The differential aeration cell and the corrosion paradox

Abstract

Steel with a low volume of both anolyte and catholyte was used for monitoring of steel electrode corrosion rates under conditions in the environment of a differential aeration cell under the conditions of both stagnating and exchanging electrolyte. A 3% NaCl solution was used as the electrolyte. A low content of oxygen near the anode was ensured by purging the solution with nitrogen while a high content of oxygen near the cathode was obtained by saturating the solution with oxygen. All experiments were performed for 72 h. Corrosion rates of steel electrodes were identified by using the weight loss method in comparison with the resistometric method. The corrosion rate of an aerated electrode drops in stagnating electrolyte to a hundredth of millimetre per year (0.01 mm/a) as a result of the catholyte alkalization and subsequent surface passivation, and that it is lower than the corrosion rate of a non‐aerated electrode (0.12 mm/a). In a stagnating solution, the catholyte is alkalized (pH 11) whereas acidification of the anolyte is negligible. In the case of continuous exchange of the aerated catholyte and the oxygen‐depleted anolyte in the environment of a differential aeration cell, the cathode's corrosion rate is up to seven times higher (0.7 mm/a) than the corrosion rate of the anode (0.1 mm/a). The difference between the electrodes' corrosion rates is given by the IR drop in the electrolyte between the cathode and anode. It was experimentally proved that in the case of continuous electrolyte exchange in the differential aeration cell the cathode corrodes faster than the anode and that no corrosion paradox is observed.