Stainless steels are widely used in structural applications due to their superior corrosion resistance. However, stainless steels sometimes suffer from pitting corrosion and stress corrosion cracking in chloride environments. It was reported that applied stress promoted pitting corrosion and changed the morphology of pitting corrosion ¹. Moreover, it is known that stress corrosion cracking is often initiated at pits in stress environment ². Elucidating the effect of applied stress on pitting corrosion behavior is important to make clear the initiation mechanisms of the stress corrosion cracking of stainless steels. As yet, however, the effect of applied stress on pitting corrosion behavior remains unclear.

　A commercial AISI 304 (18Cr-8Ni) austenitic stainless steel sheet was used in this study. The chemical composition is shown in mass percentages: 0.06% C, 0.39% Si, 1.11% Mn, 0.029% P, 0.0026% S, 8.02% Ni, 18.0% Cr, 0.13% Mo, 0.22% Cu, 0.002% Ti, 0.002% Al, 0.038% M, 0.003% O, Fe bal. The 0.2% proof stress of the steel was 222 MPa. This steel was heat-treated at 1373 K for 30 min and then quenched in water as solution treatment. Sensitization was conducted by annealing the steel at 923 K for 2 h and then quenching in water after the solution treatment. After these heat-treatments, the surfaces of specimens were successively polished with a diamond paste down to 1 μm.

　Potentiodynamic anodic polarization measurements were carried out in 0.01 M, 0.1 M, 1M, 4 M MgCl₂ solution to clarify the effect of chloride concentration on pitting corrosion. All the potentials reported in this study refer to an Ag/AgCl (3.33 M KCl) electrode. The electrode areas were ca. 10 mm ×10 mm. The potential scan rate as was set at 23 mV / min. Figure 1 shows the polarization curves of the solution-treated steel under no applied stress, and the morphology of pitting corrosion after polarization. As shown in Fig. 1a, the pitting potential decreased with chloride concentration. In 0.01 and 0.1 M MgCl₂ solutions, a small pit with a lacy metal cover was initiated. On the other hand, a large shallow pit was generated in 4 M MgCl₂.

　To analyze the effect of applied stress on the pitting corrosion behavior and the morphology of pitting corrosion, micro-electrochemical measurements were utilized. As a result of preliminary experiments of solution-treated steels, pitting corrosion always occurred in 4 M MgCl₂ solution when the electrode area was 1 mm × 1 mm. Therefore, experiments were conducted at the condition that the electrode area was 1 mm × 1 mm.

　The effect of applied stress on the pitting corrosion behavior was analyzed in 4 M MgCl₂ at 298 K. During potentiodynamic anodic polarization, the potential scan was stopped after pit initiation. After that, the specimen was left at open circuit potential for 2 days under applied stress. The applied stress was 80% of 0.2% proof stress. Figure 2 shows the change of the corrosion potential with time. It was observed that the corrosion potential increased with time. From the surface observation, no crack occurred in this study. It was suggested that the effect of temperature is also the important factor affecting the initiation of the pit-induced stress corrosion cracking of austenitic stainless steels.

References

1. N. Shimahashi, I. Muto, Y. Sugawara, N. Hara, J. Electrochem. Soc., 161 (2014), C494-C500
2. H. Masuda, Corros. Sci., 49 (2007), 120-129