In the aqueous corrosion of metals such as aluminum, local changes in pH often occur close to the corroding surface. Measurement of these values is useful in interpreting corrosion mechanisms.
A microelectrode assembly has been developed for this specific purpose. A membrane of pH-sensitive glass is fused on the end of drawn-down capillary glass tubing, of 0.5 to 1 mm diameter. Silver is plated on the outside of the capillary and partially converted to silver chloride or silver iodide. In distilled water, the silver-silver chloride and the silver-silver iodide electrodes are found to act as stable reference electrodes. Reproducible and normal, or nearly normal, response of the electrode assembly to pH are observed. Readings can readily be made as close as 0.1 mm from the metal surface.
Variations in pH adjacent to 1100 aluminum corroding in distilled water at 50 C are reported as functions of time and position, both lateral and normal to the surface.
