Stationary cathodic current measurements in a thin layer cell : application to mass transport study in confined mediums.

Abstract : In many practical situations, electrochemical processes occur within restricted volumes of electrolyte. This case is notably encountered during the corrosion of the armour steel wires in the annulus space of flexible offshore pipelines (V/S values would be in the order of 0.03 mL/cm 2). In such small electrolyte volumes, it has been widely reported that the results classically obtained in bulk conditions (infinite electrolyte volume) do not necessarily remain valid. However, to our knowledge, the continuous monitoring of well controlled corrosion experiments in similar conditions as those encountered in the annulus (V/S, CO 2 /H 2 S) remains unrealised. In order to investigate the influence of confinement on the corrosion behaviour of metallic materials, an innovative thin layer electrochemical cell was developed. This cell includes a device which allows a continuous gas supply within the electrolyte film covering the metallic specimen under study. A methodology based on high frequency impedance measurements is also proposed to control and precisely adjust the setting of the electrolyte film geometry. The experimental setup was validated by performing measurements of stationary limiting cathodic current on a steel disk immersed in a K 2 SO 4 aerated solution for various electrolyte layer thicknesses. The results obtained demonstrate the efficiency of the gas supply method and the high accuracy of the positioning procedure. This work provides an improved thin-layer electrochemical cell suitable for further studies dealing with the corrosion behaviour of steels in confined electrolytes as for instance acidic aqueous solutions encountered in the oil and gas industry. 2