High-Temperature Electrochemical Sensor for Online Corrosion Monitoring

Electrochemical sensors incorporating diamond-like carbon (DLC)-coated electrodes are effective tools for online, real-time corrosion monitoring at high temperatures. A vacuum-based chemical vapor deposition process was used to deposit a DLC thin film coating on the surface of the sensing electrodes. The chemical inertness of the DLC coating on the sensing electrodes produced a crevice-free electrode that can be used at temperatures above 100°C. In this paper, we present the coating deposition process for sensor electrodes and fabrication of coupled multielectrode array sensors (CMAS) for high-temperature, high-pressure applications. Probes were fabricated using uncoated and DLC-coated Alloy 22 (Ni-22Cr-13Mo-3Fe-3W [UNS N06022]) and titanium Grade 7 (Ti-0.2Pd [UNS R52400]) electrodes. The Alloy 22 probes were tested in high-temperature, high-pressure conditions in a pH 10 caustic solution. The measured nonuniform corrosion rate using the Alloy 22 probe with the DLC-coated electrodes was approximately 2 μm/y. The titanium Grade 7 probes were tested in a dilute sodium chloride (NaCl) solution saturated with hydrogen sulfide-carbon dioxide (H2S-CO2) at high pressures in the temperature range from 39°C to 81°C and in a saturated solution containing sodium chloride-sodium nitrate-potassium nitrate (NaCl-NaNO3-KNO3) at 150°C. The stabilized localized corrosion rates for probes with or without the DLC coating were less than 1 μm/y in the H2S-CO2 environment, and less than 0.03 μm/y in the NaCl-NaNO3-KNO3 salt mixture at 150°C. The test results indicate that titanium Grade 7 is not subject to crevice corrosion under the testing conditions.