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International Journal of Corrosion
Volume 2012 (2012), Article ID 482730, 6 pages
http://dx.doi.org/10.1155/2012/482730
Research Article

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

1Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-02 Aobayama, Aoba-ku, Sendai 980-8579, Japan
2Department of Materials Science and Engineering, School of Engineering, Tohoku University, 6-6-02 Aobayama, Aoba-ku, Sendai 980-8579, Japan
3ISFnetlife Ltd., Akasaka 7-1-16, Minato-ku, Tokyo 107-0052, Japan

Received 26 March 2012; Accepted 15 May 2012

Academic Editor: Rokuro Nishimura

Copyright © 2012 Nobuyoshi Hara et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.