Development of Electrochemical Sensors for High-Temperature
Corrosion Monitoring, 20-R9727

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Principal Investigators
Kuang-Tsan K. Chiang
Lietai Yang
Ronghua Wei
Kent Coulter

Inclusive Dates:  07/02/07 – Current

Background - The existing multielectrode sensors are promising devices for application as an online tool for corrosion monitoring. However, these devices have an upper operating temperature limit of approximately 70 °C. The sensor has an array of sensing electrodes that are made from the material of interest. The electrodes are electrically insulated from each other with a mounting material (epoxy) but coupled by an external circuit and arranged so that a small area of the electrode contacts a corrosive environment. The anodic current from each electrode is measured and used as the signal to indicate localized corrosion. Applications of the existing sensor at temperatures above 70 °C, however, are severely limited by degradation of the epoxy on the sensing electrodes. Crevice formation between the electrode and the epoxy material results in erroneous corrosion rate measurements. As a result, the existing corrosion monitoring system has not been used as quantitative real-time sensor for highly corrosion resistant metals at elevated temperatures. In this focused IR&D project, we are developing a new electrochemical corrosion monitoring system that has high-temperature (>100 °C) capability. By depositing a novel diamond-like amorphous carbon coating on the sensing electrodes our new method will prevent crevice formation at elevated temperatures.

Approach - The diamond-like carbon (DLC) thin film was deposited using a SwRI unique vacuum-based deposition technique—plasma immersion ion deposition (PIID) method. Our specific objectives are to (1) Develop deposition parameters for adherent, pinhole-free, DLC coatings on corrosion-resistant materials, such as Alloy 22, and other alloys commonly used in the high-temperature industrial systems, (2) Characterize the microstructure, electrical impedance, and corrosion resistance properties of the coating, (3) Validate that the coated electrochemical sensor can be used for online corrosion monitoring at high temperatures (e.g., 150 °C or higher) in simulated hostile environments, such as a NaCl-NaNO3-KNO3 salt mixture, and (4) Demonstrate that the coating processes and the fabrication method can be scaled up for full-scale sensor production.

Accomplishments - The requirements for high-temperature electrochemical sensors for applications in both acidic and caustic corrosive environments were assessed. The PIID method was selected for depositing a DLC coating on the wire electrodes. Coating process parameters for depositing DLC coating on wire electrodes have been developed. Silicon (Si) was selected as the interface bond layer between Alloy 22 (Ni-22Cr-13Mo-3Fe-3W) wire substrate and the DLC thin film to enhance coating adherence. The coating properties were characterized by a number of surface analysis techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and laser Raman spectroscopy. The effectiveness of the DLC coating to produce a crevice-free electrode was demonstrated in an Alloy 22 coupled multielectrode array sensor in a saturated solution containing a NaCl- NaNO3-KNO3 salt mixture at 150 °C. Figure 1 presents cross-sectional SEM micrographs of an uncoated and a DLC-coated electrode after exposure to a saturated NaCl-NaNO3-KNO3 solution for 7 days at 150 °C. Indeed, as expected, crevices formed between the uncoated Alloy 22 wires and the epoxy mounting material (Figure 1a). In contrast, the DLC coating remained intact and prevented crevice formation between the Alloy 22 and the mounting material (Figure 1b). Fabrication of DLC-coated Alloy 600 (Ni-15.5Cr-8Fe), aluminum, and titanium wire electrodes, as well as corrosion testing of the these new sensors are in progress.

Figure 1. Post-test appearance of the (a) uncoated electrode and (b) DLC-coated electrode after exposure to a saturated solution containing a NaCl-NaNO3-KNO3 salt mixture for 7 days at 150 °C.

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