18Cr-8Niステンレス鋼の孔食および間隙腐食に関する電気化学的研究

Abstract

In order to understand the electrochemical mechanism of pitting and crevice corrosion of 18Cr-8Ni stainless steel, the electrochemical behaviour of 18Cr-8Ni stainless steel on the open surface and that in the crevice were studied in neutral sodium-chloride solution by means of a potentiostatic technique, preventing the crevice corrosion under insulation coating by prepassivation of the specimen. The presence of the pit-initiation potential, critical pitting potential and crevice corrosion potential was made clear. The results obtained may be summarized as follows. (1) Both the pit-initiation potential and the crevice corrosion potential of 18Cr-8Ni stainless steel in 0.1 M NaCl solution at 25°C were estimated at ca. +0.30 V (Eh), and the critical pitting potential at +0.70 V (Eh). (2) In the potential region between the pit-initiation potential and the critical pitting potential, a frequent oscillation of current was observed on account of the pit-initiation and repassivation, which was not affected by the dissolved oxygen concentration and the stirring of the solution. (3) The pit-initiation potential and the crevice corrosion potential depended upon the potential sweep rate and were not good in reproducibility, whereas the critical pitting potential was almost independent of the potential sweep rate and in a good reproducibility. (4) Increasing the NaCl concentration causes the pit-initiation potential, the critical pitting potential and the crevice corrosion potential to shift to a more negative potential. (5) The protection potentials of pitting and crevice corrosion were estimated at +0.3 and +0.03 V (Eh), respectively.