PROOF BY UV-VISIBLE MODULATED REFLECTANCE SPECTROSCOPY OF THE BREAKDOWN BY CARBONATION OF THE PASSIVATING LAYER ON IRON IN ALKALINE-SOLUTION

Carbonation of concrete by atmospheric CO2 may lead to breakdown of the passivating layer of the steel reinforcements, leading to catastrophic corrosion, if the pH of the electrolyte in the concrete pores decreases below 8. The nature of this carbonation process has been studied by means of potential-modulated reflectance (PMR) spectroscopy, following the changes in the PMR spectrum of air-passivated iron in 1M NaOH produced by sparging the electrolyte with a CO2 + air mixture. Decreasing the pH from 8.02 to 7.69 produced the disappearance of the Fe2O3 PMR signal and nearly a doubling of the PMR maximum of FeOOH, pointing to a hydration of Fe2O3 to FeOOH preceding the breakdown of the passivating layer. A further decrease of only 0.07 pH units, down to pH 7.62, effected an increase of the corrosion rate of nearly two orders of magnitude and the disappearance of the PMR spectrum of the passivating layer, this being the first spectroscopic proof that depassivation by carbonation is due to a complete electrochemical dissolution of the passivating layer.