Development of Crevice-Free Electrodes for Multielectrode Array Sensors for Applications at High Temperatures

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 150 degrees C in corrosive environments. however. is limited by degradation of the insulating mounting material. such as epoxy. Degradation of the mountng material can lead to crevice corrosion and causes an increase in the exposed surface area of the electrodes. In this study. the effectiveness of a diamond-like carbon film to protect the Alloy 22 (UNS N06022) electrodes in an acidic sodium chloride-sodium nitrate-potassium nitrate (NaCl-NaNO3-KNO3) salt mixture was demonstrated at 150 degrees C. The average localized corrosion rate measured from the probe with coated electrodes in the NaCl-NaNO3-KNO3 salt mixture at 150 degrees C was approximately 0.43 mu m/y. 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. Posttest 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-mouruing material.