Potential and pH Measurements in a Crevice of the Steel BS 4360 50D in 3·5% NaCl and in Artificial Sea Water

Abstract

An artificial crevice has been developed which enables the measurement of the pH and potential to be made at three locations in the crevice. Experiments have been conducted using the steel BS 4360 50D in 3·5% NaCl and in artificial sea water. The crevice specimen was coupled to a steel section of much larger area located in the bulk solution, and the pH and electrode potential within the crevice monitored as afunction of the external electrode potential which was controlled potentiostaiically. The potential drop in the crevice was significant under anodic polarisation conditions but under cathodic polarisation the potential drop was less than afew mV despite polarising to about −1 V(SCE). The pH in the crevice was always in excess of 6, the bulk solution value for 3·5% NaCl, for potentials in the crevice more negative than −660mV(SGE). The values of pH in the crevice obtained in sea water differedfrom those in 3·5% NaCl only at pH values ≳9, where the pH was observed to be somewhat lower in sea water.