2018 Volume 67 Issue 9 Pages 381-385
In-situ measurement of electrical conductivity of solution within crevice of SUS316L stainless steel in 288℃ water has been conducted with newly developed electrochemical sensor system. The sensor consists of electrode （φ≈ 250μm stainless steel） and insulator （high purity alumina） directory imbedded into crevice former plate. The sensor measures local electrical conductivity of crevice solution beneath the electrode （κcrev） with electrochemical impedance method. The sensors were installed at different positions within tapered crevice of SUS316L stainless steel. The crevice specimen with the sensors were immerged into 288℃, 8 MPa, pure oxygen saturated high purity water for 100 h. κcrev at a position with crevice gap（g） of ≈ 59.3μm was 8～11μS･cm－1, least deviate from conductivity of 288℃ pure water （3.7μS･cm－1） and no localized corrosion occurred. On the contrary, κcrev at a position with g ≈4.4μm increased with time and showed maximum value of ≈1600μS･cm－1 at 70 h. Localized corrosion occurred in the vicinity of this position. Thermodynamic equilibrium calculation based on oxide in the crevice and the maximum κcrev（1600μS･cm－1） showed that pH of crevice solution can be reached to 3.53. It can be concluded that acidification occurred in tight crevice even under high purity bulk water and resulted in localized corrosion.
