Corrosion detection of carbon steel in water/oil two phases environment by electrochemical noise analysis

The aim of this paper is to demonstrate the effectiveness of the Electrochemical Noise Analysis to detect the onset of corrosion phenomena in a very high resistivity medium. Tests were carried out on carbon steel electrodes immersed in a water/mineral oil two phases environment with high concentration of CO2, different aqueous/organic phase ratio, sulphide content between 0 and 0.5 g/l and pH between 1 and 5. The evolution of corrosion phenomena were followed by collecting current and potential noise between three nominally identical electrodes. The noise data were analysed in the time and in the frequency domain. In spite of a great loss of sensitivity of the method with respect to tests performed in aqueous solution, the data indicate a good agreement between the standard deviations and the power level of Power Spectra Density (PSD) of current and potential noise signals and corrosion rates by means of weight loss. The values of the PSD slope, indicate the form of corrosion. The effect of water/oil ratio, sulphide concentration and pH on the corrosion rate was determined. Finally two methods to increase the sensitivity of the Electrochemical Noise are proposed.