HCO3. − was attributed to the formation of the soluble Fe(CO3)2. 2- complex [4]. When the concentration .... in which pK represents an equilibrium constant for chemical equation (1). ... In 0.1mol/L NaHCO3 solution at pH 8.33, –750mV, the predicted ... ion seems effective for inhibiting the enhanced corrosion of carbon steel.
Mat. Res. Soc. Symp. Proc. Vol. 713 © 2002 Materials Research Society
Corrosion Behavior of Carbon Steel in Bicarbonate (HCO3-) Solutions Junhua DONG, Toshiyasu NISHIMURA, Toshiaki KODAMA Corrosion Resistant Materials Research Group, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, 305-0047, Japan ABSTRACT Carbon steel is considered in Japan the most promising candidate material for overpacks in high-level radioactive waste disposal. Effects of bicarbonate solutions on the corrosion behavior and corrosion products of carbon steel were investigated by electrochemical measurements; FT-IR spectra and XRD pattern analyses. The results of the anodic polarization measurements showed that bicarbonate (HCO3-) accelerates the anodic dissolution and the outer layer film formation of carbon steel in the case of high concentrations, whereas it inhibits these processes in the case of low concentrations. The FTIR and XRD analyses of the anodized film showed that siderite (FeCO3) was formed in 0.5 to 1.0mol/L bicarbonate solution, and Fe2(OH)2CO3 in 0.1 to 0.2mol/L bicarbonate solution, while Fe6(OH)12CO3 was formed in 0.02 to 0.05mol/L bicarbonate solution. In all cases the pH value was around 8.3. The stability of these chemical compositions was discussed using a potential – pH diagram for the Fe-H2O-CO2 system. INTRODUCTION Carbon steel, a candidate material of the overpacks for long-term geological disposal of high-level radioactive waste, can be corroded by the attack of surrounding groundwater. However, it is a requirement that the containment of radioactive nuclides should be maintained for more than several hundred years [1,2]. The corrosion of carbon steel depends mainly on the chemical composition of water, pH and the film formed on the metal surface [3]. While extensive studies have been carried out on the anodic behavior of iron in bicarbonate solutions [4], very little information is available on the corrosion products formed on the metal surfaces. In the present study, we have examined systematically the anodic products in bicarbonate solutions at various electrode potentials and concentrations. Experimental observations were compared with newly developed potential-pH (E-pH) diagrams at room temperature (25°C). EXPERIMENTAL Specimens were cut from a carbon steel sheet with the following composition: 0.05% C, 0.3% Si, 0.7%Mn, 0.01%P, 0.003% S, 0.03% Al, 0.003%N and 0.002% O. Measurements of anodic polarization curves was carried out from the rest potential in the noble direction at a scan rate of 10mV/min. Electrolytes containing HCO3- in the range of 0.02 to 1.0 mol/L, pH 8.33, were deaerated by argon gas prior to the anodic polarization studies. The concentration of 0.1mol/L NaHCO3 is estimated to be the highest level that can be encountered in a geological environment. Saturated calomel electrode (SCE) was used as a reference electrode in all experiments, which were conducted at 25°C. After the removal of JJ11.8.1
air-formed oxides by cathodic reduction (50µA/cm2 for 30mins), specimens were anodically polarized at different potentials for one week. For corrosion product analyses both x-ray diffraction (XRD) and Fourier transform infrared (FTIR) methods were employed. The XRD conditions were 40kV x 300mA using a Cu target and the scan rate of 5 degrees/min rate. Nicolet 760 FT-IR spectrometer was used for IR spectrometry in which samples were processed into KBr tablets. Prior to XRD and FT-IR analyses, the corroded steel samples were transferred into a glove box oxygen-purged by argon gas, where they were stored until the complete dry out. RESULTS AND DISCUSSION Fig.1 shows the anodic polarization curves of carbon steel in different concentrations of HCO3- solutions at pH 8.33. Two current peaks were observed in the curves. Fig.2 shows the change of the first peak current density as a function of NaHCO3 concentration. When the concentration of HCO3− was higher than 0.1mol/L, the first peak current density increased with increasing HCO3− concentration, meaning that HCO3− has apparently two opposite effects, dissolution accelerator and passivator on iron. The enhanced dissolution effect of HCO3− was attributed to the formation of the soluble Fe(CO3)22- complex [4]. When the concentration of HCO3− was less than 0.1mol/L, the peak current density decreased with an increase in HCO3- concentration, due to the fact that the corrosion products consisting of CO32− and OH− show some protectiveness leading to a decrease of the peak current density.
Fig.1 Polarization curves of carbon steel in various concentrations of NaHCO3
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Fig.2 The effects of CNaHCO3 on the first peak current densities in Fig.1 Fig.3 shows a potential-pH diagram of the Fe-CO2-H2O system calculated using standard thermodynamics data [5]. The FeCO3 –dominant region appears in the diagram, in which the stability of FeCO3 was dependent on pH and HCO3- concentration. Magnetite (Fe3O4) becomes less stable with increasing HCO3− concentration. Fig.4 shows the current decay plots of carbon steel measured in 0.1mol/L NaHCO3 solution at various potentials under potentiostatic conditions. Experimental E-pH conditions indicated by marks plotted in Fig 3 corresponded to FeCO3 or Fe3O4 stability regions depending on potential. The current density showed a maximum at the early stage of polarization at –700 to –600 mV, followed by a decay reaching very low current density values after the formation of the anodic film. At – 550mV, however, the current density decreased rapidly showing completely passivation.
Fig.3 E-pH diagram of Fe-H2O-CO2 system based on standard thermodynamics data.
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Fig.4 Current decay at different potential in 0.1mol/L NaHCO3 solution Fig.5 and Fig.6 show the XRD patterns and FTIR spectra of the specimens after potentiostatic polarization measurements in 0.1mol/L NaHCO3 solutions. When the potential was set at –750mV, XRD patterns of the film showed Fe2(OH)2CO3 (International Center for Diffraction Data – powder diffraction data (JCPDS) 330650) phase. Both Fe6(OH)12CO3 (JCPDS 460098) and Fe2(OH)2CO3 were detected at –700mV, and at -650mV, both Fe6(OH)12CO3 and magnetite (Fe3O4) (JCPDS 851436) were observed, while only Fe3O4 was identified at –600 mV. At –550 mV, no specific peaks were detected. Because of passivation with no visible anodic film forming on metal surface ~ the level of detection in XRD was inadequate. The FTIR absorption bands of anodized film in 0.1mol/LNaHCO3 solutions were observed at wave numbers of 1540cm-1, 1360cm-1, 1070cm-1, 860cm-1 and 750cm-1, respectively, which corresponded to the typical IR spectra for carbonates [6]. The split of absorption band of CO32- at 1440cm-1 is attributed to the degeneracy of vibration symmetry which was caused by the mixed coordination of CO32- and OH- to Fe in Fe2(OH)2CO3. This characteristic split is observed in the IR spectra of basic carbonates such as Cu2(OH)2CO3. It is, thus, confirmed with FTIR the existence of both CO32- and OH- in the corrosion product formed in 0.1mol/L NaHCO3 solution. In solutions of 1.0 and 0.5 mol/L HCO3-, XRD patterns agreed with that of to FeCO3 (JCPDS 831764). The FTIR spectra of the anodized film were in agreement with the standard IR spectra of carbonate species. In a 0.2 mol/L HCO3- solution, FeCO3 coexist with Fe2(OH)2CO3 in the anodic film formed at –750mV. In solutions of 0.05 and 0.02 mol/L HCO3-, Fe2(OH)2CO3, Fe6(OH)12CO3 and Fe3O4 mixture appeared in changing proportions depending on the bicarbonate concentration and applied potentials. In this case, FTIR absorption bands were observed at 1360cm-1, 1040 cm-1, and 800 cm-1. XRD and FT-IR data are listed in Table 1 for the anodic films formed in 0.02, 0.1 and 1.0mol/LNaHCO3 solutions with the applied potential as a parameter.
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Fig.5 XRD patterns of the anodized films formed in 0.1mol/LNaHCO3 solutions (pH 8.33)
Fig.6 FT-IR spectra of the anodized films formed in 0.1mol/L NaHCO3 solutions (pH 8.33) Table 1 Compositions of the anodized films formed in NaHCO3 solutions (pH 8.33) at different potentials. The letters given in this table correspond to those given in E-pH diagrams 1.0mol/L 0.1mol/L 0.02mol/L NaHCO3 solution Fe2(OH)2CO3 Fe6(OH)12CO3 FeCO3 a -750mV Fe2(OH)2CO3+ Fe6(OH)12CO3 (FeCO3) b -700mV(-680mV) Fe6(OH)12CO3 Fe3O4 ~ Fe6(OH)12CO3 +Fe3O4 c -650mV Fe3O4 Fe3O4 (FeCO3) d -600mV(-580mV) Passivation ~ FeCO3 e -550mV
The standard Gibbs formation energy values of Fe2(OH)2CO3 and Fe6(OH)12CO3 were estimated as follows, respectively. The coexistence of FeCO3 and Fe2(OH)2CO3 in HCO3solution can be expressed by the following equilibrium reaction: Fe2(OH)2CO3 + HCO3- + H+ = 2FeCO3 + 2H2O JJ11.8.5
(1)
The thermodynamical equilibrium equation would be pK = pH + log[HCO3-] = 9.03
(2)
in which pK represents an equilibrium constant for chemical equation (1). The conditions for the coexistence of Fe2(OH)2CO3 and FeCO3 are expressed as 0.2mol/L HCO3- and pH 8.33. By substituting Eq (2) with the experimental values of the coexistence, the pK value in Eq (2) is evaluated to be 9.03. The standard Gibbs energy of formation (∆Gf) was estimated to be -1169.3 kJ/mol for Fe2(OH)2CO3. Similarly, ∆Gf of Fe6(OH)12CO3 was estimated to be -3650 kJ/mol under the condition of 0.05mol/L HCO3- solution at pH 8.33 in Eq (3). In the estimation the following standard values were employed; for Fe+2, Fe3O4, FeCO3, HCO3-, and H2O, ∆Gf = -78.9, -1015.4, -666.67, -586.85, and -237.129 kJ/mol, respectively. 3 Fe2(OH)2CO3 + 6H2O = Fe6(OH)12CO3 + 2 HCO3− + 4 H+ + 2 e−
(3)
Using the estimated values for basic carbonates, new E-pH diagrams have been constructed for the Fe-H2O-CO2 system in 1.0; 0.1and 0.01 mol/L HCO3− solutions. Fig.7 shows the E-pH diagram in 0.1 mol/L HCO3− solution. The anodic products predicted by the E-pH diagrams showed a good agreement with those observed by XRD and FTIR, as summarized in Table 1. In 0.1mol/L NaHCO3 solution at pH 8.33, –750mV, the predicted product is Fe2(OH)2CO3 alone (Fig.7), which is in agreement with the experimental result. Similarly, in 1.0 mol/L HCO3− solution, FeCO3 is also the only product deposited on carbon steel. It is, however, suggested that a soluble complex (Fe(CO3)22−) becomes stable in the aqueous phase. The solubility of Fe(CO3)22− is dependent on the concentration of carbonate species and pH value. In the potential-pH diagram (Fig.8a) the stability zone of Fe(CO3)22− becomes wider with decreasing concentration of complex Fe(CO3)22−, as is shown in Fig. 8a.
Fig.7 Practical E-pH diagram of Fe-H2O-CO2 system, [carbonate] = 10-1 mol/L. JJ11.8.6
It should be noted that under a condition of very dilute complex ion, iron carbonates and basic carbonates in the solid phase coexist with the aqueous phase. As Fig.8b shows, the dominant region of complex Fe(CO3)22− diminishes when higher concentration is assumed. Since the highest concentration of HCO3− in actual geological environment is 0.1mol/L, thus the soluble complex Fe(CO3)22− does not occur in underground water environment. In 0.02 mol/L HCO3- solution, Fe6(OH)12CO3 and Fe3O4 were detected, which corresponds to the potential-pH diagram of Fe-H2O-CO2 system in 0.01 mol/L HCO3−.
Fig.8a Practical E-pH diagram of Fe-H2O-CO2 system, [carbonate] = 1.0 mol/L, [Fe(CO3)22-] = 10-5 mol/L.
Fig.8b Practical E-pH diagram of Fe-H2O-CO2 system, [carbonate] = 1.0 mol/L, [Fe(CO3)22-] = 10-2 mol/L.
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Fig.9 Practical E-pH diagram of Fe-H2O-CO2 system, [carbonate] = 10-2 mol/L CONCLUSION 1) Anodic films formed on steel surface in bicarbonate solutions were analyzed with reference to new potential-pH diagrams. The corrosion products of carbon steel in NaHCO3 solutions are dependent on the concentration of HCO3 and the applied potentials. By potentiostatic polarization, FeCO3, Fe2(OH)2CO3, Fe6(OH)12CO3 and Fe3O4 are detected successively with increasing concentrations of HCO3− and at potentials 2 2) Soluble complex Fe(CO3)2 − is assumed to be formed in concentrated NaHCO3 solutions, and could be the main reason of the accelerated anodic dissolution by carbonates. 3) Since the highest value of [HCO3 ] is estimated to be 0.1mol/L in actual geological disposal environments, HCO3 ion seems effective for inhibiting the enhanced corrosion of carbon steel 4) Potential-pH diagrams for the Fe-H2O-CO2 system have been constructed at various concentrations of HCO3 . The corrosion products observed in experiments to be deposited on the steel agree with those predicted from the diagrams.
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