228 DOI 10.1002/mawe.201000578 Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4 behavior of carbon steel in simulated soil solution Einfluss der Erdbodenzusammensetzung auf das elektrochemische Verhalten von Kohlenstoffstählen in simulierten Erdbodenlösungen T. M. Liu 1,Y.H.Wu 1, 2, S. X. Luo 2, C. Sun 3 In this study, effect of cations, Ca 2+,Mg 2+,K +, and anions, SO 4, HCO 3,NO 3 on electrochemical corrosion behavior of carbon steel in simulated soil solution was investigated through potentiodynamic polarization curves and electrochemical impedance spectroscopy. The results indicate that the Ca 2+ and Mg 2+ can decrease the corrosion current density of carbon steel in simulated soil solution, and K +,SO 4,HCO 3, and NO 3 can increase the corrosion density. All the above ions in the simulated soil solution can decrease its resistivity, but they have different effect on the charge transfer resistivity. This finding can be useful in evaluating the corrosivity of certain soil through chemical analysis, and provide data for construction engineers. Keywords: Soil corrosion / simulated soil solution / carbon steel / corrosion behavior / Schlüsselwörter: Korrosion im Erdboden / simulierte Erdbodenlösungen / Kohlenstoffstähle / Korrosionsverhalten / 1 Introduction With the development of Chinese economy and society, more and more pipelines for natural gas and oil transport and land buried structures are constructed. They are often expected to have a longer working life. The fundamental cause of the deterioration of land buried structures is soil corrosion. Soil corrosion is an electrochemical interaction between underground structures and the ambient soil environment. Actually, the concern with the environment is of great importance and a better understanding of the soil as a corrosive agent becomes necessary for the use of adequate protection for buried structures, avoiding the occurrence of leakiness and, as a consequence, the contamination of the soil [1]. An increasing awareness and understanding of the soil corrosion concept has been noticed since the National Association of Corrosion Engineers (NACE) was founded in 1943. Nowadays, in China, with the rapid development of petroleum industries and constant increase in the numbers of steel pipelines buried underground, more and more attention needs to be paid to this area of study [2]. 1 College of Materials Science and Engineering, Chongqing University, Congqing 400044, P.R. China 2 Department of Chemistry, Zunyi Normal College, Zunyi 563002, P.R. China 3 State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China Correspondence author: Y. H. Wu, College of Materials Science and Engineering, Chongqing University, Congqing 400044, P.R. China E-mail: wuyuanhuil@126.com Due to the complexity of soil and its porous, heterogeneous and discontinuous environment constituted by mineral or organic solid phase, water liquid phase, air and other gas phases [3 4], corrosion behavior mechanism for underground structure is still unclear. The factors that influence corrosion in soil are numerous, such as, soil type, moisture content, and position of water table, soil resistivity, soluble ion content, soil ph, oxidation-reduction potential and, the role of microbes in soil environment [3]. So, the studies of soil corrosion are generally studied in simulated solutions. Chemical composition plays a key role in understanding how a soil influences the corrosion of buried steel. The chemical compositions of soil usually include NaCl, CaCl 2, MgCl 2, KCl, Na 2 SO 4, NaHCO 3, and NaNO 3. Carlos et al. [1] studied the corrosivity of the soil in the Southeastern region of Brazil. Nie et al. [5] studied the effect of temperature on the electrochemical corrosion characteristics of carbon steel in a salty soil. However, there has been little work investigating the effect of soil compositional cations (Na +,Ca 2+,Mg 2+,K + ) and anions (Cl, SO 4, HCO 3, NO 3 ) on the electrochemical corrosion behavior of carbon steel in simulated soil solution directly. On account of its physical, mechanical and economic advantages, steel is almost always used in underground structures. In this study, 0.01 M NaCl solution, which was selected as the simulated soil solution based on the composition of soil in Yingtan, China, was used as matrix soil solution, and the effect of compositional cations and anions on the corrosion behavior of Q235 carbon steel in simulated soil solution was discussed. The soil corrosivity can be considered as the capacity of this environment to produce and to develop the phenomenon of corrosion. The soil is defined as an electrolyte and this can be understood by means of the electrochemical theory [1]. So, the potentiodynamic polarization curves technique and electrochemical impedance spec-
Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4 229 Table 1. Chemical compositions of the carbon steel studied (wt.%) Steel C S P Mn Si Cu Q235 0.176 0.023 0.019 0.057 0.233 0.033 troscopy (EIS) were used in this investigation. It is expected that a better understanding will be obtained about the compositional ions on the corrosion of buried carbon steel. 2 Experimental Specimens for electrochemical tests were made from Q235 carbon steel whose chemical composition was shown in Table 1. Samples were cut into small squares of 10 mm 6 10 mm, and then covered with epoxy resin except test surface, with a working area of 1 cm 2. Each sample was successively polished using silicon carbide emery papers from grit 150, 240, 400 to 600, then rinsed with deionized water, and degreased with acetone. The corrosion of buried structures is related to soil conditions in which they are buried. The simulated soil solution was based on the soil composition from Yingtan, China. 0.01 M NaCl solution was used as the matrix corrosion electrolyte, and other cations or anions were added into it in order to study the effect of compositional ions on the corrosion behavior of carbon steel in soil solution. The chemical compositions of the used corrosion electrolytes were listed in Table 2. Table 2. Corrosion electrolyte type and compositions used in this test Electrolyte type Compositions 1 0.01 M NaCl 2 0.01 M NaCl + 0.01 M CaCl 2 3 0.01 M NaCl + 0.01 M MgCl 2 4 0.01 M NaCl + 0.01 M KCl 5 0.01 M NaCl + 0.01 M Na 2SO 4 6 0.01 M NaCl + 0.01 M NaHCO 3 7 0.01 M NaCl + 0.01 M NaNO 3 All the electrochemical measurements were carried out by means of Parstat 2273 equipment at room temperature. Potentiodynamic polarization measurements were carried out in a conventional three electrodes glass cell with a platinum counter electrode and a saturated calomel electrode (SCE) as reference electrode with luggin capillary bridge. All tests have been performed in aerated solutions. The potentiodynamic polarization curves were recorded by a constant sweep rate of 10 mv/min. Before recording the polarization curves, the open-circuit potential was stable for 30 min. The cathodic branch was always determined first, the open-circuit potential (OCP) was then re-established and the anodic branch determined. The samples were polarized from 0.3V to 0.6 V versus OCP. Electrochemical impedance spectroscopy (EIS) has proved its usefulness to follow the degradation of metals [6 7]. EIS measurements were performed in a frequency range of 10 5 10 2 Hz Figure 1. Potentiodynamic polarization curves and EIS diagram about effect of Ca 2+,Mg 2+,K + on Q235 carbon steel in NaCl solution.
230 T. M. Liu et al. Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4 Figure 2. Potentiodynamic polarization curves and EIS diagram about effect of SO 4 on Q235 carbon steel in NaCl solution. with 51 points settled. Square sheet steel of the same size as described above was used as the working electrode. The corrosion electrolyte and test equipment were the same as that used of potentiodynamic polarization curves measurement. The above electrochemical measurements in all the tested solutions have been conducted for three times, but almost no differences were obtained. So, only was one curve for each test given in this study. 3 Results Effect of Ca 2+,Mg 2+,K + on electrochemical corrosion behavior of Q235 carbon steel in simulated soil solution is shown in Figure 1. Figure 1(a) stands for the potentiodynamic polarization curves. Figure 1(b) and Figure 1(c) stand for the Nyquist diagram and Bode plot for EIS, respectively. Effect of SO 4 on electrochemical corrosion behavior of Q235 carbon steel in simulated soil solution is shown in Figure 2. Effect of HCO 3 on electrochemical corrosion behavior of Q235 carbon steel in simulated soil solution is shown in Figure 3. Effect of NO 3 on electrochemical corrosion behavior of Q235 carbon steel in simulated soil solution is shown in Figure 4. In order to further understand the corrosion mechanisms, the corresponding equivalent circuit for EIS tests is shown in Figure 5. The capacitance loop can be described with Rs, Rt and Q. Rt was charge transfer resistance representing the resistance of electron transfer during electrochemical reaction process. Q was the electric double layer capacitance. Rs referred to the solution resistance between working electrode and reference electrode. The plots were fitted using ZSimpWin software and the results were listed in Table 3. Effect of Ca 2+,Mg 2+,K + on the Rt of Q235 carbon steel in NaCl solution is shown in Figure 6. Effect of SO 4, HCO 3,NO 3 on the Rt of Q235 carbon steel in NaCl solution is shown in Figure 7. It was well known that the low frequency capacitance loop was mainly related to the characteristics of electric double layer formed in the interface of metal surface and corrosion electrolyte, which can be described by Rt. 4 Discussion 4.1 Effect of Ca 2+,Mg 2+,K + on electrochemical corrosion behavior of carbon steel In order to study the effect of Ca 2+,Mg 2+, and K + on electrochemical corrosion behavior of carbon steel in simulated soil solution. 0.01 M CaCl 2, 0.01 M MgCl 2 and 0.01 M KCl were added into the 0.01 M NaCl solution, respectively. Clearly, for all the potentiodynamic polarization curves, the cathodic process are all controlled by the reduction of dissolved oxygen, and the anodic process are all controlled by the dissolution of carbon steel electrode. For the addition of CaCl 2, it reduces the cathodic current density a little, and increases the anodic current density obviously. For the addition of MgCl 2, it reduces the cathodic current slightly, but greatly increases the anodic current density. For the addition of KCl, it
Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4 231 Figure 3. Potentiodynamic polarization curves and EIS diagram about effect of HCO 3 on Q235 carbon steel in NaCl solution. Table 3. EIS fitting results of Q235 carbon steel in all the simulated soil solutions Solutions Equivalent circuit Rs (ohm/cm 2 ) Capacitance Q Q-Y o (ohm/cm 2 /s) Q-n Rt (ohm/cm 2 ) 0.01 M NaCl R(QR) 118.1 0.001084 0.7925 1522 0.01 M NaCl + 0.01 M CaCl 2 R(QR) 40.08 0.0008365 0.8278 1684 0.01 M NaCl + 0.01 MgCl 2 R(QR) 36.61 0.000976 0.8579 1586 0.01 M NaCl + 0.01 KCl R(QR) 57.82 0.001126 0.8011 1496 0.01 M NaCl + 0.01 Na 2SO 4 R(QR) 44.50 0.001532 0.8179 1296 0.01 M NaCl + 0.01NaHCO 3 R(QR) 61.12 0.0004054 0.7229 1374 0.01 M NaCl + 0.01NaNO 3 R(QR) 44.66 0.001.070 0.8058 1459 increases the cathodic current density obviously, and firstly decreases and then increases the anodic current density. From the potentiodynamic polarization curves, it can be concluded that the aggressiveness of the three added compositions are in the order of KCl A MgCl 2 A CaCl 2. As the anions are the same, we can say the aggressiveness of the cations is in order of K + A Mg 2+ A Ca 2+. For the Nyquist diagram and Bode plot, it is seen that there is a similar feature. Usually, the magnitude of impedance at high frequency stands for the solution resistance, and low frequency stands for the charge transfer resistance. As can be seen from Figure 1(b), the size of the high frequency semicircle decreased in the order of effect of CaCl 2 A MgCl 2 A NaCl A KCl. It means that, for the NaCl matrix solution, the aggressiveness of the three added compositions are in the order of KCl A MgCl 2 A CaCl 2.For the Figure 1(c), the addition of the KCl, MgCl 2, and CaCl 2 all decreased the solution resistance. From the results in Table 3 and Figure 6, it can be seen that the fitting result is in accordance with the results in Figure 1(b) and Figure 1(c). So, the EIS study results are in accordance with the potentiodynamic polarization results. 4.2 Effect of SO 4, HCO 3,NO 3 on electrochemical corrosion behavior of carbon steel In order to study the effect of SO 4, HCO 3,NO 3 on the electrochemical corrosion behavior of carbon steel in simulated soil solution. 0.01 M Na 2 SO 4, 0.01 M NaHCO 3 and 0.01 M NaNO 3 were
232 T. M. Liu et al. Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4 Figure 4. Potentiodynamic polarization curves and EIS diagram about effect of NO 3 on Q235 carbon steel in NaCl solution. Figure 5. Equivalent circuit for EIS plots of Q235 carbon steel in all the test solutions. Rs is the resistance of corrosive electrolyte; Q is constant phase element parameter; Rt is the charge transfer resistance. Figure 6. Effect of Ca 2+,Mg 2+,K + on the Rt of Q235 carbon steel in NaCl solution. added into 0.01 M NaCl solution, respectively. For all the cases, the cathodic processes are characterized by reduction process of dissolved oxygen, and the anodic processes are dissolution of carbon steel electrode. For Figure 2, the addition of Na 2 SO 4 can increase the cathodic and anodic current density of carbon steel in NaCl solution (Figure 2(a)), decrease the size of capacitive semicircle (Figure 2(b)), and can decrease the charger transfer resistance in low frequency and solution resistance in high frequency (Figure 2(c)). For the results of addition of NaHCO 3 in Figure 3, it can increase both the cathodic and anodic current density of carbon steel in NaCl solution as seen in Figure 3(a). The diffusive impedance in low frequency range in Figure 3(b) indicates that the diffusion process is the step control process. Figure 3(c) also indicates that the addition of NaHCO 3 can decrease the solution resistance. Higher corrosion current density and lower charge transfer resistance indicate the corrosivity of NaHCO 3 on the corrosion behavior of carbon steel in NaCl solution. The presence of low-frequency diffusive impedance in Nyquist diagram in Figure 3(b) suggests that mass-transfer of dissolved oxygen plays an essential role in carbon steel corrosion, and the whole corrosion process is mixed-controlled by activation and diffusion steps, and this may due to the emission of carbon dioxide. For the results of addition of NaNO 3 in Figure 4, including Figure 4(a), Figure 4(b), and Figure 4(c), the NaNO 3 can slightly
Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4 233 higher corrosion current density. The lower aggressiveness of cations than anion lies in the ability of their precipitation on the carbon steel electrode surface, as a sequence, inhibited its dissolution rate. Figure 7. Effect of SO 4,HCO 3,NO 3 on the Rt of Q235 carbon steel in NaCl solution. increase the corrosion current density, decrease the charger transfer resistance, and decrease the solution resistance. From the results in Table 3 and Figure 7, it can be seen that the fitting results are in accordance with the results in Figure 2, Figure 3, and Figure 4. Based on the above discussion, we can gain the conclusion that the corrosivity of the addition chemicals are in the order of Na 2 SO 4 A NaHCO 3 A NaNO 3. As the cations are the same, we can say the aggressiveness of the anions is in order of SO 4 A HCO 3 A NO 3. 5 Conclusions Effect of cations, Ca 2+,Mg 2+,K +, and anions, SO 4, HCO 3,NO 3, on electrochemical corrosion behavior of carbon steel in simulated soil solution was investigated via potentiodynamic polarization curves and electrochemical impedance spectroscopy. The obtained conclusions are as follows: 1. In simulated soil solution, the aggressiveness of the compositional cations is order of K + A Mg 2+ A Ca 2+, and that of anions is in order of SO 4 A HCO 3 A NO 3. 2. Addition of Ca 2+ and Mg 2+ can increase the charge transfer resistance and K + can decrease it. All the anions, SO 4, HCO 3, and NO 3 can decrease the charge transfer resistance. But, both the cations and anions can reduce the solution resistivity in 0.01 M NaCl solution. 3. For soil solution, the corrosivity of cations is more aggressive than that of anions. This may due to their difference in radius. As soil is a very complex mixture, it is quite difficult to understand the real corrosion mechanism and the effect of different ions, and we studied this in simulated soil solutions. This may provide information for the forecast of resistance of certain soil, and can guide the engineers to select more suitable construction materials. 4.3 Discussion on the carbon steel corrosion mechanism in simulated soil solution Generally, the anodic and cathodic reactions of carbon steel corrosion in aerated solution can be expressed as follows: Fe fi Fe 2+ + 2e (1) O 2 +2H 2 O+4efi 4OH (2) The dissolution of carbon steel from the steel matrix to corrosive electrolyte stands for the anodic reaction, and oxygen dissolving and diffusion through the soil solution towards the steel electrode surface stands for the cathodic reaction. The corrosion rate is often expressed by corrosion current density. The corrosion current density has relation to the resistance of electrolyte besides the anodic dissolution rate and cathodic dissolved oxygen reduction rate. Both the cations and anions can decrease the solution resistance in simulated soil solution as can be seen in Figure 1(c), Figure 2(c), Figure 3(c), Figure 4(c), and Table 3. The difference of the ions on the aggressiveness of corrosion electrolyte may lies in their radii and adsorption energy. Ion with shorter radius and high adsorption energy can preferentially adsorb at the special sites of carbon steel electrode surface, leading to Acknowledgement This work was supported by Science and Technology Foundation of Guizhou Province of China (No. 20082008) and Science and Technology Foundation of Zunyi City of China (No. 200724). 6 References [1] C. Alberto, M. Ferreira, A. C. Ponciano, Science of the Total Environment 2007, 388, 250. [2] Y. T. Li, Corrosion Engineering Science and Technology 2008, 44, 91. [3] A. Benmoussa, M. Hadjel, M. Traisnel, Materials and Corrosion 2006, 57, 77. [4] Y. H. Wu, T. M. Liu, S. X. Luo, C. Sun, Materialwissenschaft und Werkstofftechnik 2010, 41, 142. [5] X. H. Nie, X. G. Li, C. W. Du, Y. F. Cheng, Journal of Applied Electrochemistry 2007, 39, 277. [6] E. O. Olorunniwo, I. B. Imasogie, A. A. Adeniyi, Anti-Corrosion Methods and Materials 2004, 54, 346. [7] K. Belmokre, N. Azzouz, F. Kermiche, Materials and Corrosion 1998, 49, 108. Received in final form: March 18 th 2010 T 578