オーステナイト系ステンレス鋼の中性塩水溶液中におけるアノード分極挙動と界面インピーダンスの関係

抄録

The impedance of stainless steel electrodes in neutral aqueous solutions containing sulphate and chloride ions have been measured under anodic polarization. The types of steels used are 18Cr-8Ni, 17Cr-13Ni, 17Cr-13Ni-3Mo, 20Cr-25Ni, 20Cr-25Ni-1Mo, 20Cr-25Ni-3Mo, 20Cr-25Ni-5Mo and 18Cr. The conclusions obtained are summarized as follows.
(1) In 1M-Na₂SO₄ solution, two minimum values (at+0.40V and +0.95V vs. SCE) and one maximum value (at +0.75V vs. SCE) appear in the capacitive component-potential curves of all the specimens used, and these curves can be devided into four regions at these potentials, i. e. region I: -0.18V (corrosion potential)-+0.40V, region II: +0.40V- +0.75V, region III: +0.75- +0.95V, and region IV: +0.95V- +1.50V (V vs. SCE). It is thought that the region I corresponds mainly to the growth of passive films with potential and the regions II to IV probably resulted not only from the changes of passive films but also from the changes of Faradaic impedances caused by the dissolution of electrodes.
(2) In 1M-NaCl solutions, the pitting potentials of the steels used, except for the 20Cr-25Ni-5Mo steel, are wholly in the region I. The 20Cr-25Ni-5Mo steel does not sustain pitting corrosion by the anodic polarization in IM-NaCl solution, and in this case all regions I to IV appear in the capacitive component-potential curves. The changes of capacitances in the region II and III, however, are not clear in comparison with the changes in the same regions obtained with the 20Cr-25Ni steel in 1M-Na₂SO₄ solution. The same phenomenon is found in the capacitive component-potential curve of 17Cr-13Ni steel obtained in the solution of 0.1M-NaCl+0.2M-Na₂MoO₄. These phenomena are thought to result from the adsorption of MoO₄^2- ions.