EFM Current and Voltage Conventions

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 Electrochemical Frequency Modulation (EFM) software’s default 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 EFM software, the equilibrium potential at a metal’s surface 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 net 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 experiments is changes in the working electrode’s surface during the scan.

In the EFM, 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 off 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.