Microelectrochemical Investigation on Pit Initiation at Sulfide and Oxide Inclusions in Type 304 Stainless Steel

Abstract

MnS and MnO inclusions with a small amount of chromium were obtained by heat-treatments of Type 304 stainless steel at 1353, 1573, and 1673 K, and the anodic dissolution behavior of the inclusions was investigated using a microelectrochemical technique. In the case of the sulfide inclusions, those with a high dissolution potential were found to provide high pitting potentials in the macroscopic measurements in 0.1 M NaCl. The initiation sites of metastable and stable pits were at the boundaries of the inclusions and the matrix in 3 M NaCl, 3 M MgCl 2 , and 8 M LiCl. The dissolution current densities on the inclusion surface at the moment of the initiation of a stable pit decreased with increasing chloride-ion concentration. The synergistic effect of chloride ions and sulfur-containing species released from the inclusions was thought to play an important role in the pit-initiation process. The oxide inclusions did not dissolve in the passive region of Type 304 stainless steel. In the solutions with chloride-ion concentrations of up to 6 M, pitting did not occur at the oxide inclusions in microscopic polarization measurements. The oxide inclusions exhibited a comparatively inert characteristic as pit-initiation sites.Sulfide inclusions in stainless steels, particularly manganese sulfide, are well known to act as initiation sites for pitting in chloride environments. 1 It is generally agreed that the most significant phenomenon which precedes pitting at the inclusions is sulfide dissolution, and also that the complete dissolution of the inclusions is not required for pit-initiation sites to be produced. 2,3 Sulfur-containing species are released during MnS dissolution, and various aggressive compounds, such as SO 4 2− , HSO 3 − , S 2 O 3 2− , S, and H 2 S, have been proposed as the reaction products. 2-9 Park et al., Paik et al., and Webb et al. demonstrated that the thiosulfate ions released from sulfide inclusions accelerated the dissolution of the inclusions and resulted in stable pitting when their concentration exceeded a critical value. 7-9 Krawiec et al. and Vignal et al. experimentally revealed that the sulfur species produced during MnS dissolution reacted with the passive film on a metal matrix to increase pitting susceptibility and to enhance cathodic reaction kinetics. 10-12 Park and Böhni detected the local acidification on MnS inclusions during the dissolution and pointed out its relationship to pitting at the inclusions. 13 Many researchers have revealed that the MnS/matrix boundaries mainly dissolved, and metastable pits were generated at the boundaries. 3,14-22 In our previous study, the synergistic effect of sulfur-containing species and chloride ions on pit initiation was suggested from the morphological observation of metastable and stable pits in chloride and chloride-free environments. 21 Suter and Böhni examined the anodic polarization behavior of different zones of a single MnS inclusion and revealed that the dissolution potential of the inclusion/matrix boundary was lower than that of the ...

Local Electrochemistry and In Situ Microscopy of Pitting at Sensitized Grain Boundary of Type 304 Stainless Steel in NaCl Solution

“…For the solution-treated specimen, many current spikes were generated above 0.4 V, and the current density rapidly increased and exceeded 0.1 A m −2 at 0.67 V. The potential region for MnS dissolution has been reported to be around 0.5 V in NaCl solutions. 13,14,[17][18][19]24 The many current spikes around 0.5 V were due to the dissolution of the sulfide inclusions, suggesting that the inclusions were likely the initiation sites for pitting. 17 In the case of the sensitized specimen, a sharp increase in the current density appeared suddenly at 0.32 V. The pitting potential of the sensitized specimens were determined to be around 0.3 V.…”

Real-Time Microelectrochemical Observations of Very Early Stage Pitting on Ferrite-Pearlite Steel in Chloride Solutions

“…21,22 The development of this technique led to advances in the past twenty years regarding the understanding of pitting corrosion at non-metallic inclusions in stainless steels. [23][24][25][26][27][28] Chiba et al developed a micro-electrochemical system for in situ high-resolution optical microscopy, which can be used for the morphological analysis of pitting during the very beginning of the initiation process. [29][30][31] In this study, we used this technique to investigate the initiation site of pitting of AISI 1045 ferrite-pearlite steel.…”

Pitting Corrosion of Steel Induced by Al2O3 Inclusions

“…Compared with Al 2 O 3 inclusions, pitting corrosion occurred more easily at the junction of the steel matrix and Mg, Al, Ca composite oxide inclusion. Muto et al [16] conducted pitting corrosion micro-electrochemical experiments of sulfide and oxide inclusions in 304 stainless steel. They observed that MnS/CrS inclusions were more likely to be sites of pitting initiation rather than the MnO/CrO inclusions.…”