(Invited) Relationship between Protective Film on Sulfide Inclusions and Pitting Corrosion Resistance on Type 304 Stainless Steel Under Applied Stress
Authors: Tokuda Shimpei , Muto Izumi , Sugawara Yu , Hara Nobuyoshi 
Published: December 2020

Stainless steels are widely used due to their superior corrosion resistance. In actual applications, stainless steels tend to suffer from localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride environments. Among others, SCC is dangerous because its growth rate is often rapid and SCC readily causes material failure due to a combination of a material and a corrosive environment under tensile stress. Pitting corrosion is well known to be the initiation sites for SCC.1 Previous research shows that the chemical compositions of sulfide inclusions play an important role in preventing pitting corrosion on Type 304 stainless steels. 2, 3 Muto et.al. reported that CrS and Ti4C2S2 inclusions have higher pitting corrosion resistance than MnS inclusion without stress. 2, 3 However, the pitting corrosion resistance of the sulfide inclusions under applied stress remains unclear. In this research, electrochemical measurements were conducted for stainless steels with different sulfide inclusions with applied stress, and the pitting corrosion resistance of the sulfide inclusions under applied stress was analyzed.
Three types of Fe-18Cr-8Ni austenitic stainless steels (Type 304) were used in this study. Each stainless steel has MnS, CrS, and Ti4C2S2 inclusions, respectively. A solution treatment was conducted at 1373 K for 0.5 h, and then water-quenched. After the heat-treatment, the surfaces of the specimen was mechanically ground with 1500-grit SiC paper and polished down to 1 μm with a diamond paste. The surfaces of the specimen, except for the electrode area, was covered with a resin. Tensile tests were performed using a Beben microtest 5 kN module that was a horizontal type tensile test machine. In the air, the tensile stress increased to 180 MPa (=75% of the 0.2% proof stress of the steels) and then was kept constant. After that, a small acrylic cell was attached to the gauge section of the specimen surface with a micro-scale electrode area. Micro-scale electrochemical measurements and immersion tests were carried out in 0.1 M MgCl2.
To reveal the relationship between the inclusion compositions and electrochemical behavior of the inclusions, potentiodynamic polarization was conducted for Type 304 stainless steels with different sulfide inclusions with and without applied stress. The pitting potential of the CrS and the Ti4C2S2 inclusions was higher than that of the MnS inclusion. The polarization was repeated three times for the stainless steel with the Ti4C2S2 inclusion, no pitting was observed for two times until 1.4 V vs. Ag/AgCl (3.33 M KCl) even under applied stress. After polarization, the specimen surfaces were observed by SEM, and sulfide inclusions were found inside the pits for all the specimens. The MnS, CrS, and Ti4C2S2 inclusions were thought to act as the pit initiation sites. Moreover, the oxide films were formed on the Ti4C2S2 inclusions after polarization. The potential-pH diagrams were calculated to ascertain the possibility of oxide film formation on the inclusion surfaces and compare the thermodynamic stability of MnS, CrS, and Ti4C2S2 inclusions. Ti4C2S2 is stable at lower potentials in the all pH range, and the experimental conditions of the polarization corresponded to the TiO2 region. The high pitting corrosion resistance of the Ti4C2S2 inclusion is attributed to the formation of TiO2 on the Ti4C2S2 inclusions. The pitting corrosion behavior at sulfide inclusion such as CrS and Ti4C2S2 under applied stress was discussed.

Reference

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I. Muto, S. Kurokawa, and N. Hara, J. Electrochem. Soc., 156, C395-C399 (2009).

3. N. Shimahashi, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C262-C269 (2013).