Effect of HCO3− on electrochemical kinetics of carbon steel corrosion in CO2-saturated brines. (10th November 2018)

• Record Type: Journal Article
• Authors: Wright, Ruishu F.; Brand, Edward R.; Ziomek-Moroz, Margaret; Tylczak, Joseph H.; Ohodnicki, Paul R.
• Is Part Of: Electrochimica acta. Volume 290(2018)
• Page Start: 626
• Page End: 638
• Publication Date: 2018-11-10
• Subjects: Electrochemical corrosion -- Kinetics -- Bicarbonate ion -- Half reactions -- Tafel slopes; Electrochemistry -- Periodicals; Electrochemistry, Industrial -- Periodicals; 541.37
• DOI: 10.1016/j.electacta.2018.09.114
• Languages: English
• ISSNs: 0013-4686

Abstract

Abstract: The effect of HCO3 − (aq) on CO2 corrosion of carbon steel was investigated in deaerated 3.5 wt% NaCl solutions at 30 °C from pH 3.96 to 7.15. In the CO2 -saturated solutions, the pH was adjusted with different HCO3 − (aq) concentrations, c [HCO3 − (aq)]. The corrosion rate decreased by a factor of 2 as the pH and c [HCO3 − (aq)] increased. The cathodic current density during polarization increased at higher pH with higher c [HCO3 − (aq)], indicating that HCO3 − (aq) acted as an additional hydrogen source for the hydrogen evolution reaction. As the pH increased, the active dissolution regions displayed similar anodic Tafel slopes and suggested a modified Bockris mechanism for the Fe oxidation reaction. The exchange current densities for the half reactions were calculated to study kinetics of the anodic and cathodic half reactions independently. The anodic exchange current density ( j 0, a ) increased by one order of magnitude in the presence of CO2, indicating the involvement of CO2 (aq) in the Fe oxidation reaction. As the pH and c [HCO3 − (aq)] increased, the cathodic exchange current density ( j 0, c ) decreased by a factor of 50 because the increase in j 0, c [HCO3 − (aq)] was not high enough to compensate the decline from the other hydrogen sources, especially j 0, c [H + (aq)]; and j 0, a decreased by a factor of 2.4 because HCO3 − (aq) may have competed with CO2 (aq) for the surface coverage and the increase in j 0, a [HCO3 − (aq)] could not compensate the decrease in j 0, a [CO2 (aq)]. It suggests that the reaction rate constant of HCO3 − (aq) was smaller than CO2 (aq) for the anodic half reaction and was smaller than H + (aq) for the cathodic half reaction. The XPS results verified that the corrosion products transitioned from iron carbonate to hydroxide as the pH increased while iron carbonate remained the major product. As the pH increased with HCO3 − (aq), a second time constant was observed at lower frequencies of the electrochemical impedance spectroscopy (EIS) results. Highlights: HCO3 − acted as an additional hydrogen source for the cathodic reaction. Active dissolution mechanism was discussed based on anodic Tafel slopes. Half reactions were studied independently on the effect of HCO3 − . CO2 (aq) was verified to participate in the Fe oxidation reaction. Corrosion products transitioned from iron carbonate to hydroxide as pH increased.