Monitoring corrosion and flow characteristics in oil/brine mixtures of various compositions

The feasibility of monitoring corrosion processes by means of electrochemical impedance and noise measurements in paraffin oil/brine mixtures of various compositions between 0 and 80 vol % of oil was studied. Controlling the electrode potential in low-conductivity solutions was a major difficulty because of the drift of the corrosion potential that progressively polarized the electrode. This drift was at the origin of some uncertainty in the measurement of the impedance and current noise power spectral density. In contrast, electrolyte resistance (ER) measurements, performed with a home-made device delivering an analog signal allowing the mean value as well as the fluctuations of the ER to be measured in real time, were not sensitive to the corrosion potential drift. Measurements could be carried out for volumetric oil contents (VOC) ≤ 80%, above which the potentiostat could not control the electrode potential due to intermittent interruptions of electrical continuity in the solution. The power spectral density (PSD) of the ER fluctuations increased monotonically with the VOC on the whole frequency range, on > 6 amplitude decades. Thus, in steady-state hydrodynamic conditions the level of the PSD at a given frequency allowed the VOC to be assessed, and thus the aggressiveness of the oil/brine mixture to be estimated indirectly. Analysis of the ER fluctuations gave information on the hydrodynamic flow pattern close to the electrode.