Stainless steels are widely used due to their superior corrosion resistance. In the actual applications of stainless steels, sensitization is sometimes caused by welding. When Cr-depleted zones are formed, intergranular stress corrosion cracking (IGSCC) readily occurs at heat affected zone in chloride environments under applied stress. Pitting corrosion is well known to be the initiation site for SCC ¹. Elucidation of the effect of applied stress on pitting corrosion is necessary to ascertain the initiation mechanism of IGSCC. In this research, electrochemical measurements were conducted under applied stress, and the effect of applied stress on pitting corrosion of sensitized stainless steels was analyzed.

Two types of commercial AISI 304 (18Cr-8Ni) austenitic stainless steel sheets were used in this study (low S and re-sulfurized steels). The chemical compositions are shown in Table 1 (mass %). After solution treatment (1373 K, 30 min, W.Q.), the specimens were subjected to sensitization treatment (923 K, 2 h, W.Q.). After these heat-treatments, the surfaces of the specimens were successively polished with a diamond paste down to 1 μm. The surfaces of the specimen, except for the electrode area, were covered with 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 was increased to 180 MPa (=80% of the 0.2% proof stress of the steels) with a crosshead speed of 0.02 mm min⁻¹, and then kept constant. After that, in order to preserve the electrolyte, a small acrylic cell was attached to the gauge section of the specimen surface with a micro-scale electrode area. Electrochemical measurements were performed at 180 MPa.

To elucidate the effect of applied stress on pit initiation of sensitized Type 304 (low S), immersion tests in 4 M MgCl₂ (pH was adjusted at 5.0) were conducted under applied stress for 24 h. Electrode areas were changed from 100 to 500 square µm. In the absence of applied stress, the critical electrode area in which pitting corrosion was observed was ca. 230 square µm. Under applied stress, pitting corrosion was observed even when the electrode area was ca. 100 square µm. The critical electrode area required for pit initiation decreased by applied stress, indicating applied stress promotes pit initiation. Previous research suggested that the pit initiation mechanism can be divided into three steps: acidification around sulfide inclusion; depassivation of stainless steel; and local active dissolution of stainless steel². In situ observations were conducted during potentiodynamic polarization in 0.1 M Na₂SO₄ to elucidate the dissolution behavior of the sulfide inclusion of Type 304 (re-sulfurized). Around the corrosion potential, the dissolution of the sulfide inclusion was promoted under applied stress. The depassivation pH values in naturally aerated in 1 M MgCl₂ were measured with and without stress. The pH of the solution was lowered stepwise with HCl. Depassivation pH without and with stress were 0.6 and 0.8, respectively. It is thought that the stainless steel was readily depassivated under applied stress. To investigate the rate of active dissolution, potentiodynamic polarization was carried out in 4 M MgCl₂ at pH 0.0. However, no difference in current densities was observed without and with applied stress. It is supposed that applied stress affects depassivation pH of the steel matrix and accelerates the MnS dissolution, and hence pit initiation is seemed to be promoted by applied stress.

References

1. H. Masuda, Corros. Sci., 49, 120–129 (2007).
2. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, 511–520 (2013).