In-Situ Fabrication of Tungsten Oxide Film Ph Micro-Sensor and its Application on the Ph Monitoring of Fe-Cu Galvanic Corrosion

Abstract

Localized and interface pH value are important parameters for studying the corrosion progress and complex interface reactions. Although commercial glass pH sensors have been widely used to measure solution pH value due to their advantages of long stability and high sensitivity, they were unable to determine the interface and microregion pH value for their difficulty in miniaturization. In this work, an all-solid-state pH micro-sensor with strong adhesion strength wasin-situ fabricated on diameter 15 μm tungsten ultramicroelectrode (UME) using constant current anodizing. The pH micro-sensor showed good linear response (R2 = 0.999), excellent sensitivity (with a slope of -53.8 mV/pH), quick response (＜0.1 s), long lifetime (＞44 days) and great anti-interference ability to most of the ions in marine (selectivity coefficients pKij＞7), such as SO42−, Cl−, NO3−, K+, Mg2+ and Fe2+. Then, the pH micro-sensor, combined with the potentiometric mode of SECM, was further employed to explore the localized pH value distribution during the galvanic corrosion of Fe-Cu couple in 0.1 M NaCl solution. Results showed that both acidic sites and alkalizing sites appeared on the Fe surface, while homogeneous alkalization took place overall Cu surface. This was consistent with the SVET results that anodic and cathodic areas existed simultaneously on Fe surface. Thus, it could be concluded that local corrosion occurred on Fe surface and only part of the Fe surface acted as the anodic even under galvanic effect. However, Cu acted as cathode with the homogeneously distributing cathodic current on the whole surface during the galvanic corrosion.

Keywords: Ultra-micro pH sensor, Anti-interference ability, galvanic corrosion, Localized corrosion, SECM