CHARACTERISTICS OF POTENTIAL MEASUREMENTS IN THE FIELD OF AC CORROSION

The problems observed in the instantaneous off potential measurement were attributed to a non-sufficient time resolution. By increasing the data acquisition rate up to 2 MHz it was possible to determine the polarization of the metal surface within 2 µs after disconnecting the coupon from the ac and dc power source. This increase in resolution represents an acceleration of the measurement for a factor of thousand compared to the commonly used instruments. However, it was found that the depolarization of the surface is faster than 1 µs in calcium ion free soil solution. As a consequence, the threshold of-0.85 V CSE was not exceeded although ac corrosion occurs on the metal surface. In the presence of calcium ions, however, the threshold is exceeded due to a different depolarization behavior. As the depolarization and therefore the interpretation of the instantaneous off potential measurement depends strongly on the composition of the soil it cannot be recommended for the characterization of coupons in field applications. In order to address the influence of the soil composition and to investigate the chemical processes occurring on the coupon surface, continuous data acquisition was installed in two locations in Switzerland. The results of the first 6 months of data acquisition are presented