The Effect of Pre-Corrosion and Steel Microstructure on Inhibitors Performance in CO2 Corrosion

The effect of pre-corrosion and steel microstructure on inhibitors performance in CO2 corrosion.

L.D. Paolinellia, T. Pérezb and S.N. Simisona a División Corrosión - INTEMA, UNMdP, Av. Juan B. Justo 4302, B7608FDQ Mar del Plata, Argentina. Phone: 00542234816600, Fax: 00542234810046, e-mail:[email protected]. b Centro de Investigación Industrial (CINI), FUDETEC, Simini 250, 2804 Campana, Argentina. Phone: 00543489433592-, Fax: 00543489427928, e-mail: [email protected].

The importance of chemical composition and microstructure on CO2 corrosion of carbon and low alloy steels has been widely recognized, but different aspects are still uncertain and contradictory results can be found in the literature [1, 2]. The effect of these variables on the corrosion product film formation and properties and on inhibitor efficiency are far from being understood. This is mainly due to the complexity of the problem and the difficulty to describe the involved mechanisms. In addition test conditions also vary widely, making them almost impossible to compare. As a consequence of the above depicted situation, the necessity of more systematic work to clarify the involved mechanisms is evident. In previous works we have shown that inhibitors efficiency is affected by microstructure and that this effect could be specific to inhibitor molecular structure [3-5]. The aim of the present work is to advance in the characterization of the relationship among microstructure-surface condition-inhibitor efficiency in CO2 corrosion. A carbon steel with two different microstructures (ferrite-perlite and tempered martensite) was tested in a deoxygenated 5% wt. NaCl solution, saturated with CO2 at 40 °C, pH 6; low speed stirring (100 rpm) was used to ensure laminar flow conditions. The performance of a CO2 corrosion commercial imidazoline-based product was studied by means of electrochemical measurements employing a.c. and d.c. techniques. The inhibitor was added after various pre-corrosion periods: 24, 48 and 72 hrs. Surface films characteristics were studied using SEM. Electrochemical Impedance Spectroscopy (EIS) and Linear Polarization Resistance (LRP) studies showed that the properties of surface films and the efficiency of the inhibitor depends on microstructure, being superior for tempered martensite than for ferrite-perlite. In addition, pre-corrosion decreases efficiency and its impact is also microstructure dependent.

Keywords: CO2 corrosion; steel microstructure; corrosion inhibitors; imidazolines; EIS

1 1. Introduction

The carbon and low alloyed steels are widely used in the petroleum industry, mainly due to economic reasons. Nevertheless their corrosion resistance is limited in certain oil and gas environments that contain high chloride and carbon dioxide concentration. According to steel chemical composition and fabrication processes, they may have different microstructures which influence their mechanical properties and corrosion resistance. As a consequence of corrosion processes, a layer of corrosion products is formed on the surface of the steel. Its protection properties depend on the environmental conditions and the characteristics of the material. The presence of iron carbonate (FeCO3) is commonly related with the formation of protective layers [6]. Because of its low solubility (pKsp = 10.54 at 25 2+ 2- °C [7]), FeCO3 precipitates when ferrous cations (Fe ) interacts with carbonate (CO3 ) and - bicarbonate (HCO3 ) ions in the solution [8]. The surface scales formed below 40 °C in chloride media containing carbon dioxide consists mainly of cementite (Fe3C) with some FeCO3 and alloying elements of the steel [3,6]. Fe3C is part of the original steel microstructure and accumulates on the surface after the preferential dissolution of ferrite (α- Fe) into Fe2+. It is suggested that cementite provides an available area for the cathodic reactions [9]. When the environment is highly aggressive or the scales formed on the steel are non- protective, the use of corrosion inhibitors is required. Organic compounds containing nitrogen such as amines, amides, quaternary ammonium salts and specially imidazolines and their derivatives, are widely used in the petroleum industry with this purpose. They usually are adsorbed on the metallic surface generating a protective film that interferes with the electrochemical reactions involved in the corrosion processes. Despite their extensive use, their action mechanisms are mostly unknown. The importance of chemical composition and microstructure on CO2 corrosion of carbon and low alloy steels has been widely recognized, but different aspects are still uncertain and contradictory results can be found in the literature [1,2]. The effect of these variables on the corrosion product films formation and properties, and on inhibitors efficiency are far from being understood. This is mainly due to the complexity of the problem and the difficulty to describe the involved mechanisms. Additionally test conditions also vary widely, making them almost impossible to compare. According to Rosenfeld et. al. [10], inhibitors incorporate to the corrosion product layer and form a protective barrier between the base material and the corrosive media. French et. al. [11] presented SEM results showing that the structure of the corrosion product layer is modified by the inhibitors. They suggested that the structure of the inhibitor must be the appropriate one to interact with the corrosion products and that they can be effective on iron carbonates or sulfides, but not effective on oxides. Oblonsky et. al. [12] studied the adsorption of octadecyldimethylbenzylammonium chloride (ODBAC) on carbon steel with two different microstructures. They found that ODBAC physisorbs strongly on the ferritic-perlitic microstructure and weakly on the martensitic microstructure. They considered that the passive film formed on the martensitic steel is more stable and prevents optimal adsorption of the inhibitor. In previous works we have shown that inhibitor efficiency is affected by microstructure and that the effect could be specific to inhibitor molecular structure [3-5]. EIS has proved to be a powerful technique to study corrosion processes and inhibitor performance in different environments [4,13,14]. Due to the lack of a full understanding of the effect of steel microstructure on inhibitor performance in CO2 corrosion, it is important to have a better knowledge of the inhibition mechanisms considering the influence of both corrosion products and the microstructural steel

2 properties. Because the widespread usage of imidazoline derivatives as inhibitors in the petroleum industry this type of inhibitor was selected in the present work. Both EIS and some standard d.c. measurements (LRP, Ecorr) were employed to study the performance of an oleic imidazoline-based inhibitor on a carbon steel with two different microstructures (ferrite- perlite and tempered martensite) in a high chloride, CO2 saturated solution. Surface films characteristics were studied by means of SEM.

2. Experimental

A carbon steel with the following composition (% wt.): 0.38 C - 0.99 Mn- 0.33 Si - 0.17 Cr - 0.09 Cu - 0.04 Ni -0.02 Mo - <0.01 P- <0.01 S - Fe balance was used. Two different heat treatments were performed on the samples: annealing: austenized at 890 °C and furnace cooled (Sample H); and quenching and tempering (Q&T): austenized at 890 °C, water Q&T 1h at 700 °C (Sample T). The heat treatment conditions were chosen in order to obtain two very distinctive cementite morphologies (coarse laminar and globular). Working electrodes were machined from these heat-treated materials into 5-mm diameter bars, cut and mounted with epoxy resin in a disc electrode holder. Electrical contact between sample and holder was obtained with silver loaded epoxy resin. For electrochemical studies surfaces were polished with 600-grit SiC paper. Experiments were conducted at atmospheric pressure, 40 0C and with low speed stirring (100 rpm) to ensure laminar flow conditions. Three-electrode jacketed test cells with a working volume of 0.5 liters and a concentric Pt ring as counter electrode were used. A saturated calomel electrode was chosen as reference. Test solution was 5% wt. NaCl (analytical-reagent grade), saturated with deoxygenated CO2. 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 a DCR OXI200 (Chemetrics) and it was kept below 40 ppb during the experiments. To minimize the air ingress, a positive pressure of deoxygenated CO2 was maintained in the cells during the experiments.  Chemetrics colorimetric ampoules were used to measure the dissolved CO2 concentration as H2CO3 and the values were 1000 ppm in all the tests. The pH was adjusted to 6 adding 15 ml of deoxygenated 1.0 M aq. NaHCO3. A commercial imidazoline-based product (R1 = (CH2)n-CH3 n = 10-17, R2 = (CH2)2-NH2) was used as inhibitor. Taking into account inhibitor manufacturer specifications, a concentration of 50 ppm was used. A Solartron 1280B unit was used for the electrochemical measurements. EIS was measured at the corrosion potential (Ecorr) using an applied potential of ± 0.005 V rms and a frequency range of 20000 to 0.05 Hz. Linear Polarization Resistance (LRP) was measured by polarizing -4 the working electrode ± 0.015 V vs. Ecorr with a sweep rate of 10 V/s. The corrosion potential was also monitored before and after d.c. and a.c. analyses. The inhibitor was added after various pre-corrosion periods: 24, 48 and 72 hours. During the pre-corrosion period, measurements were performed every 24 hours. The formation of the inhibitor film was evaluated after 2 and 24 hrs. A scanning electron microscope (SEM Phillips XL 30) was used for surface film characterization. For the calculation of the corrosion rate the Stearn Geary equation was used with B = 0.022V [14].

3 3. Results and Discussion

Table I shows the d.c. electrochemical study results obtained from at least 10 replicas of each experimental condition. For all the experimental conditions, the inhibitor addition moves the corrosion potential to more positive values, indicating that an anodic type inhibitor is being used. Moreover, the inhibitor addition strongly decreases the corrosion rate, being this effect dependent on both the steel microstructure and the pre-corrosion period length. It is worth noting that pre-corrosion time decreases inhibitor efficiency. Regarding the microstructure, there are not great differences in corrosion rate previous to the inhibitor addition.

2 Table I: Ecorr [mV vs. SCE], RP [Ω.cm ], SRp (± RP dispersion), r [mm/year]. 2 and 24 hours of inhibition on H and T samples pre-corroded for 24, 48, and 72 hours.

H samples T samples Condition Ecorr RP SRp r Ecorr RP SRp r 24hrs pre- corrosion -741 169 21 0.76 -745 173 14 0.74 2hrs inhibitor -682 1448 304 0.09 -672 1585 512 0.08 24hrs inhibitor -621 7248 1571 0.018 -626 8144 2368 0.016

H samples T samples Condition Ecorr RP SRp r Ecorr RP SRp r 48hrs pre- corrosion -736 151 20 0.85 -742 146 16 0.88 2hrs inhibitor -679 861 240 0.15 -651 1455 552 0.09 24hrs inhibitor -624 5013 891 0.026 -604 6081 1466 0.021

H samples T samples Condition Ecorr RP SRp r Ecorr RP SRp r 72hrs pre- corrosion -740 193 78 0.67 -739 138 18 0.93 2hrs inhibitor -713 445 148 0.29 -703 478 168 0.27 24hrs inhibitor -635 4692 957 0.027 -643 5962 2877 0.022

Polarization resistance (RP) dispersions are higher for Q & T than for annealed samples, indicating that the film formed on these samples after inhibitor addition is very sensitive to minor perturbations of the experimental conditions. Nevertheless, even having higher dispersion the T samples show a better behavior for most of the evaluated conditions. Corrosion products characteristics can be observed in Figures 1 and 2. Figure 1 shows surface morphology of 24 hours pre-corrosion and 24 hours inhibition samples. The existence of a homogeneous thin film due to inhibitor action can be inferred since the surface roughness decreases but the morphology of the corroded surface is not completely erased. For longer exposure to the corrosive medium, previous to the inhibitor addition, the surface roughness increases, but still a homogeneous corrosion film can be observed on sample surfaces. The analysis in a cross section shows that corrosion product films on H samples are 10 µm thickness average while in the case of T samples are less than 3 µm. The presence of cementite was detected in the films. The laminar structure is clearly visible in the annealed

4 sample (high magnification is needed for globular cementite in T samples). The inhibitor film could not be differentiated from the corrosion product film in any of the specimens.

Figure 1: Surface morphology corresponding to 24 hrs pre-corrosion (left) and 24 hrs inhibition after 24 hrs of pre-corrosion (right) (a) Sample H (b) Sample T.

Figure 2: Cross section of surface films formed after 72hrs pre-corrosion and 24hrs inhibition: Sample H (top) Sample T (bottom).

5 The impedance diagrams for samples H and T after 2 and 24 hours of inhibition and pre- corrosion of 24, 48, and 72 hours are presented in Nyquist and Bode plots in Figures 3 to 5 respectively.

Figure 3: H and T sample impedance Figure 4: H and T sample impedance

diagrams, obtained after 2 and 24 hours diagrams, obtained after 2 and 24 hours

of inhibition on samples pre-corroded for of inhibition on samples pre-corroded for

24 hours. Bode plots: a) Modulus b) 48 hours. Bode plots: a) Modulus b)

Phase angle, c) Nyquist plot. Phase angle, c) Nyquist plot.

All experimental Nyquist plots have a depressed semicircular shape in the complex impedance plane, with the center under the real axis. This is the typical behavior of solid metal electrodes that show frequency dispersion of the impedance data [15]. Electrical equivalent circuits are generally used to model the electrochemical behavior and to calculate important parameters such as electrolyte resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Cdl) [16]. 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:

-1 -n ZCPE =Y (iω) (2)

Where “Y “is a proportional factor, “i” is √–1, “ω” is 2πƒ and “n” represents a phase shift [15]. Often a CPE is used in a model instead of a capacitor to compensate for non-

6 homogeneity in the system. For example, a rough or porous surface can cause a double-layer capacitance to appear as a constant phase element with an n value between 0.9 and 1 [17]. -1 For n = 0, ZCPE represents a resistance with R = Y , for n = 1 a capacitance with C = Y.

a) Rs ZCPEDL

Rct

Rs ZCPEF

RF ZCPEDL

Rct

Figure 6: Electrical equivalent circuits, used to fit the impedance curves.

Figure 5: H and T sample impedance diagrams, obtained after 2 and 24 hours of inhibition on samples pre-corroded for 72 hours. Bode plots: a) Modulus b) Phase angle, c) Nyquist plot.

The electrical equivalent circuits used to analyze the impedance plots are shown in Figure 6 and the values of the circuit parameters for all the conditions studied are shown in Table II. In Figure 6a, RS is the electrolyte resistance, Rct is the charge transfer resistance and ZCPEdl is related to the non ideal capacitance of the surface products film. In Figure 6b, RS represents again the electrolyte resistance, ZCPEF is related to the non ideal capacitance of the inhibitor film, RF is related to the resistance of the inhibitor film, ZCPEdl is related to the non ideal capacitance of the double layer of the metal/corrosion product film and Rct is the charge transfer resistance. Optimum fit with the model was obtained for all experimental data. The circuit shown in Fig 6 a was used to fit 2 hrs inhibition experiments after 24, 48 and 72 hrs of pre- corrosion. This circuit was also used for 24 hrs of inhibition after 72 hrs of pre-corrosion. To fit the results corresponding to 24 hrs of inhibition after 24 hrs and 48 hrs of pre-corrosion a two time constant circuit (Figure 6b) was employed.

7 2 Table II: Equivalent circuit parameters from fitting impedance curves. RS, Rdl, RF [Ω.cm ]. 2 YCPEdl, YCPEF [F/cm ], nCPEdl, nCPEF constant phase element exponent. 2 and 24 hours of inhibition on H and T samples pre-corroded for 24, 48, and 72 hours.

24hrs pre-corrosion 48hrs pre-corrosion 72hrs pre-corrosion Circuit 2hrs inhibition 2hrs inhibition 2hrs inhibition Parameters H Samples T Samples H Samples T Samples H Samples T Samples Rs 2.6 1.9 1.9 2.1 2.5 2.6 Rct 1226 1506 856 1422 443 455

YCPEdl 0.000694 0.000545 0.001592 0.001280 0.001529 0.001487 nCPEdl 0.81 0.82 0.85 0.79 0.86 0.90

24hrs pre-corrosion 48hrs pre-corrosion 72hrs pre-corrosion Circuit 24hrs inhibition 24hrs inhibition 24hrs inhibition Parameters H Samples T Samples H Samples T Samples H Samples T Samples Rs 2.5 2.2 2.6 2 2.4 2.7

RF 11 30 8 14 ****** ******

YCPEF 0.000284 0.000107 0.000471 0.000255 ****** ****** nCPEF 0.84 0.82 0.85 0.81 ****** ****** Rct 7046 7610 5002 6008 4565 5595

YCPEdl 0.000130 0.000136 0.000348 0.000188 0.001092 0.000752 nCPEdl 0.85 0.69 0.82 0.68 0.88 0.83

High frequency time constants could be related to the formation of a thin and protective film on the sample surface due to longer times in contact with the inhibitor [5, 18]. The circuit parameter values in Table II support this hypothesis. In fact, for all the pre- corrosion conditions after a short period of exposure to the inhibitor (2 hours) and also for 24 hrs of inhibition after 72 hrs of pre-corrosion no high frequency time constants were obtain. These results could indicate the presence of a less protective film. For 2 hrs inhibition, the charge transfer resistance decreases with longer pre-corrosion periods while the non ideal capacitance shows a tendency to increase indicating poorer properties as protective film. Concerning microstructure, the comparison between Rct and YCPEdl values could indicate that films on samples T are more protective than on samples H, but the detrimental effect of the pre-corrosion is stronger in samples T. For longer inhibition periods (24 hrs) and 24 hrs and 48 hrs of pre-corrosion (Table II) the film resistance (RF) is higher for samples T and decreases for longer periods of pre-corrosion, being not detectable for 72 hrs pre-corrosion. The film capacity is higher than for 2 hours of inhibition. The above results indicate that after 24 hours in contact with the inhibitor a very thin and isolating film has been formed on the surface of the samples. From the RF and Y CPEF values, it can be seen that the protective properties are superior for the Q &T ones. Although other authors have also found a detrimental influence of pre-corrosion on inhibitors efficiency [9,14] the proposed mechanism is related to the accumulation of laminar cementite on the steel surface that provides a larger cathodic area. The results presented above do not support this mechanism because the effect of pre-corrosion is present in both microstructures, being the one with much less cementite accumulation on surface more sensitive to this effect.

8 4. Conclusions

From the above results some conclusions can be withdrawn:

• Steel microstructure affects inhibitors efficiency • For both microstructures, a protective film is formed after the inhibitor addition that greatly decreases the corrosion rate. • The effect is more important for Q & T samples but is dependent on the length of the pre-corrosion period. • Pre-corrosion has a negative effect on inhibitor efficiency on both microstructures, although Q & T seems to be more sensitive to the surface conditions. • EIS allows obtaining “in situ” information concerning surface films properties. • More work has to be done in order to provide more evidence on the effect of microstructure and corrosion products films characteristics on inhibitors efficiency. Particularly, in order to clarify the influence of cementite accumulation on surface it will be very useful to monitor the system for longer periods of pre-corrosion and of inhibitor film formation.

5. Acknowledgements

This work was supported by the Argentine Research Council for Science and Technology (CONICET, grant PIP 0413/98), FUDETEC and the University of Mar del Plata. The authors are grateful to Dr. Pablo Castro and Eng. Guillermo Pantiu (CINI) for their valuable assistance in the SEM studies.

6. References

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