Electrochemical methods in combination with in-situ microscopy were considered for studying precursor sites for pitting on nickel alloy UNS N07718 in chloride-containing environments at high temperatures. Therefore, the optimum size of the working electrode and a suitable pseudo-reference electrode must be selected for the electrochemical setup. Simultaneous in-situ micro-visualization and electrochemical testing was done on small electrodes from UNS N07718 immersed in artificial seawater at room temperature. The tests were performed using millimeter-size electrodes (minielectrodes) and micron-size electrodes (microelectrodes). Passivity with little or no metastable activity was observed during potentiodynamic polarization tests conducted using the microelectrodes. In contrast, several metastable events were observed in the minielectrodes during potentiodynamic polarization tests. The differences encountered when using a platinum (Pt) pseudo-reference electrode versus the Ag/AgCl reference electrode were established. A shift of 100 mV or more in the corrosion potential and in the potential where metastable pitting initiated was obtained for the Ag/AgCl reference electrode compared to the values obtained when using a Pt pseudo-reference electrode.

Subject
Scanning electron microscopy, Test methods, Materials, Microelectrodes, Microscopy, Precipitates, Reference electrodes, Artificial seawater, Pitting, Alloys, Electrodes, Electrode potential, Inclusions
Keywords:
Pitting, Critical Pitting Temperature, UNS N07718, Electrochemical Noise, Potentiodynamic
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2018
Association for Materials Protection and Performance (AMPP)
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