R_p measurements: Potentiostatically or galvanostatically?

The estimation of corrosion current in field applications is mostly based on the Stern-Geary relationship and direct measurement of the polarization resistance R_p mainly with the linear polarization resistance technique but also with the electrochemical impedance technique. Technical advances in the last years effectively made available very simple and practical devices for field corrosion engineers. In spite of their convenience, the measurements reliability is based upon basic assumptions, such as the system linearity for example, which are not a priori ensured and can be the source of measurement errors. One of these drawbacks is related to the drift of the corrosion potential during the measurements. The present paper investigates the influence of the non-stationarity of the system on R_p measurements. Values of R_p measured with the electrochemical impedance and linear polarization resistance techniques under potentiostatic or galvanostatic control were compared in the case of iron corrosion in stagnant and circulating brine solutions. It is shown that the galvanostatic control at a zero current gives more stable and hence reliable R_p values than the ever-used potentiostatic control, in which the applied potential progressively differs from the actual corrosion potential during the measurement. It can then be seen as a more adequate interface control in non-stationary corrosion applications.