R_p measurements with EN in oil-brine mixtures when LPR fails

Measuring the corrosion rate with the linear polarisation resistance (LPR) technique in multiphase flows is often impossible in practice because of the high level of electrochemical noise or because of the non-stationarity of the flows. LPR, electrochemical noise (EN) and impedance measurements were carried out in oil-brine mixtures to determine polarization resistance R_p values, using three nominally identical electrodes in a flow-loop cell so as to approach in-situ monitoring conditions. The potential of the working electrode (WE) was controlled with respect to the reference electrode (RE) with a potentiostat or with respect to the counter electrode (CE) with a zero-resistance ammeter. It was found that no reliable values of R_p could be obtained in the presence of oil with the former configuration because the potential of the WE continuously drifted and could not remain equal to the corrosion potential. With the latter configuration, the WE and CE did not work exactly at their corrosion potential but consistent values of R_p could be measured with the LPR technique or derived from the noise resistance or the noise impedance in stationary flows. For non-stationary flows, in which the electrical continuity between the electrodes was intermittently lost, only the EN technique was able to provide reliable values of R_p.