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The influence of steel microstructure on CO2 corrosion. EIS studies on the inhibition efficiency of benzimidazole
Electrochimica Acta 48 (2003) 845 /854 www.elsevier.com/locate/electacta
The influence of steel microstructure on CO2 corrosion. EIS studies on the inhibition efficiency of benzimidazole Damia´n A. Lo´pez *, S.N. Simison, S.R. de Sa´nchez Divisio´n Corrosio´n-INTEMA, Facultad de Ingenierı´a-UNMdP, Av. Juan B. Justo 4302, B7608FDQ Mar del Plata, Argentina Received 23 September 2002; received in revised form 22 November 2002
Abstract Electrochemical measurements employing a.c. and d.c. techniques were used to study CO2 corrosion of carbon steel with two different microstructures: annealed, and quenched and tempered (Q&T), with and without inhibitors. The corrosive media was a deoxygenated 5% NaCl solution, saturated with CO2 at 40 8C and pH 6. Benzimidazole was employed as inhibitor, with a concentration of 100 ppm. Electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LRP) results showed that without inhibitor, Q&T samples have a better corrosion resistance than the annealed ones. On the other hand, the presence of inhibitor improves the corrosion resistance for the annealed samples whereas for the Q&T samples the opposite effect is observed. From the Bode phase angle plots it can be concluded that there is no evidence of inhibitor film formation in any microstructure condition. Based on the experimental findings in the present work, a mechanism of action for the inhibitor is proposed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: EIS; Carbon dioxide; Steel microstructure; Corrosion inhibitor; Benzimidazole
1. Introduction
occurring in the cathode are the reduction of H2CO3 and HCO3 (Eqs. (2a) and (2b)) [2].
Carbon and low alloy steels are the most commonly used construction materials for pipelines in the oil and gas industry. They are, however, very susceptible to corrosion in environments containing CO2. In order to improve their performance, corrosion inhibitors are frequently used. However, a clear understanding of the mechanism of action of the corrosion inhibition process is still needed for reliable field applications. When carbon dioxide dissolves in the presence of a water phase, carbonic acid forms, which is corrosive to carbon steel:
2H2 CO3 2e 0 H2 2HCO 3
CO2 H2 O l H2 CO3
(1)
Several mechanisms have been proposed for the dissolution of iron in aqueous, deareated CO2 solutions [1]. The main corrosion process can summarized by three cathodic (Eqs. (2a), (2b) and (2c)) and one anodic (Eq. (3)) reactions. At pH 6 the main processes
* Corresponding author. Tel.: /54-223-48-16600x244; fax: /54223-48-10046. E-mail address: [email protected] (D.A. Lo´pez).
2HCO 3 2e
0 2H 2e 0 H2
H2 2CO2 3
Fe 0 Fe2 2e
(2a) (2b) (2c) (3)
Due to these processes, a corrosion layer is formed on the steel surface. The properties of these layers and their influence on the corrosion rate are important factors to take into account when studying the corrosion of steels in CO2 aqueous solutions. Some evidence suggests that iron carbonate, FeCO3, may be important in the formation of protective layers [1,3]. Its formation can be explained using equations Eqs. (4), (5a) and (5b) [4]. Because of its low solubility, FeCO3 falls out of solution as a precipitate (pKsp /10.54 at 25 8C [5]) 0 FeCO3 Fe2 CO2 3 2
2HCO 3
Fe 0 Fe(HCO3 )2 Fe(HCO3 )2 0 FeCO3 CO2 H2 O
(4) (5a) (5b)
The morphology of the scale influences the level of protection observed [6]. Below 40 8C, surface scales are
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 7 7 6 - 4
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formed mainly of cementite (Fe3C) with some FeCO3 and alloying elements of the steel [3,7,8]. Fe3C is part of the original steel in the non-oxidized state that accumulates on the surface after the preferential dissolution of ferrite (a-Fe) [9]. It is suggested that cementite provides an available area for the cathodic reactions [10,11]. Steels may have different microstructures depending on the chemical composition and on the fabrication process. These various microstructural components (ferrite, perlite, bainite, martensite) could influence not only the mechanical properties but also the corrosion resistance of the material [12]. The protectiveness of the surface scale depends on the nature of the base alloy (composition, heat treatment) and the environment (temperature, CO2 partial pressure, pH). Several authors have studied the influence of steel microstructure on the corrosion process in aqueous solutions containing CO2, although there is not a general agreement on this issue [11,13,14]. Dugstad [15] studied the formation of protective scales during CO2 corrosion and found that small differences in microstructure can affect anchoring properties, which could explain large performance differences of similar steels in field applications. The surface condition and microstructure also affect the inhibitor efficiency. Oblonsky et al. [16] studied the adsorption of octadecyldimethylbenzylammonium chloride (ODBAC) to carbon steel with two different microstructures. They found that ODBAC physisorbs strongly to the ferritic /perlitic microstructure and weakly to the martensitic microstructure. They attributed the differences to the persistency of the passive films on the two microstructures, with the more stable passive film on the martensitic steel preventing optimal adsorption of the inhibitor. Malik [17] studied the influence of precorrosion on the performance of a C16 quaternary amine as inhibitor for CO2 corrosion of carbon steels. He found that the improved inhibition on a precorroded surface was related to the concentration of inhibitor and to its entry into regions of high blocking effect on CO2 3 Fe2. Benzimidazole and its derivatives are commonly employed as corrosion inhibitors for protecting steel and has been the subject of several investigations [18 / 22]. Kuznetzov [23] studied the mechanism of action of inhibitors considering that most chemical reactions can be treated as acid /base interactions. Based on this concept, benzimidazole would act as a rather strong base due to its pyridine-like nitrogen, which can serve as an electron donor. On the other hand, Fe3, Fe2 and metallic Fe would behave like acids, as they act as electron acceptors, with higher acidity corresponding to higher oxidation state. This interaction with the inhi-
bitor would proceed mainly due to the presence of longrange electrostatic forces [23]. Because there is no general explanation for the influence of microstructure on CO2 corrosion and on inhibitor performance, it is very important to study basic aspects concerning the relationship between microstructure, surface scales and inhibitors efficiency. Electrochemical impedance spectroscopy (EIS) is a powerful technique to study corrosion processes and inhibitor performance in different environments [6,10,18,24 /32]. In the present work, both EIS and some standard d.c. measurements (LRP, Ecorr) were employed to study the corrosion process in carbon steel with two different microstructures (annealed and quenched and tempered (Q&T)), as well as the effect of the heat treatment on the efficiency of benzimidazole as a corrosion inhibitor in CO2 saturated brine media.
2. Experimental Carbon steel with the composition 0.99 Mn /0.38 C / 0.33 Si /0.17 Cr /0.09 Cu /0.04 Ni /0.02 Mo / B/0.01 P / B/0.01 S /Fe balance was used. Two different heat treatments were performed: annealing: austenized at 890 8C and furnace cooled (Sample H); and quenching and tempering: austenized at 890 8C, water Q&T 1 h at 700 8C (Sample T). Fig. 1 shows the microstructures of both samples under study. Working electrodes were machined from these heattreated materials into 5-mm diameter bars, cut and mounted with epoxy resin in a disc electrode holder. Electrical contact between sample and holder was made with silver loaded epoxy resin. Surfaces were polished with 600-grit SiC paper for electrochemical studies and with 0.05 mm alumina until mirror finish for microstructure characterization. In the last case the samples were etched with Nital and observed with scanning electron microscope (SEM Phillips XL 30). Experiments were conducted at atmospheric pressure, 40 8C and with low speed stirring (100 rpm) to ensure laminar flow conditions. Three-electrode jacketed test cells were used with a concentric Pt ring as counter electrode. A saturated calomel electrode was chosen as reference. Test solution was 5 wt.% NaCl (analyticalreagent grade), saturated with deoxygenated CO2 and a working volume of 0.5 l. A set of glass columns containing silica gel and CrO3 treated with hot O2, N2 and CO was employed as an oxygen scavenger to remove impurities from commercial CO2 (99.98%). The oxygen concentration of the solution was measured with DCR OXI200 (Chemetrics† ) and was kept below 40 ppb during the experiments. A positive pressure of deoxygenated CO2 was maintained in the cells during the experiments minimizing the possibility of air ingress. Chemetrics† colorimetric ampoules were used to mea-
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Fig. 1. Microstructure of the carbon steels specimens (Nital etched). (a) Sample H, (b) Sample T.
sure the dissolved CO2 concentration as H2CO3 and the values were 1000 ppm in all the tests. Fifteen milliliters of deoxygenated 1.0 M aq. NaHCO3 were added to adjust pH to 6. The samples were immersed in the working solution and evaluated during 144 h. Benzimidazole analytical-reagent grade 98% (BZI) was employed as inhibitor in a concentration of 100 ppm. A Solartron 1280B unit was used for the electrochemical measurements. EIS was measured at the corrosion potential (Ecorr) using an applied potential of 9/0.005 V rms and a frequency range of 20 000/0.05 Hz. Linear polarization resistance (LRP) was measured by polarizing the working electrode 9/0.015 V versus Ecorr with a sweep rate of 10 4 V s 1. The corrosion potential was also monitored before and after d.c. and a.c. analyses. Measurements were taken at 2, 24, 48, 72 and 144 h after exposure. The efficiency of inhibition (h) was calculated by [31]: h (%) [(Rct;i Rct;b )=Rct;i ]100
(6)
where, Rct,i is the charge transfer resistance in test with inhibitor; Rct,b is the charge transfer resistance in test without inhibitor.
3. Results and discussion 3.1. Uninhibited solutions The impedance diagrams for samples H and samples T obtained after 2, 24, 48, 72 and 144 h of immersion are presented in the Nyquist plots in Figs. 2 and 3, respectively. The magnitude of the impedance increases with time up to 72 h and then the values stabilize for both microstructural conditions. From both figures, it can be seen that after 48 h of exposure the diameters of the plots get bigger for sample T. All experimental plots have a depressed semicircular shape in the complex impedance plane, with the center
Fig. 2. EIS Nyquist plots at different times of immersion for sample H without inhibitor.
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Fig. 3. EIS Nyquist plots at different times of immersion for sample T without inhibitor.
under the real axis. This behavior is typical for solid metal electrodes that show frequency dispersion of the impedance data [29]. Electrically equivalent circuits are generally used to model the electrochemical behavior and to calculate the parameters of interest such as electrolyte resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Cdl) [33]. When a non-ideal frequency response is present, it is commonly accepted to employ distributed circuit elements in an equivalent circuit. The most widely used is constant phase element (CPE), which has a non-integer power dependence on the frequency. The impedance of a CPE is described by the expression: ZCPE Y 1 (iv)n
(7)
where Y is a proportional factor, i is /1, v is 2pf and n has the meaning of a phase shift [29]. Often a CPE is used in a model in place of a capacitor to compensate for non-homogeneity in the system. For example, a rough or porous surface can cause a double-layer capacitance to appear as a CPE with an n value between 0.9 and 1 [34]. For n /0, ZCPE represents a resistance with R /Y 1, for n/1 a capacitance with C /Y , for n /0.5 a Warburg element and for n //1 an inductance with L /Y 1.
Fig. 4. Equivalent circuit used to represent the impedance results.
Fig. 4 shows the electrical equivalent circuit employed to analyze the impedance plots. From the equivalent circuit Rs is the electrolyte resistance (V cm2), Rct is the charge transfer resistance (V cm2) and Y (V1 cm 2 sn ) and n are the parameters depicted for ZCPE in equation (7). Excellent fit with the model was obtained for all experimental data. As an example, the Nyquist and Bode plots at 48 h for sample H are presented in Fig. 5a and b, respectively. It is observed that the fitted data follow almost the same pattern as the original results along the whole diagrams, with an average error of about 1% in all cases. The fitted parameter results for both microstructural conditions are shown in Tables 1 and 2. Fig. 6 shows the main parameters obtained with d.c. and a.c. measurements for both microstructures without inhibitor. It was observed that, despite slight differences, both Rp and Rct follow the same pattern. It is also worth nothing that the Ecorr values are not influenced by heat treatment and show a slender drift in the anodic direction at longer times of exposure. The fitting results show that the proportional factor Y of CPE increases with time without significant differences in the values obtained at each time for both microstructural conditions. According to Turgoose et al. [10] this increase in the capacitance (Y ) value is associated with an increase in the surface area available for the cathodic reaction, which is related to the electrochemical activity of the non-oxidized cementite (Fe3C) residue exposed after the corrosion process. However, this would lead to a decrease in the values of Rct with time (higher corrosion rates), in contrast to the results obtained in this work, which present a rising tendency through time for these values. To explain this behavior an important aspect of this process must be considered. As corrosion takes place, the area of Fe3C
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Fig. 5. EIS Nyquist (a) and Bode (b) plots at 48 h for sample H: (k) experimental data, ( /) fitting model.
Table 1 Circuit parameters for sample H without inhibitor Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
n
2 24 48 72 144
123 176 192 210 211
6.98 11.96 18.52 24.09 30.10
0.83 0.90 0.91 0.91 0.92
2.1 2.9 3.1 2.8 2.2
Table 2 Circuit parameters for sample T without inhibitor Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
n
2 24 48 72 144
137 182 205 257 260
7.17 11.27 17.65 21.13 31.03
0.83 0.90 0.91 0.92 0.93
2.0 3.2 2.3 2.0 1.8
exposed increases for the annealed samples. That is because the dissolution of the surrounding ferrite (a-Fe) from pearlite leaves a laminar structure of non-oxidized Fe3C, which is not removed from the sample surface. In the case of the Q&T samples, when the surrounding ferrite is dissolved, the globular Fe3C detaches from the metal and is removed from the sample surface leaving a new area of exposed cementite equivalent to the initial one [14]. Thus, the increase in the capacitance values cannot be related to an increasing area of Fe3C exposed because a significant difference in the values of capacitance should have been observed between the annealed samples and the Q&T ones.
Therefore, another process should be taken into account to explain the obtained results. It was previously demonstrated that in the experimental conditions studied, deposits of FeCO3 are formed [3,7,8,35] and their presence is usually associated with a reduction in the corrosion rate. As iron carbonate precipitates, it develops a surface layer over the corroding metal. At 40 8C these scales are porous and inhomogeneous, allowing the access of the corroding solution to the base material. These scales, however, seem to provide some corrosion protection to the metal beneath them by restricting the mass transfer of reactants and products between the bulk solution and the metal [28]. On the other hand, a second capacitive arch in the EIS Nyquist plot or a second time constant in the EIS Bode plot would be expected if a protective FeCO3 film was forming on the surface [28]. However, this is not observed in the impedance plots presented in Figs. 2 and 3. This could be due to the formation of a porous thin layer of FeCO3 with a resistance that is much smaller than the charge transfer resistance (Rct). The semicircle representing the FeCO3 film merges with the charge transfer loop and hence the EIS data are described by a simple capacitive semicircle [30,36]. Consequently, an increase in the capacitance (Y ) values could be related to the growing area of an iron carbonate deposit over the surface of the samples, which is accompanied by an increase in the corresponding Rct values. The growing of the n factor agrees with this assumption because it can be attributed to a decrease of the surface inhomogeneity resulting from the formation of a deposit on the surface [18]. Finally, the differences in the Rct values observed with the two microstructural conditions can be explained considering the increasing area of exposed Fe3C in time for the annealed samples, which leads to higher corrosion rates compared with those for the Q&T samples.
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Fig. 6. Ecorr, Rct and Rp for both microstructures without inhibitor.
3.2. Inhibited solutions A set of several independent experiments was conducted in order to investigate the influence of the microstructure of the samples in the efficiency of a corrosion inhibitor (100 ppm BZI) in long-term experiments. The impedance diagrams obtained at various immersion times for samples H and T with inhibitor are presented as Nyquist plots in Figs. 7 and 8, respectively. An increasing trend is observed in the magnitude of the impedance for both microstructural conditions. It is important to note that these values for samples H are larger than those obtained in the uninhibited solutions, whereas for samples T a decrease of the impedance magnitude is observed. It can be observed that for short times of exposure (2 h) there are no considerable differences for Rct values for both microstructures
without and with benzimidazole addition (Figs. 2 and 3 and Figs. 7 and 8, respectively). The efficiency calculated by equation (6) at 144 h of immersion time for sample H is 27.5% and for sample T is /17.7%. Thus, BZI has a protective effect in the annealed samples and an activation effect for the Q&T ones. The same equivalent circuit (Fig. 4) was employed to fit the experimental data. The corresponding fitted parameter results are shown in Tables 3 and 4. Fig. 9 presents the evolution of Rp, Rct and Ecorr over time for both microstructures with BZI. As in the uninhibited solutions (Fig. 6), it was observed that both Rp and Rct follow the same pattern. It was also observed that the inhibitor addition does not cause any shift in the corrosion potential. The fitting results show that the capacitance values (Y ) increase with time in the same manner for sample H
Fig. 7. EIS Nyquist plots at different times of immersion for sample H with BZI 100 ppm.
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851
Fig. 8. EIS Nyquist plots at different times of immersion for sample T with BZI 100 ppm.
Table 3 Circuit parameters for sample H with 100 ppm of BZI Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
n
2 24 48 72 144
122 184 200 223 291
6.77 10.97 13.80 16.68 21.93
0.82 0.89 0.91 0.93 0.93
2.0 2.6 2.6 2.0 2.3
Table 4 Circuit parameters for sample T with 100 ppm of BZI Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
N
2 24 48 72 144
119 186 196 204 214
7.55 9.14 15.88 17.84 24.36
0.80 0.90 0.92 0.92 0.93
3.4 2.1 2.5 2.5 2.4
during the filming process. This new phase shift would mean that the formation of the inhibitor film has changed the electrode interfacial structure and therefore, an additional time constant should be considered [32]. Fig. 10 presents the Bode u versus log f plots for both microstructural conditions without and with inhibitor at the longest immersion time, which were plotted using the same data presented in the Nyquist plots. It is evident that a single time constant is present without and with the addition of BZI, thus, there is no evidence to support the formation of a protective inhibitor film. According to Cao [24], the invariability of the Ecorr with the inhibitor addition could be associated with a geometric blocking mechanism. In such a case, the inhibition efficiency (h ) equals the coverage of the adsorbed inhibiter species (U ) on the metal surface. In order to verify this point, the relationship between h and the relative coverage (m ) should be calculated. The latter is defined by: m1Cd =Cdo lU
and T, although these values are smaller than the ones obtained in the uninhibited solutions. This tendency can be interpreted by considering that the inhibitor is somehow affecting the kinetics of FeCO3 formation, which is independent of the microstructure of the samples. The n factor values also present an increasing trend with time, which corroborates the decrease of the surface inhomogeneity due to the deposition of FeCO3 over the sample. If a protective inhibitor film forms, the Bode u versus log f plots should evidence the presence of two time constants [18,26,27,32]. One would expect to see a new phase angle shift at higher frequency range and a continuous increase in the phase angle shift with time
(8)
where Cd and Cod are the interfacial capacitance for the metal electrode in solutions with and without inhibitor, respectively. The expression of the coefficient l is: l 1Cds =Cdo Csd
(9)
and is the interfacial capacitance of the electrode when U equals 1. If h equals U , the plot of m versus h must be a straight line passing through the origin [24]. In our particular case, none of the microstructural conditions yielded such a result (plots not shown) and therefore the action mechanism of BZI must be explained in a different manner. The inhibition of iron and steel corrosion by benzimidazole and its derivatives has been the subject of several investigations [18 /21]. The efficiency of BZI has
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Fig. 9. Ecorr, Rct and Rp for both microstructures with BZI (100 ppm).
ð10Þ
Fig. 10. EIS Bode u vs. log f plot for both microstructural conditions without and with inhibitor at 144 h immersion time.
been found to be acceptable in acidic media, yielding values of 70% with U around 0.72 [20]. The protective action of the azoles has been explained by the formation of a localized metal-inhibitor bond, in which the main role is played by the pyridine-like nitrogen of the imidazole ring which acts as a strong base [22,23]. In acidic media the benzimidazole molecules are protonated and they act as cations. Keris et al. [21] found that the introduction of a proton in BZI increases the charge redistribution in the imidazole and benzene rings. This suggests that the main contribution to the formation of metal-inhibitor bonds is made by the pyridine-like nitrogen, the benzene ring, and to a lesser extent by the other nitrogen atom. The cation form of BZI can be reduced to neutral molecules at the cathode according to equation (10) [21].
In the current experimental conditions, at pH 6 and considering a pKa /5.53 for BZI [20], only 34% of the inhibitor is protonated. The experimental results can be explained considering that the interaction mechanism between the inhibitor and the sample surface occurs through the reduction of the protonated species at the cathodic sites (Fe3C), which leads to the chemisorption of the reduced inhibitor species. As a consequence, a blockage of these active sites leads to a reduction of the corrosion process. On the other hand, this protective effect slows down the formation of FeCO3 deposits due to lesser amounts of ionic species available to generate this product. The smaller capacitance values observed, compared with those obtained in the uninhibited solutions support this theory. If the inhibitor acts mainly at the cathode, the reduction of the cathodic current would generate a drift in the Ecorr in the negative direction [37]. However, the results show that the shift of corrosion potential due to the inhibitor addition is negligible. This behavior could be explained considering that in the proposed mechanism the protonated inhibitor follows a reduction process on the surface. Then, the additional cathodic current generated would mask the current drop and the consequent change in the corrosion potential. The effect of the steel microstructure is essential to explain the efficiency of the inhibitor under the proposed mechanism of action in each case. When the
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annealed samples are exposed to the corrosive fluid, the corrosion process yields an increase in the area of Fe3C exposed as stated before. Here, BZI interacts with the laminar Fe3C and the adsorbed molecules block the active site for further cathodic reactions. The balance between this obstruction and the slower formation of FeCO3 gives a favorable relation with the average reduction of the corrosion rate (increase of Rct) compared with the condition without inhibitor. The low efficiency acquired (27.5%) can be related to the quite small amount of protonated BZI at pH 6. In the case of the Q&T samples, the inhibitor interacts with the globular Fe3C generating the same blocking and retarding effect as in the annealed samples. The main difference is that as the corrosion process takes place, the carbides detach from the metal and are removed from the sample surface. Corrosion continues and a new area of uncovered cementite appears at the surface. Consequently, in the Q&T samples the inhibitor would only produce a delay in the FeCO3 precipitation and does not provide any appreciable protection due to cathodic blocking. Higher corrosion rates (lower Rct) would then be expected for the inhibited samples compared with those immersed in uninhibited solutions. This would explain the negative efficiency or activation process calculated for this microstructural condition.
4. Conclusions EIS has been shown to be a useful tool for studying the corrosion process in CO2 corrosion and evaluating the mechanism of action of benzimidazole as corrosion inhibitor. The behavior of steel in NaCl 5 wt.% solution saturated with deoxygenated CO2 depends on the microstructure of the material. Without inhibitor addition, the Q&T samples have a better corrosion resistance than the annealed ones. From the data obtained it can be seen that benzimidazole is not a particularly useful inhibitor in the experimental conditions studied. However, the efficiency of BZI is affected by the microstructure of the sample. For the annealed samples, the presence of inhibitor improves the corrosion resistance of the material whereas for the Q&T samples the opposite effect is observed. It is proposed that the protection mechanism of benzimidazole is based on the reduction of the protonated species at the cathodic sites (Fe3C), leading to the adsorption and blockage of these active areas and a relatively small delay in FeCO3 precipitation. The morphology of the cementite in each microstructure determines if such blockage is useful to reduce or to activate corrosion rate compared with the condition in which the inhibitor is absent.
853
From these results, it was demonstrated that when two different heat treatments are applied to steel, the corrosion processes and the efficiency of the inhibitor are appreciably influenced.
Acknowledgements This work was supported by the Argentine Research Council for Science and Technology (CONICET, grant PIP 0413/98) and the University of Mar del Plata. One of the authors (D.A.L.) is grateful to Dr. Silvia Cere´, Dr. Omar Lo´pez and Dr. William Schmitz for their valuable assistance and helpful comments during the revision of this paper. D.A.L. is a recipient of a CONICET fellowship for post-graduate studies.
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The influence of steel microstructure on CO2 corrosion. EIS studies on the inhibition efficiency of benzimidazole Damia´n A. Lo´pez *, S.N. Simison, S.R. de Sa´nchez Divisio´n Corrosio´n-INTEMA, Facultad de Ingenierı´a-UNMdP, Av. Juan B. Justo 4302, B7608FDQ Mar del Plata, Argentina Received 23 September 2002; received in revised form 22 November 2002
Abstract Electrochemical measurements employing a.c. and d.c. techniques were used to study CO2 corrosion of carbon steel with two different microstructures: annealed, and quenched and tempered (Q&T), with and without inhibitors. The corrosive media was a deoxygenated 5% NaCl solution, saturated with CO2 at 40 8C and pH 6. Benzimidazole was employed as inhibitor, with a concentration of 100 ppm. Electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LRP) results showed that without inhibitor, Q&T samples have a better corrosion resistance than the annealed ones. On the other hand, the presence of inhibitor improves the corrosion resistance for the annealed samples whereas for the Q&T samples the opposite effect is observed. From the Bode phase angle plots it can be concluded that there is no evidence of inhibitor film formation in any microstructure condition. Based on the experimental findings in the present work, a mechanism of action for the inhibitor is proposed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: EIS; Carbon dioxide; Steel microstructure; Corrosion inhibitor; Benzimidazole
1. Introduction
occurring in the cathode are the reduction of H2CO3 and HCO3 (Eqs. (2a) and (2b)) [2].
Carbon and low alloy steels are the most commonly used construction materials for pipelines in the oil and gas industry. They are, however, very susceptible to corrosion in environments containing CO2. In order to improve their performance, corrosion inhibitors are frequently used. However, a clear understanding of the mechanism of action of the corrosion inhibition process is still needed for reliable field applications. When carbon dioxide dissolves in the presence of a water phase, carbonic acid forms, which is corrosive to carbon steel:
2H2 CO3 2e 0 H2 2HCO 3
CO2 H2 O l H2 CO3
(1)
Several mechanisms have been proposed for the dissolution of iron in aqueous, deareated CO2 solutions [1]. The main corrosion process can summarized by three cathodic (Eqs. (2a), (2b) and (2c)) and one anodic (Eq. (3)) reactions. At pH 6 the main processes
* Corresponding author. Tel.: /54-223-48-16600x244; fax: /54223-48-10046. E-mail address: [email protected] (D.A. Lo´pez).
2HCO 3 2e
0 2H 2e 0 H2
H2 2CO2 3
Fe 0 Fe2 2e
(2a) (2b) (2c) (3)
Due to these processes, a corrosion layer is formed on the steel surface. The properties of these layers and their influence on the corrosion rate are important factors to take into account when studying the corrosion of steels in CO2 aqueous solutions. Some evidence suggests that iron carbonate, FeCO3, may be important in the formation of protective layers [1,3]. Its formation can be explained using equations Eqs. (4), (5a) and (5b) [4]. Because of its low solubility, FeCO3 falls out of solution as a precipitate (pKsp /10.54 at 25 8C [5]) 0 FeCO3 Fe2 CO2 3 2
2HCO 3
Fe 0 Fe(HCO3 )2 Fe(HCO3 )2 0 FeCO3 CO2 H2 O
(4) (5a) (5b)
The morphology of the scale influences the level of protection observed [6]. Below 40 8C, surface scales are
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 7 7 6 - 4
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formed mainly of cementite (Fe3C) with some FeCO3 and alloying elements of the steel [3,7,8]. Fe3C is part of the original steel in the non-oxidized state that accumulates on the surface after the preferential dissolution of ferrite (a-Fe) [9]. It is suggested that cementite provides an available area for the cathodic reactions [10,11]. Steels may have different microstructures depending on the chemical composition and on the fabrication process. These various microstructural components (ferrite, perlite, bainite, martensite) could influence not only the mechanical properties but also the corrosion resistance of the material [12]. The protectiveness of the surface scale depends on the nature of the base alloy (composition, heat treatment) and the environment (temperature, CO2 partial pressure, pH). Several authors have studied the influence of steel microstructure on the corrosion process in aqueous solutions containing CO2, although there is not a general agreement on this issue [11,13,14]. Dugstad [15] studied the formation of protective scales during CO2 corrosion and found that small differences in microstructure can affect anchoring properties, which could explain large performance differences of similar steels in field applications. The surface condition and microstructure also affect the inhibitor efficiency. Oblonsky et al. [16] studied the adsorption of octadecyldimethylbenzylammonium chloride (ODBAC) to carbon steel with two different microstructures. They found that ODBAC physisorbs strongly to the ferritic /perlitic microstructure and weakly to the martensitic microstructure. They attributed the differences to the persistency of the passive films on the two microstructures, with the more stable passive film on the martensitic steel preventing optimal adsorption of the inhibitor. Malik [17] studied the influence of precorrosion on the performance of a C16 quaternary amine as inhibitor for CO2 corrosion of carbon steels. He found that the improved inhibition on a precorroded surface was related to the concentration of inhibitor and to its entry into regions of high blocking effect on CO2 3 Fe2. Benzimidazole and its derivatives are commonly employed as corrosion inhibitors for protecting steel and has been the subject of several investigations [18 / 22]. Kuznetzov [23] studied the mechanism of action of inhibitors considering that most chemical reactions can be treated as acid /base interactions. Based on this concept, benzimidazole would act as a rather strong base due to its pyridine-like nitrogen, which can serve as an electron donor. On the other hand, Fe3, Fe2 and metallic Fe would behave like acids, as they act as electron acceptors, with higher acidity corresponding to higher oxidation state. This interaction with the inhi-
bitor would proceed mainly due to the presence of longrange electrostatic forces [23]. Because there is no general explanation for the influence of microstructure on CO2 corrosion and on inhibitor performance, it is very important to study basic aspects concerning the relationship between microstructure, surface scales and inhibitors efficiency. Electrochemical impedance spectroscopy (EIS) is a powerful technique to study corrosion processes and inhibitor performance in different environments [6,10,18,24 /32]. In the present work, both EIS and some standard d.c. measurements (LRP, Ecorr) were employed to study the corrosion process in carbon steel with two different microstructures (annealed and quenched and tempered (Q&T)), as well as the effect of the heat treatment on the efficiency of benzimidazole as a corrosion inhibitor in CO2 saturated brine media.
2. Experimental Carbon steel with the composition 0.99 Mn /0.38 C / 0.33 Si /0.17 Cr /0.09 Cu /0.04 Ni /0.02 Mo / B/0.01 P / B/0.01 S /Fe balance was used. Two different heat treatments were performed: annealing: austenized at 890 8C and furnace cooled (Sample H); and quenching and tempering: austenized at 890 8C, water Q&T 1 h at 700 8C (Sample T). Fig. 1 shows the microstructures of both samples under study. Working electrodes were machined from these heattreated materials into 5-mm diameter bars, cut and mounted with epoxy resin in a disc electrode holder. Electrical contact between sample and holder was made with silver loaded epoxy resin. Surfaces were polished with 600-grit SiC paper for electrochemical studies and with 0.05 mm alumina until mirror finish for microstructure characterization. In the last case the samples were etched with Nital and observed with scanning electron microscope (SEM Phillips XL 30). Experiments were conducted at atmospheric pressure, 40 8C and with low speed stirring (100 rpm) to ensure laminar flow conditions. Three-electrode jacketed test cells were used with a concentric Pt ring as counter electrode. A saturated calomel electrode was chosen as reference. Test solution was 5 wt.% NaCl (analyticalreagent grade), saturated with deoxygenated CO2 and a working volume of 0.5 l. A set of glass columns containing silica gel and CrO3 treated with hot O2, N2 and CO was employed as an oxygen scavenger to remove impurities from commercial CO2 (99.98%). The oxygen concentration of the solution was measured with DCR OXI200 (Chemetrics† ) and was kept below 40 ppb during the experiments. A positive pressure of deoxygenated CO2 was maintained in the cells during the experiments minimizing the possibility of air ingress. Chemetrics† colorimetric ampoules were used to mea-
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847
Fig. 1. Microstructure of the carbon steels specimens (Nital etched). (a) Sample H, (b) Sample T.
sure the dissolved CO2 concentration as H2CO3 and the values were 1000 ppm in all the tests. Fifteen milliliters of deoxygenated 1.0 M aq. NaHCO3 were added to adjust pH to 6. The samples were immersed in the working solution and evaluated during 144 h. Benzimidazole analytical-reagent grade 98% (BZI) was employed as inhibitor in a concentration of 100 ppm. A Solartron 1280B unit was used for the electrochemical measurements. EIS was measured at the corrosion potential (Ecorr) using an applied potential of 9/0.005 V rms and a frequency range of 20 000/0.05 Hz. Linear polarization resistance (LRP) was measured by polarizing the working electrode 9/0.015 V versus Ecorr with a sweep rate of 10 4 V s 1. The corrosion potential was also monitored before and after d.c. and a.c. analyses. Measurements were taken at 2, 24, 48, 72 and 144 h after exposure. The efficiency of inhibition (h) was calculated by [31]: h (%) [(Rct;i Rct;b )=Rct;i ]100
(6)
where, Rct,i is the charge transfer resistance in test with inhibitor; Rct,b is the charge transfer resistance in test without inhibitor.
3. Results and discussion 3.1. Uninhibited solutions The impedance diagrams for samples H and samples T obtained after 2, 24, 48, 72 and 144 h of immersion are presented in the Nyquist plots in Figs. 2 and 3, respectively. The magnitude of the impedance increases with time up to 72 h and then the values stabilize for both microstructural conditions. From both figures, it can be seen that after 48 h of exposure the diameters of the plots get bigger for sample T. All experimental plots have a depressed semicircular shape in the complex impedance plane, with the center
Fig. 2. EIS Nyquist plots at different times of immersion for sample H without inhibitor.
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Fig. 3. EIS Nyquist plots at different times of immersion for sample T without inhibitor.
under the real axis. This behavior is typical for solid metal electrodes that show frequency dispersion of the impedance data [29]. Electrically equivalent circuits are generally used to model the electrochemical behavior and to calculate the parameters of interest such as electrolyte resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Cdl) [33]. When a non-ideal frequency response is present, it is commonly accepted to employ distributed circuit elements in an equivalent circuit. The most widely used is constant phase element (CPE), which has a non-integer power dependence on the frequency. The impedance of a CPE is described by the expression: ZCPE Y 1 (iv)n
(7)
where Y is a proportional factor, i is /1, v is 2pf and n has the meaning of a phase shift [29]. Often a CPE is used in a model in place of a capacitor to compensate for non-homogeneity in the system. For example, a rough or porous surface can cause a double-layer capacitance to appear as a CPE with an n value between 0.9 and 1 [34]. For n /0, ZCPE represents a resistance with R /Y 1, for n/1 a capacitance with C /Y , for n /0.5 a Warburg element and for n //1 an inductance with L /Y 1.
Fig. 4. Equivalent circuit used to represent the impedance results.
Fig. 4 shows the electrical equivalent circuit employed to analyze the impedance plots. From the equivalent circuit Rs is the electrolyte resistance (V cm2), Rct is the charge transfer resistance (V cm2) and Y (V1 cm 2 sn ) and n are the parameters depicted for ZCPE in equation (7). Excellent fit with the model was obtained for all experimental data. As an example, the Nyquist and Bode plots at 48 h for sample H are presented in Fig. 5a and b, respectively. It is observed that the fitted data follow almost the same pattern as the original results along the whole diagrams, with an average error of about 1% in all cases. The fitted parameter results for both microstructural conditions are shown in Tables 1 and 2. Fig. 6 shows the main parameters obtained with d.c. and a.c. measurements for both microstructures without inhibitor. It was observed that, despite slight differences, both Rp and Rct follow the same pattern. It is also worth nothing that the Ecorr values are not influenced by heat treatment and show a slender drift in the anodic direction at longer times of exposure. The fitting results show that the proportional factor Y of CPE increases with time without significant differences in the values obtained at each time for both microstructural conditions. According to Turgoose et al. [10] this increase in the capacitance (Y ) value is associated with an increase in the surface area available for the cathodic reaction, which is related to the electrochemical activity of the non-oxidized cementite (Fe3C) residue exposed after the corrosion process. However, this would lead to a decrease in the values of Rct with time (higher corrosion rates), in contrast to the results obtained in this work, which present a rising tendency through time for these values. To explain this behavior an important aspect of this process must be considered. As corrosion takes place, the area of Fe3C
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849
Fig. 5. EIS Nyquist (a) and Bode (b) plots at 48 h for sample H: (k) experimental data, ( /) fitting model.
Table 1 Circuit parameters for sample H without inhibitor Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
n
2 24 48 72 144
123 176 192 210 211
6.98 11.96 18.52 24.09 30.10
0.83 0.90 0.91 0.91 0.92
2.1 2.9 3.1 2.8 2.2
Table 2 Circuit parameters for sample T without inhibitor Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
n
2 24 48 72 144
137 182 205 257 260
7.17 11.27 17.65 21.13 31.03
0.83 0.90 0.91 0.92 0.93
2.0 3.2 2.3 2.0 1.8
exposed increases for the annealed samples. That is because the dissolution of the surrounding ferrite (a-Fe) from pearlite leaves a laminar structure of non-oxidized Fe3C, which is not removed from the sample surface. In the case of the Q&T samples, when the surrounding ferrite is dissolved, the globular Fe3C detaches from the metal and is removed from the sample surface leaving a new area of exposed cementite equivalent to the initial one [14]. Thus, the increase in the capacitance values cannot be related to an increasing area of Fe3C exposed because a significant difference in the values of capacitance should have been observed between the annealed samples and the Q&T ones.
Therefore, another process should be taken into account to explain the obtained results. It was previously demonstrated that in the experimental conditions studied, deposits of FeCO3 are formed [3,7,8,35] and their presence is usually associated with a reduction in the corrosion rate. As iron carbonate precipitates, it develops a surface layer over the corroding metal. At 40 8C these scales are porous and inhomogeneous, allowing the access of the corroding solution to the base material. These scales, however, seem to provide some corrosion protection to the metal beneath them by restricting the mass transfer of reactants and products between the bulk solution and the metal [28]. On the other hand, a second capacitive arch in the EIS Nyquist plot or a second time constant in the EIS Bode plot would be expected if a protective FeCO3 film was forming on the surface [28]. However, this is not observed in the impedance plots presented in Figs. 2 and 3. This could be due to the formation of a porous thin layer of FeCO3 with a resistance that is much smaller than the charge transfer resistance (Rct). The semicircle representing the FeCO3 film merges with the charge transfer loop and hence the EIS data are described by a simple capacitive semicircle [30,36]. Consequently, an increase in the capacitance (Y ) values could be related to the growing area of an iron carbonate deposit over the surface of the samples, which is accompanied by an increase in the corresponding Rct values. The growing of the n factor agrees with this assumption because it can be attributed to a decrease of the surface inhomogeneity resulting from the formation of a deposit on the surface [18]. Finally, the differences in the Rct values observed with the two microstructural conditions can be explained considering the increasing area of exposed Fe3C in time for the annealed samples, which leads to higher corrosion rates compared with those for the Q&T samples.
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850
Fig. 6. Ecorr, Rct and Rp for both microstructures without inhibitor.
3.2. Inhibited solutions A set of several independent experiments was conducted in order to investigate the influence of the microstructure of the samples in the efficiency of a corrosion inhibitor (100 ppm BZI) in long-term experiments. The impedance diagrams obtained at various immersion times for samples H and T with inhibitor are presented as Nyquist plots in Figs. 7 and 8, respectively. An increasing trend is observed in the magnitude of the impedance for both microstructural conditions. It is important to note that these values for samples H are larger than those obtained in the uninhibited solutions, whereas for samples T a decrease of the impedance magnitude is observed. It can be observed that for short times of exposure (2 h) there are no considerable differences for Rct values for both microstructures
without and with benzimidazole addition (Figs. 2 and 3 and Figs. 7 and 8, respectively). The efficiency calculated by equation (6) at 144 h of immersion time for sample H is 27.5% and for sample T is /17.7%. Thus, BZI has a protective effect in the annealed samples and an activation effect for the Q&T ones. The same equivalent circuit (Fig. 4) was employed to fit the experimental data. The corresponding fitted parameter results are shown in Tables 3 and 4. Fig. 9 presents the evolution of Rp, Rct and Ecorr over time for both microstructures with BZI. As in the uninhibited solutions (Fig. 6), it was observed that both Rp and Rct follow the same pattern. It was also observed that the inhibitor addition does not cause any shift in the corrosion potential. The fitting results show that the capacitance values (Y ) increase with time in the same manner for sample H
Fig. 7. EIS Nyquist plots at different times of immersion for sample H with BZI 100 ppm.
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851
Fig. 8. EIS Nyquist plots at different times of immersion for sample T with BZI 100 ppm.
Table 3 Circuit parameters for sample H with 100 ppm of BZI Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
n
2 24 48 72 144
122 184 200 223 291
6.77 10.97 13.80 16.68 21.93
0.82 0.89 0.91 0.93 0.93
2.0 2.6 2.6 2.0 2.3
Table 4 Circuit parameters for sample T with 100 ppm of BZI Time (h) Rs (V cm2)
Rct (V cm2)
Y (V 1 cm2 sn /104)
N
2 24 48 72 144
119 186 196 204 214
7.55 9.14 15.88 17.84 24.36
0.80 0.90 0.92 0.92 0.93
3.4 2.1 2.5 2.5 2.4
during the filming process. This new phase shift would mean that the formation of the inhibitor film has changed the electrode interfacial structure and therefore, an additional time constant should be considered [32]. Fig. 10 presents the Bode u versus log f plots for both microstructural conditions without and with inhibitor at the longest immersion time, which were plotted using the same data presented in the Nyquist plots. It is evident that a single time constant is present without and with the addition of BZI, thus, there is no evidence to support the formation of a protective inhibitor film. According to Cao [24], the invariability of the Ecorr with the inhibitor addition could be associated with a geometric blocking mechanism. In such a case, the inhibition efficiency (h ) equals the coverage of the adsorbed inhibiter species (U ) on the metal surface. In order to verify this point, the relationship between h and the relative coverage (m ) should be calculated. The latter is defined by: m1Cd =Cdo lU
and T, although these values are smaller than the ones obtained in the uninhibited solutions. This tendency can be interpreted by considering that the inhibitor is somehow affecting the kinetics of FeCO3 formation, which is independent of the microstructure of the samples. The n factor values also present an increasing trend with time, which corroborates the decrease of the surface inhomogeneity due to the deposition of FeCO3 over the sample. If a protective inhibitor film forms, the Bode u versus log f plots should evidence the presence of two time constants [18,26,27,32]. One would expect to see a new phase angle shift at higher frequency range and a continuous increase in the phase angle shift with time
(8)
where Cd and Cod are the interfacial capacitance for the metal electrode in solutions with and without inhibitor, respectively. The expression of the coefficient l is: l 1Cds =Cdo Csd
(9)
and is the interfacial capacitance of the electrode when U equals 1. If h equals U , the plot of m versus h must be a straight line passing through the origin [24]. In our particular case, none of the microstructural conditions yielded such a result (plots not shown) and therefore the action mechanism of BZI must be explained in a different manner. The inhibition of iron and steel corrosion by benzimidazole and its derivatives has been the subject of several investigations [18 /21]. The efficiency of BZI has
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Fig. 9. Ecorr, Rct and Rp for both microstructures with BZI (100 ppm).
ð10Þ
Fig. 10. EIS Bode u vs. log f plot for both microstructural conditions without and with inhibitor at 144 h immersion time.
been found to be acceptable in acidic media, yielding values of 70% with U around 0.72 [20]. The protective action of the azoles has been explained by the formation of a localized metal-inhibitor bond, in which the main role is played by the pyridine-like nitrogen of the imidazole ring which acts as a strong base [22,23]. In acidic media the benzimidazole molecules are protonated and they act as cations. Keris et al. [21] found that the introduction of a proton in BZI increases the charge redistribution in the imidazole and benzene rings. This suggests that the main contribution to the formation of metal-inhibitor bonds is made by the pyridine-like nitrogen, the benzene ring, and to a lesser extent by the other nitrogen atom. The cation form of BZI can be reduced to neutral molecules at the cathode according to equation (10) [21].
In the current experimental conditions, at pH 6 and considering a pKa /5.53 for BZI [20], only 34% of the inhibitor is protonated. The experimental results can be explained considering that the interaction mechanism between the inhibitor and the sample surface occurs through the reduction of the protonated species at the cathodic sites (Fe3C), which leads to the chemisorption of the reduced inhibitor species. As a consequence, a blockage of these active sites leads to a reduction of the corrosion process. On the other hand, this protective effect slows down the formation of FeCO3 deposits due to lesser amounts of ionic species available to generate this product. The smaller capacitance values observed, compared with those obtained in the uninhibited solutions support this theory. If the inhibitor acts mainly at the cathode, the reduction of the cathodic current would generate a drift in the Ecorr in the negative direction [37]. However, the results show that the shift of corrosion potential due to the inhibitor addition is negligible. This behavior could be explained considering that in the proposed mechanism the protonated inhibitor follows a reduction process on the surface. Then, the additional cathodic current generated would mask the current drop and the consequent change in the corrosion potential. The effect of the steel microstructure is essential to explain the efficiency of the inhibitor under the proposed mechanism of action in each case. When the
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annealed samples are exposed to the corrosive fluid, the corrosion process yields an increase in the area of Fe3C exposed as stated before. Here, BZI interacts with the laminar Fe3C and the adsorbed molecules block the active site for further cathodic reactions. The balance between this obstruction and the slower formation of FeCO3 gives a favorable relation with the average reduction of the corrosion rate (increase of Rct) compared with the condition without inhibitor. The low efficiency acquired (27.5%) can be related to the quite small amount of protonated BZI at pH 6. In the case of the Q&T samples, the inhibitor interacts with the globular Fe3C generating the same blocking and retarding effect as in the annealed samples. The main difference is that as the corrosion process takes place, the carbides detach from the metal and are removed from the sample surface. Corrosion continues and a new area of uncovered cementite appears at the surface. Consequently, in the Q&T samples the inhibitor would only produce a delay in the FeCO3 precipitation and does not provide any appreciable protection due to cathodic blocking. Higher corrosion rates (lower Rct) would then be expected for the inhibited samples compared with those immersed in uninhibited solutions. This would explain the negative efficiency or activation process calculated for this microstructural condition.
4. Conclusions EIS has been shown to be a useful tool for studying the corrosion process in CO2 corrosion and evaluating the mechanism of action of benzimidazole as corrosion inhibitor. The behavior of steel in NaCl 5 wt.% solution saturated with deoxygenated CO2 depends on the microstructure of the material. Without inhibitor addition, the Q&T samples have a better corrosion resistance than the annealed ones. From the data obtained it can be seen that benzimidazole is not a particularly useful inhibitor in the experimental conditions studied. However, the efficiency of BZI is affected by the microstructure of the sample. For the annealed samples, the presence of inhibitor improves the corrosion resistance of the material whereas for the Q&T samples the opposite effect is observed. It is proposed that the protection mechanism of benzimidazole is based on the reduction of the protonated species at the cathodic sites (Fe3C), leading to the adsorption and blockage of these active areas and a relatively small delay in FeCO3 precipitation. The morphology of the cementite in each microstructure determines if such blockage is useful to reduce or to activate corrosion rate compared with the condition in which the inhibitor is absent.
853
From these results, it was demonstrated that when two different heat treatments are applied to steel, the corrosion processes and the efficiency of the inhibitor are appreciably influenced.
Acknowledgements This work was supported by the Argentine Research Council for Science and Technology (CONICET, grant PIP 0413/98) and the University of Mar del Plata. One of the authors (D.A.L.) is grateful to Dr. Silvia Cere´, Dr. Omar Lo´pez and Dr. William Schmitz for their valuable assistance and helpful comments during the revision of this paper. D.A.L. is a recipient of a CONICET fellowship for post-graduate studies.
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