Pitting Corrosion Behavior at Sulfide Inclusions on Type 304 Stainless Steel with Applied Stress
Authors: Tokuda Shimpei , Muto Izumi , Sugawara Yu , Hara Nobuyoshi 
Published: February 2020

Stainless steels are widely used due to their excellent 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 To ascertain the initiation mechanism of SCC, elucidation of the effect of applied stress on pitting corrosion is necessary. In this research, electrochemical measurements were conducted under applied stress, and the role of applied stress in pitting corrosion of stainless steels was analyzed.
AISI 304 (18Cr-8Ni) austenitic stainless steels were used in this study. A solution treatment was conducted at 1373 K for 0.5 h and then sensitized at 923 K for 2 h. 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 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 4 M MgCl2 and 0.1 M Na2SO4. The pH value of 4 M MgCl2 and 0.1 M Na2SO4 was 4.3 and 5.6, respectively.
To elucidate the role of applied stress in the pit initiation, the distribution density of pit initiation sites was investigated in the immersion tests in 4 M MgCl2 with and without applied stress. The size of the electrode area was changed from ca. 4.3 × 103 µm2 (65 µm × 65 µm) to ca. 1.0 × 106 µm2 (1 mm × 1 mm). In the absence of applied stress, the occurrence of pitting decreased with the size of the exposed area, and the threshold between pitting and no pitting was approximately 5.1 × 104 µm2 (226 µm × 226 µm). Under applied stress, pitting corrosion occurred regardless of the size of the exposed area. After the immersion tests, the pits were found to occur at the sensitized grain boundary by electrolytic etching. Additionally, the residual of the MnS inclusion was observed inside the pit. It is thought that applied stress likely promoted the pit initiation at the MnS inclusion at the sensitized grain boundary. In situ observation during micro-scale polarization was conducted at the electrode area containing a single MnS inclusion to reveal the dissolution behavior of MnS inclusion under applied stress. Before polarization, the color of the MnS surface was gray. Without applied stress, almost no change on the appearance of the inclusion surface was detected during the polarization although the electrode potential increased. On the other hand, the surface color of MnS inclusion changed from gray to black at open-circuit potential under applied stress. Moreover, many round products were observed during polarization, and the number of them increased with electrode potential. The round products were thought to be gas bubbles originated from the MnS dissolution as a by-product. This is evidence that MnS dissolution was accelerated under applied stress. The promotion of pit initiation on Type 304 stainless steels under applied stress contributed to the acceleration of the dissolution of the MnS inclusions. To improve the pitting corrosion resistance of Type 304 stainless steels at sulfide inclusions, the relationship between the inclusion compositions and electrochemical behavior of the inclusions were investigated. It has been reported that CrS, TiS, and Ti-carbosulfide are resistant to pitting in the absence of applied stress.2, 3 Pitting corrosion behavior at sulfide inclusion such as CrS and Ti-carbosulfide under applied stress is also discussed.

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