VALCARCE Maria beatriz
The role of chloride, nitrite and carbonate ions on carbon steel
M. B. VALCARCE; C. LÓPEZ; M. VÁZQUEZ
JOURNAL OF THE ELECTROCHEMICAL SOCIETY
ELECTROCHEMICAL SOC INC
Lugar: Pennington; Año: 2012 vol. 159 p. 244 - 251
The effect of nitrite ions on the breakdown of passive films formed on steel in highly carbonated solutions is investigated. The solutions contain 0.015 mol l-1 Na2CO3 and 0.3 mol l-1 NaHCO3 (pH=9). To simulate contamination, the solutions incorporate 0.05 and 0.1 mol l-1 NaCl. The inhibition efficiency is evaluated measuring the corrosion potential, cyclic voltammograms, anodic polarization curves, pitting potentials, polarization resistance and performing electrochemical impedance and Raman spectra, together with weight-loss measurements. High concentrations of CO3-2 favor the development of a passive film that becomes more compact when nitrite ions are added. As the [NO2-]/[Cl-] increases, the films become more resistant to pitting attack. No repassivation was observed independently of the chloride and nitrite content. A linear relationship was established between Epit and log ([NO2-]*([HCO3-] + [CO3=]) / [Cl-]). The role of nitrite and carbonate ions has been rationalized by a mechanism dominated by the formation of adsorbed FeCO3 and the competitive adsorption between NO2- and CO32− inside the pit. It is shown that the [NO2-]/[Cl-] ratio cannot be taken as an appropriate parameter to evaluate the inhibitor efficiency for different pH values and different carbonate and chloride ions concentrations. Instead, [NO2-]=0.2 mol l-1 guarantees the inhibitor efficiency in the different electrolytes studied.