Abstract
The surface blocking properties of calcareous deposit have been shown to have a beneficial effect for cathodic protection. Though this may be true, the role of calcareous deposit has never been quantified. The purpose of this study was to investigate the effect of the formation of calcareous deposit on carbon steel induced by the application of cathodic polarisation in simulated soil solution. The solution was derived from Evian water, with [Ca2+]:[Mg2+] ratio maintained at 2:1 and chloride concentration of 0.01 mol/L. The behaviour of the metal/electrolyte interface was studied using non-invasive in-situ electrochemical techniques at OCP and at various applied potentials Eapp from –0.7 VSCE to –1.2 VSCE. A combination of voltammetry, electrochemical impedance spectroscopy with X-ray diffraction analysis of the mineral layer were utilised. One week long experiments were conducted. Results showed that at OCP, lepidocrocite, carbonated green rust, calcite and aragonite were found as the corrosion process involved anodic and cathodic zones. For cathodic protection Eapp –0.7 VSCE to –1.2 VSCE, calcite and aragonite was found. At Eapp –1.2 VSCE, brucite was found due to increased interfacial pH and as a result evolution of hydrogen bubble was observed. Normalised current curves for Eapp –1.2 VSCE showed surface coverage not exceeding 60% after 7 days. In contrast –1.1 VSCE ≤ Eapp ≤ –0.7 VSCE surface coverage was 0% at day 7. Quantification of the cathodic protection efficiency done via modelling of polarisation curves illustrated a “passivation” phenomenon which resulted from formation of hydroxyl cations. Modelling of polarisation curves allowed for acquisition of the expression for the anodic component of the current ja as a function of the applied potential Eapp. It could be predicted that the interfacial behaviour had a direct influence on the anodic Tafel coefficient. The influence of the calcareous deposit on cathodic protection efficiency can be quantified by plotting log ja vs Eapp. Eapp –0.7, –0.8 and –1.0 VSCE were shown to offer the best current inhibition with formation of calcareous layer allowing very low current density to be maintained.
M.Tech. (Chemical Engineering)