Inhibition of Bicarbonate-Chloride Corrosion and Passivation of Carbon Steel under Open-Circuit Conditions by Molybdate

Abstract

Molybdate's inhibition of localized corrosion and passivation of AISI 1020 carbon steel in pH 9 and pH 10 solutions containing bicarbonate/carbonate and chloride anions was investigated under open-circuit conditions. The critical molybdate concentration required to prevent pitting corrosion in these solutions was determined. Using Design of Experiments and statistical analysis, the main and interaction effects of these anions on the polarization resistance were identified. Results show that the immersion time (a measure of the degree of passivation) and the presence of other ions can affect the effectiveness of a corrosion inhibitor. A combination of XPS and electrochemical methods was used to assess the effect of ions on the passive film formation. Increasing chloride concentration in the presence of bicarbonate/carbonate ions increased the total charge passed (extent of oxidation) but decreased the passive film thickness. This indicates that a significant part of the Fe atoms oxidized do not contribute to passive film formation, suggesting a harmful synergy between bicarbonate/carbonate and chloride. However, when molybdate is present, the total charge passed decreased and the passive film thickened slightly, showing the beneficial effect of molybdate on the passive film growth process, which is most probably due to competitive adsorption against bicarbonate/carbonate and chloride.

The roles carbonate and bicarbonate ions play on pre‐passive HRB400 rebars in simulated pore solutions of deteriorating concrete

“…As the concentration of carbonate and bicarbonate ions increased, the critical chloride threshold for depassivation also increased gradually. In addition, in the solution containing higher concentration of carbonate and bicarbonate ions, the measured OCP was more positive after depassivation, which indicated a lower demand for cathodic current and implied a lower anodic current . It was obvious that carbonate and bicarbonate ions had an inhibitory effect on the depassivation of HRB400 rebars in pH 11 solutions.…”

“…In addition, in the solution containing higher concentration of carbonate and bicarbonate ions, the measured OCP was more positive after depassivation, which indicated a lower demand for cathodic current and implied a lower anodic current. [25] It was obvious that carbonate and bicarbonate ions had an inhibitory effect on the depassivation of HRB400 rebars in pH 11 solutions. Figure 1b shows the effect of pH on pre-passive HRB400 rebars in solutions with 0.03 carbonate and bicarbonate ions and different chloride content.…”

“…The influence of carbonate and bicarbonate content on the pre‐passive HRB400 rebars is quite interesting in this study. On the one hand, the presence of carbonate and bicarbonate ions does have a negative effect on the resistance of passive film, resulting in a higher steady‐state dissolution rate before depassivation . On the other hand, higher concentration of carbonate and bicarbonate ions can tolerate more chloride before the depassivation of steel rebars, which indicates that carbonate and bicarbonate ions compete with chloride ions on the surface of the sample .…”

“…Many researchers have devoted to studying the corrosion behavior of carbon steel rebars in carbonate and bicarbonate solution . Tan et al believed that carbonate and bicarbonate ions had an inhibitory effect on pitting, although the inhibition ability of carbonate and bicarbonate ions was one order magnitude lower than that of hydroxide.…”

“…[22][23][24] Many researchers have devoted to studying the corrosion behavior of carbon steel rebars in carbonate and bicarbonate solution. [17,23,[25][26][27][28][29][30] Tan et al [17] believed that carbonate and bicarbonate ions had an inhibitory effect on pitting, although the inhibition ability of carbonate and bicarbonate ions was one order magnitude lower than that of hydroxide. Moreno et al [23] found no passivity for carbon steel in solution (pH = 9) with 0.02 M carbonate and bicarbonate, but the steel could be passivated in 0.4 M carbonate and bicarbonate solution with same pH, even in the presence of 0.01% Cl − .…”