DEVELOPMENT OF CREVICE-FREE ELECTRODES FOR MULTIELECTRODE ARRAYSENSORS FOR APPLICATIONS AT HIGH TEMPERATURES

摘要

Coupled multielectrode array sensors (CMAS) have been used as online and real-time monitors for localized corrosion in both laboratory and field systems. In a CMAS probe, multiple miniature electrodes made of materials identical to the engineering component are used as sensing electrodes. The application of CMAS at high temperatures up to l50°C in corrosive environments, however; is limited by degradation of the mounting material, such as epoxy, that allows crevice formation and causes an increase in the exposed surface area of the electrodes and the initiation of crevice corrosion. In this study, the effectiveness of a diamond-like carbon film in protecting the Alloy 22 electrodes in an acidic NaCI-NaNO_3-KNO_3 salt mixture was demonstrated at 150 °C. The average localized corrosion rate measured from the probe with coated electrodes in the NaCI-NaNO_3-KNO_3 salt mixture at 150 °C was approximately 0.43 μm/yr. In contrast, the. average localized corrosion rate measured using uncoated Alloy 22 electrodes was approximately two orders of magnitude higher than that measured from the probe with coated electrodes. Post test scanning electron microscopy (SEM) analysis and ,cyclic polarization tests revealed that the coating was adherent and prevented crevice formation between the Alloy 22 electrode and the epoxy mounting material.