2000 Volume 49 Issue 2 Pages 99-105
The working group in JSCE : “Detection of the localized corrosions by the electrochemical noise” carried out a cooperative measurement by the group members, to investigate the potential noises on the condition under which the cracks occurred in the corroding crevice. A crevice corrosion specimen with two type-304 stainless steel slips that were spot-welded in each other was used for the specimen. 3.5 mass% NaCl solution at 80°C was employed under air exposed condition for the test solution. Potential noises were measured with an electrometer and a reference electrode every 0.5 s in typically.
From the measurement results, it was found a potential noise of a rapid shift to the noble side followed by slow recovery occurred in the condition under which the corrosion cracks were formed inside the crevice. This fluctuation pattern-calls RR-type-is a symmetrical to the pattern of the potential noise by localized corrosions that were formed on the free surface. The RR-type noise was not observed in the condition under which only the crevice corrosion occurred without the cracking.
To investigate the generation mechanism of the RR-type noise, the potential noise of the specimen was measured in scratching the specimen surface. And also, the short circuit current noise between the specimen and the counter electrode of type-304 stainless steel was measured in the same condition. Consequently, it was estimated the origin of the RR-type noise was hydrogen evolution reaction at the bare surface after the film breakdown by a crack formation, because cathodic local current, not anodic local current was required to make the potential shift noble side transitionally. The potential of inside a corroding crevice is a less noble level, and the pH in there is a lower value, so that hydrogen evolution reaction is easy to occur inside the crevice.