the endeavor to create corrosion resistant surfaces for stainless steels.
Micro-scale electrode areas with dimensions of approximately 300 μm×150 μm which contained a stripe-like MnS inclusion were prepared to analyze the effect of atmospheric aging on the dissolution of MnS inclusions and the pitting initiation process at the inclusions. A commercial re-sulfurized Type 304 stainless steel bar with a diameter of 70 mm containing large MnS inclusions was used in this study. This stainless steel was heat-treated at 1353 K for 0.5 h and quenched in water. The specimen surfaces parallel to the rolling direction were mirror polished, and then exposed to air at 298 K and 50% relative humidity (RH) for 1, 30, and 90 days.
A SEM/EDS (scanning electron microscope/energy-dispersive X-ray spectroscopy) analysis indicated that the MnS inclusions did not dissolve during the 90-day exposure, and the atomic ratios of Mn and S of the inclusions remained almost 1:1. An AES (auger electron spectroscopy) analysis indicated no outstanding changes in the thickness of passive film on the steel matrixes after the 30 days of exposure, whereas the thickness of the oxide layer on the MnS inclusions had doubled after the 30-day exposure period. In order to investigate the correlation between the oxide layer formed on the MnS inclusions and the pit initiation at the inclusions, the anodic polarization curves of a micro-scale electrode area were measured in 3 M NaCl. It is interesting to note that the exposure for in excess of 30 days improved the resistance of the investigated stainless steel to pitting at the MnS inclusions with no stable pitting found on the specimens exposed. The extent of the dissolution of the MnS inclusions on the specimens exposed for more than 30 days measured in 0.1 M Na2SO4and 0.1 M NaCl was suppressed to less than half that of the specimens exposed for one day. In addition, the exposure for 90 days inhibited the trench formation at the MnS/steel boundaries in 3 M NaCl. The over 30 days of exposure (atmospheric aging) resulted in the suppression of the MnS dissolution by the growth of the oxide layer on the inclusions, and then inhibited the formation of the trenches at the MnS/steel boundaries as a trigger for the pit initiation at the MnS inclusions. This explains the lower activity of MnS inclusions in atmospheric environments.
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