IR-compensation in DC Corrosion

When you pass current between two electrodes in a conductive solution, there are always regions of different potentials in the solution. Much of the overall change in potential occurs very close to the surface of the electrodes. Here the potential gradients are largely due to ionic concentration gradients set up near the metal surfaces. There is always a potential difference (a potential drop) caused by current flow through the resistance in the bulk of the solution.

In an electrochemical experiment, the potential that you wish to control or measure is the potential of a metal specimen (called the working electrode) versus a reference electrode. You are normally not interested in the potential drops caused by solution resistances.

The Gamry Instruments potentiostat, like all modern electrochemical instruments, can be setup as a three-electrode potentiostat. It measures and controls the potential difference between a non-current-carrying reference electrode and one of the two current-carrying electrodes (the working electrode). The potential drop near the other current carrying electrode (the counter electrode) typically does not matter when a three-electrode setup is used.

Careful placement of the reference electrode can compensate for some of the IR-drop resulting from the cell current, I, flowing through the solution resistance, R. You can think of the reference electrode as sampling the potential somewhere along the solution resistance. The closer it is to the working electrode, the closer you are to measuring a potential free from IR errors. However, complete IR compensation cannot be achieved in practice through placement of the reference electrode, because of the finite physical size of the electrode. The portion of the cell resistance that remains after placing the reference electrode is called the uncompensated resistance, Ru.

The DC Corrosion software uses current-interrupt IR-compensation to dynamically correct uncompensated resistance errors. In the current-interrupt technique, the cell current is periodically turned off for a short time. With no current through the solution resistance, its IR-drop disappears. The potential drops at the electrode surface remain constant on a short time-scale. The difference in potential with the current flowing and without is a measure of the uncompensated IR-drop.

The DC Corrosion software makes a current-interrupt measurement right after each data point is acquired. It actually takes three potential readings: E1 before the current is turned off, and E2 and E3 while it is off. See the figure below. Normally, the latter two are used to extrapolate the potential difference, ΔE, back to the exact moment when the current was interrupted. The timing of the interrupt depends on the cell current. The interrupt time is 40 µs on the higher current ranges. On lower current ranges, the interrupt lasts longer.

Current-Interrupt Potential versus Time

The software has a second current-interrupt measurement mode that involves averaging of the two points on the decay curve. See the discussion of the Pstat.SetIRuptMode() for more information.

In controlled-potential modes, the applied potential can be dynamically corrected for the measured IR error in one of several ways. In the simplest of these, the IR error from the previous point is applied as a correction to the applied potential. For example, if an IR free potential of 1 V is desired, and the measured IR error is 0.2 V, the DC Corrosion software applies 1.2 V. The correction is always one point behind, because the IR error from one point is applied to correct the applied potential for the next point. In addition to this common mode, the DC Corrosion software offers more complex feedback modes.

By default in the controlled-potential modes, the potential error measured via current-interrupt is used to correct the applied potential. In the controlled-current modes, no correction is required. If IR-compensation is selected, the measured IR error is subtracted from the measured potential. All reported potentials are therefore free from IR error.