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Improvement of Pitting Corrosion Resistance of Type 316L Stainless Steel by Potentiostatic Removal of Surface MnS Inclusions

DOI: 10.1155/2012/482730

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Abstract:

The beneficial effect of the removal of MnS inclusions on the pitting of stainless steels has been demonstrated in two ways. (1) High-purity Type 316L stainless steel with no inclusions was used as a specimen in the measurement of anodic polarization curves in 0.5?M NaCl and (2) commercial Type 316L stainless steel with MnS and slag-related inclusions was first polarized at different potentials for 30 min in 1?M Na2SO4 of pH 3 and then anodic polarization measurements were taken in 0.5?M NaCl. Pitting did not occur in the passive or transpassive region of the high-purity steel. The polarization treatment dissolved MnS and some oxide inclusions (CaO and SiO2) on the surface of the commercial steel. An increase in pitting potential of the commercial steel was noted after treatment at potentials above 0.2?V. At the same time, the number of current spikes due to metastable pits decreased significantly. These results are more likely due to the beneficial effect of removing MnS inclusions from the steel surface rather than the modification effect of the chemical composition of passive films on the surface. 1. Introduction Manganese sulfide (MnS) inclusions are known to act as the initiation sites of pitting corrosion on stainless steels [1–9], while the overall dissolution of the inclusions is not necessary for the formation of pit initiation sites [10–17]. It is therefore expected that the removal of surface MnS inclusions improves the pitting corrosion resistance of stainless steels. The aim of treating the surface of stainless steels with processes like nitric acid passivation (ASTM A380 and ASTM A967) is to form a stable Cr-enriched passive oxide film [18], which plays an important role in providing high corrosion resistance to stainless steels. Such surface treatment has an additional effect of dissolving and removing the MnS inclusions from the surface of stainless steels [19]. Therefore, the improvement of pitting corrosion resistance by passivation treatments can be attributed not only to the modification of the chemical composition of the passive films but also to the removal of MnS from the surface of stainless steels. However, these two effects cannot be distinguished from each other in practice. For the further development of passivation treatments of stainless steels which meet environmental regulations and human safety standards, it is important to understand the intrinsic mechanism of each effect provided by passivation treatments. The purpose of the present study is to examine solely the effect of removing MnS on the pitting corrosion

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