Measurement of Corrosion Kinetics by Coulometric Detection Method

The paper presents the results of comparative measurements of corrosion rate by Coulometric Detection of Corrosion Products (CDCP method) and voltammetry (VA). The corrosion kinetics was measured in the time interval where the steady potential Est is established in the “carbon steel – 0.5 M NaCl solution” system as an example. The results of measurements by both methods showed a considerable difference in mass losses in the time interval of specimen surface activation. This fact was explained taking the semiconducting nature of iron oxides into account. Thus, the disagreement may originate from different conductance mechanisms of the metal and its oxides in a strong electric field. The CDCP method was used to determine the conditions required for agreement of the results of potentiodynamic and coulometric measurements. Application of the measuring capabilities of the CDCP method to determine the amount of corrosion products allowed to: 1. record the kinetics in the microgram range, both during initial accumulation of corrosion products on the metal surface before Est is reached and in the “protective effect” region of the oxide layers formed; 2. determine the conditions required for agreement between the CDCP and VA methods: a balance should be established between the paces of accumulation of the corrosion products formed on the specimen surface and accumulated in the liquid corrosive phase; 3. find out that if polarization measurements are used, it is advisable to calibrate them by the CDCP method in order to take real mass losses in the corrosion process into account.