“Null” Methods Applied to Corrosion Measurements

The use of null methods to eliminate IR drops from measurements of electrode potential in the presence of polarizing current furnishes a new tool for corrosion studies. When IR drop corrections are made in the null circuit by using a.c., polarization potentials are measured to the outside of the “electrode boundary.” When increments of d.c. are used instead of a.c., the effects of film resistance are also nullified. It is possible to measure the film resistance directly. Following Müller's picture of the local action theory preliminary results indicate that the two polarization curves and the film resistance curve may furnish us criteria from which the amount of local action current can be calculated. Arguments in favor of such measurements are given, along with results of experiments.