Current and Voltage Conventions in DC Corrosion

A current value of –1.2 mA can mean different things to people in different areas of electrochemistry. To a corrosion scientist it represents 1.2 mA of cathodic current. To an analytical electrochemist it represents 1.2 mA of anodic current.

In the DC Corrosion’s standard techniques we follow the corrosion convention for current. Positive currents are anodic, resulting in an oxidation at the metal specimen under test. Potentials can also be a source of confusion. In the DC Corrosion software, the equilibrium potential assumed by the metal in the absence of electrical connections to the metal is called the open-circuit potential, Eoc. We have reserved the term corrosion potential, Ecorr, for the potential at which no current flows, as determined by a numerical fit of current-versus-potential data. In an ideal case, the values for Eoc and Ecorr are identical. One reason the two voltages may differ in real systems is changes in the electrode surface during the scan. In the DC105, all potentials are specified or reported as the potential of the working electrode with respect to either the reference electrode or the open-circuit potential. The former is always labeled as “vs Eref” and the latter is labeled as “vs Eoc”.

The equations used to convert from one form of potential to the other are

E vs Eoc = (E vs Eref) – Eoc

E vs Eref = (E vs Eoc) + Eoc

Regardless of whether potentials are versus Eref or versus Eoc, one sign convention is used. The more positive a potential, the more anodic it is. More anodic potentials tend to accelerate oxidation of a metal specimen.