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 °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 NaCl-NaNO3-KNO3 salt mixture was demonstrated at 150 °C. The average localized corrosion rate measured from the probe with coated electrodes in the NaCl-NaNO₃-KNO₃ 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.

In recent years, multielectrode systems have been extensively used for studying the spatiotemporal electrochemical behaviors of metals in corrosion, especially in localized corrosion environments, and for corrosion monitoring in both laboratories and industrial processes.^1-4 Typical multielectrode systems are made of an array of small electrodes imbedded in an insulation material such as epoxy, in which case they are also called coupled multielectrode array sensors (CMAS).^5-9 The electrode surface area of the multielectrode arrays is typically 0.008 to 0.8 mm² (which corresponds to a circular area with a diameter of 0.1 to 1 mm)-close to the size of a well-developed pit. Thus the multielectrode arrays or multielectrode array sensors often quantitatively measure the localized corrosion rate in laboratories or industrial fields.⁵

To apply Faraday's Law to calculate the corrosion rate, the CMAS must have a well-defined surface area. At low temperatures (<70°C), this requirement can be satisfied by using a proper sealing material such as an epoxy⁴ as the insulator, or applying a proper coating^{1, 9} around the electrodes to achieve a good bonding between the electrode and the insulating material or the coating. At elevated temperatures (e.g., 150 °C), however, it is difficult to find a proper coating or sealing material that would resist a harsh chemical environment such as the acidic NaCl-NaNO₃-KNO₃ salt mixture. Heat-shrink polytetrafluoroethylene (PTFE) tubing has been used to seal and flush-mount the electrodes of multielectrode array sensors¹⁰ for high-temperature applications. However, crevices may exist between the side surface of the electrode and the inner surface of the PTFE tube because there is not strong bonding between the electrode and the PTFE tube. For less corrosion-resistant metals such as carbon steel in untreated cooling water at room temperature or stainless steel in concentrated chloride solution boiling temperature, such crevices may not be a problem because most reactions are expected to take place at the exposed cross section of the electrode.