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
References
[1]
J. E. Castle and R. Ke, “Studies by auger spectroscopy of pit initiation at the site of inclusions in stainless steel,” Corrosion Science, vol. 30, no. 4-5, pp. 409–428, 1990.
[2]
M. A. Baker and J. E. Castle, “The initiation of pitting corrosion of stainless steels at oxide inclusions,” Corrosion Science, vol. 33, no. 8, pp. 1295–1303, 1992.
[3]
R. Ke and R. Alkire, “Surface analysis of corrosion pits initiated at MnS inclusions in 304 stainless steel,” Journal of the Electrochemical Society, vol. 139, no. 6, pp. 1573–1580, 1992.
[4]
M. A. Baker and J. E. Castle, “The initiation of pitting corrosion at MnS inclusions,” Corrosion Science, vol. 34, no. 4, pp. 667–682, 1993.
[5]
R. Ke and R. Alkire, “Initiation of corrosion pits at inclusions on 304 stainless steel,” Journal of the Electrochemical Society, vol. 142, no. 12, pp. 4056–4062, 1995.
[6]
G. S. Frankel, “Pitting corrosion of metals: a review of the critical factors,” Journal of the Electrochemical Society, vol. 145, no. 6, pp. 2186–2198, 1998.
[7]
H. Krawiec, V. Vignal, O. Heintz, R. Oltra, and J. M. Olive, “Influence of the chemical dissolution of MnS inclusions on the electrochemical behavior of stainless steels,” Journal of the Electrochemical Society, vol. 152, no. 7, pp. B213–B219, 2005.
[8]
T. L. S. L. Wijesinghe and D. J. Blackwood, “Real time pit initiation studies on stainless steels: the effect of sulphide inclusions,” Corrosion Science, vol. 49, no. 4, pp. 1755–1764, 2007.
[9]
G. S. Eklund, “Initiation of pitting at sulfide inclusions in stainless steel,” Journal of the Electrochemical Society, vol. 121, no. 4, pp. 467–473, 1974.
[10]
G. Wranglén, “Pitting and sulphide inclusions in steel,” Corrosion Science, vol. 14, no. 5, pp. 331–349, 1974.
[11]
T. Suter, E. G. Webb, H. B？hni, and R. C. Alkire, “Pit initiation on stainless steels in 1？M NaCl with and without mechanical stress,” Journal of the Electrochemical Society, vol. 148, no. 5, pp. B174–B185, 2001.
[12]
C. Punckt, M. B？lscher, H. H. Rotermund et al., “Sudden onset of pitting corrosion on stainless steel as a critical phenomenon,” Science, vol. 305, no. 5687, pp. 1133–1136, 2004.
[13]
P. Schmuki, H. Hildebrand, A. Friedrich, and S. Virtanen, “The composition of the boundary region of MnS inclusions in stainless steel and its relevance in triggering pitting corrosion,” Corrosion Science, vol. 47, no. 5, pp. 1239–1250, 2005.
[14]
H. Krawiec, V. Vignal, O. Heintz, and R. Oltra, “Influence of the dissolution of MnS inclusions under free corrosion and potentiostatic conditions on the composition of passive films and the electrochemical behaviour of stainless steels,” Electrochimica Acta, vol. 51, no. 16, pp. 3235–3243, 2006.
[15]
I. Muto, Y. Izumiyama, and N. Hara, “Microelectrochemical measurements of dissolution of MnS inclusions and morphological observation of metastable and stable pitting on stainless steel,” Journal of the Electrochemical Society, vol. 154, no. 8, pp. C439–C444, 2007.
[16]
I. Muto, D. Ito, and N. Hara, “Microelectrochemical investigation on pit initiation at sulfide and oxide inclusions in type 304 stainless steel,” Journal of the Electrochemical Society, vol. 156, no. 2, pp. C55–C61, 2009.
[17]
S. J. Zheng, Y. J. Wang, B. Zhang, Y. L. Zhu, C. Liu, and P. Hu, “Identification of MnCr2O4 nano-octahedron in catalysing pitting corrosion of austenitic stainless steels,” Acta Materialia, vol. 58, no. 15, pp. 5070–5085, 2010.
[18]
J. S. Noh, N. J. Laycock, W. Gao, and D. B. Wells, “Effects of nitric acid passivation on the pitting resistance of 316 stainless steel,” Corrosion Science, vol. 42, no. 12, pp. 2069–2084, 2000.
[19]
R. R. Maller, “Passivation of stainless steel,” Trends in Food Science and Technology, vol. 9, no. 1, pp. 28–32, 1998.
[20]
J. Shinozaki, I. Muto, T. Omura, M. Numata, and N. Hara, “Local dissolution of MnS inclusion and microstructural distribution of absorbed hydrogen in carbon steel,” Journal of the Electrochemical Society, vol. 158, no. 9, pp. C302–C309, 2011.
[21]
T. Shibata and T. Takeyama, “Stochastic theory of pitting corrosion,” Corrosion, vol. 33, no. 7, pp. 243–251, 1977.
[22]
T. Shibata, “Stochastic studies of passivity breakdown,” Corrosion Science, vol. 31, pp. 413–423, 1990.
[23]
T. Shibata, “1996 W.R. Whitney Award Lecture: statistical and stochastic approaches to localized corrosion,” Corrosion, vol. 52, no. 11, pp. 813–830, 1996.
