Plasticizer-free solid-contact pH-selective microelectrode for visualization of local corrosion

• pH-sensitive solid-contact microelectrode was developed to monitor corrosion processes with micro-resolution.
• The pH selective membrane is based on self-plasticized copolymer P(MMA-DMA).
• The developed microelectrode is more robust than traditional glass-capillary microelectrodes.
• P(MMA-DMA)-based membrane demonstrates improved properties relatively to PVC-based one.
• The developed microelectrode is efficient for quasi-simultaneous monitoring of local pH and current density.

The pH-sensitive solid-contact microelectrode (SCME) was developed for micro-potentiometric measurements for corrosion applications. A needle-shaped metallic body of the microelectrode was insulated with the exception of the 5 micron long apex where a layer of conductive polymer, poly(3-octhylthiophene-2,5-diyl) and then a pH sensitive membrane was deposited. The pH-selective membrane was based on plasticizer-free poly(methylmetacrilate–decylmetacrilate). The pH sensitivity was imparted on the membrane by a pH ionophore 4-nonadecylpyridine. The developed microelectrode is more robust than traditionally used glass capillary microelectrodes as it does not include any liquid in its construction nor is it as fragile. Thus, the main disadvantages of glass capillary microelectrodes, most importantly fragility and leakage of the liquid membrane, were eliminated. The properties of the improved microelectrodes, important for corrosion applications, were characterized. These include the slope, long- and short-term stability and reproducibility of potential, detection limits, selectivity coefficients, response time and life-time of the microelectrodes. The developed microelectrodes showed stable potential and short response time assuring reliable results during scanning measurements. Corrosion of hot dip galvanized steel sample was monitored by measuring local pH and current density quasi-simultaneously. The designed SCME showed its efficiency for practical applications for localized, scanning measurements in the field of corrosion.