2022 年 63 巻 5 号 p. 726-729
Duplex stainless steel (DSS) can be used in harsh environments owing to its excellent strength and corrosion resistance. However, the sigma (σ) phase, which reduces the corrosion resistance, precipitates during manufacturing. To elucidate the deterioration mechanism of corrosion resistance, we prepared three types of DSS (F55) with different volume fractions of the σ phase. Polarization curves were measured to investigate the corrosion characteristics. The corrosion rate and corrosion potential (Ecorr) were evaluated using Tafel extrapolation. Owing to the precipitation of the σ phase, Ecorr was low, and the corrosion rate was high, which indicates that the σ phase precipitation promotes the anodic reaction. An electricity damage corrosion test at a constant quantity of electricity was conducted on the sample, which was heat-treated at 1173 K and had the largest amount of σ phase precipitation. The X-ray diffraction results before and after the corrosion test showed that the alpha (α) phase was reduced by corrosion. In addition, the results of optical micrographs and electron probe microanalysis maps confirmed that the σ phase was precipitated in the α phase. This indicates that the precipitation of the σ phase increases the corrosion rate of the surrounding α phase. Therefore, the precipitation of the σ phase, which is harmful to DSSs, deteriorates the corrosion resistance because the periphery of the σ phase is easily corroded.
