Chemical and Electrochemical Mechanisms Behind Aqueous Co2 Corrosion of Mild Steel- a Basic Review

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Inquiry Commodity | March 23 2018

Technical Note: Electrochemistry of CO₂ Corrosion of Balmy Steel: Effect of CO₂ on Cathodic Currents

Aria Kahyarian;

*Plant for Corrosion and Multi-Stage Flow Engineering, Ohio Academy, 342 Due west Country Street, Athens, OH 45701.

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Bruce Chocolate-brown;

*Institute for Corrosion and Multi-Phase Menstruum Technology, Ohio Academy, 342 West Land Street, Athens, OH 45701.

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Srdjan Nešić

*Institute for Corrosion and Multi-Phase Flow Technology, Ohio Academy, 342 Due west State Street, Athens, OH 45701.

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CORROSION (2018) 74 (8): 851–859.

The common understanding of aqueous CO_ii corrosion mechanism considers carbonic acid equally an electroactive species. The straight reduction of carbonic acid on a steel surface is believed to be the cause of the higher corrosion rates of mild steel, every bit compared to that observed in strong-acid solutions with the aforementioned pH. However, in-depth quantitative analyses based on comprehensive mechanistic models, developed in contempo years, accept challenged this idea. In an attempt to provide explicit experimental evidence for the significance of directly reduction of carbonic acid in CO_two corrosion of mild steel, the charge transfer controlled cathodic currents in CO₂ saturated solutions were investigated in the present study. The experiments were conducted on three different surfaces: Blazon 316L stainless steel, pure fe, and API 5L X65 mild steel, in gild to examine the possible effect of alloying impurities on the kinetics and the mechanism of cathodic currents. The experimental polarization curves showed that at a constant pH, the charge transfer controlled cathodic currents did not increase with increasing fractional pressure of CO_ii from 0 bar to 5 bar. This confirmed that the direct carbonic acrid reduction was not significant at the conditions covered in the present written report, and its sole outcome was to buffer the hydrogen ion concentration.

INTRODUCTION

The mechanism of cathodic reactions involved in CO₂ corrosion, i.east., the sequence of electrochemical and chemical reactions, is a rather complex matter, in the sense that it involves a number of electroactive species that are interrelated through homogeneous chemic reactions. The CO₂ gas, upon dissolution in water (Reaction [1]), goes through hydration (Reaction [2]) and dissociation reactions (Reactions [iii] and [4]) to form an acidic, corrosive solution.

The CO₂ corrosion in aqueous acid solutions is generally believed to involve numerous electrochemical reactions as shown below.^i-3 Anodic partial of Reaction (5) is the cause of metallic deterioration, and cathodic fractional of Reactions (6) through (9) provide the electron sink required for the anodic reaction to progress spontaneously. The significance of these reactions is mainly based on the studies done by de Waard and Milliams in 1975,^4-v Schmitt and Rothmann in 1977,⁶ and Grayness, et al., in 1989 and 1990,^7-eight as reviewed in more than details elsewhere.^9-11

The profound effect of homogeneous reactions is mainly associated with the CO₂ hydration equilibrium, where simply a small fraction (∼0.2%) of CO_ii(aq) reacts to form H₂CO₃.¹ Therefore, there is a large reservoir of CO_ii(aq) present in the solution to replenish the H₂CO₃ concentration every bit it is consumed past the corrosion procedure. The higher corrosion rates observed in aqueous CO_ii solutions, equally compared to a potent acid solution (e.g., HCl) at the same pH, were therefore associated with the boosted H₂CO_iii reduction and the effect of CO₂ hydration reaction.^10-12

While the abovementioned mechanistic view of the cathodic reactions in CO_two corrosion is widely accepted, the findings in more recent studies have challenged its basis.^13-fifteen In those studies it was shown quantitatively that the limiting currents could exist adequately explained even if H_iiCO_three was not considered an electroactive species.^{thirteen-fifteen} This can exist understood when because the local concentration of chemical species at the metal surface, and the influence of the homogeneous chemic reactions. That is, the H_iiCO_iii dissociation reaction (Reaction [3]) occurs in the vicinity of the metallic surface, followed by electrochemical reduction of the produced H⁺ ions (Reaction [6]), which provides a parallel reaction pathway to the direct H₂CO₃ reduction reaction. This observation carries a significant mechanistic implication, because information technology undermines the previous commonly accepted mechanistic arguments, which were developed based on the analysis of cathodic polarization beliefs at or shut to limiting currents.^6-8 Therefore, to date, the evidence for straight H₂CO₃ reduction is mostly circumstantial. This was perchance all-time noted by Nordsveen, et al.,^thirteen who suggested that while the cathodic limiting currents can be quantitatively explained without because H₂CO₃ as an electroactive species, the predicted corrosion rates are in better agreement with the experimental information when this additional reaction was included in the model.

The electrochemical activity of H₂CO_iii has been discussed specifically in a few dissimilar studies. Linter and Burstein published 1 of the primeval manufactures suggesting that H₂CO_iii is non electrochemically active.¹⁶ The authors investigated the machinery of CO_ii corrosion on both a 13Cr stainless steel and a depression alloy steel. The arguments were developed based on the polarization curves obtained in N₂-saturated and CO_two-saturated 0.5 M NaCl solutions at pH 4.0 with additional potassium hydrogen phthalate buffer. In a nutshell, the authors were able to observe the accuse transfer controlled current densities at both N_ii-saturated and CO₂-saturated solutions. The results showed no pregnant increase in this range of current densities when comparing the 2 solutions, leading to the conclusion that H₂CO₃ is non electrochemically active. The findings of Linter and Burstein¹⁶ did not gain much attention over the years, perhaps considering of the concerns arising from the limited environmental weather covered in their study—i.e., the fact that at pH 4.0 the cathodic current is dominated by H⁺ reduction. The concentration of H₂CO₃ at 1 bar (100 kPa) CO_ii is nigh a third of H⁺ at pH iv;¹⁰ with roughly like commutation electric current densities considered for H_iiCO_iii and H⁺,¹² the expected contribution of H_twoCO₃ falls hands within the experimental error of the measurements reported by the authors. In addition, while the use of boosted buffer was an elegant way to elucidate the charge transfer cathodic currents, concerns could be raised about the secondary effects of these buffers on the electrochemical reactions.

In 2008, Remita, et al., studied the electrochemical activity of H₂CO_iii using a more than quantitative approach.¹⁷ The authors conducted a series of experiments in N₂-saturated and CO₂-saturated solutions at pH ∼ iv using a rotating disk electrode (RDE) experimental appliance. Their arguments were based on a comprehensive mathematical model, similar to those discussed to a higher place.^{thirteen-fifteen} Using the electrochemical kinetic parameters obtained for H⁺ reduction in Northward₂-saturated solutions, authors were able to predict the results obtained in CO₂-saturated solutions without considering H₂CO_three equally a pregnant species (absent-minded in their model). Their observation led to the determination that H_iiCO_iii is not electrochemically active, and its sole consequence was claimed to exist the buffering effect of H₂CO₃ on surface concentration of H⁺. It is worthwhile to mention that the arguments used in this study suffer from the same shortcomings as those in the study by Linter and Burstein.¹⁶ That is the very narrow range of experimental weather and the fact that at their conditions (pH iv and 1 bar CO₂) the cathodic currents are dominated by H⁺ reduction. In fact, i may advise that the charge transfer controlled currents were not clearly observed every bit compared to the study of Linter and Burstein,¹⁶ where an additional buffer was used to shift the mass transfer limiting current toward higher values. At the atmospheric condition in the study past Remita, et al.,¹⁷ the observed range of cathodic currents were generally under mixed charge transfer/mass transfer command, and the pure charge transfer controlled currents were covered by the anodic reaction at lower currents and by the mass transfer limiting current at higher currents. That makes the distinction of the possible outcome of electrochemical action of H₂CO_three even harder. The determination made past Remita, et al., that H₂CO₃ acts as a buffer in this system, is in agreement with what was suggested earlier—that the limiting electric current densities could exist reasonably predicted without the straight reduction of H_twoCO₃.^13-xv The consequence of flow velocity that was discussed extensively, and the practiced agreement obtained with the model prediction is merely a further confirmation of the buffering power of H₂CO_iii as a weak acid.

Information technology is important to realize that the clearly demonstrated buffering ability of H_iiCO₃ (or any other weak acid) does non exclude the possibility of H_iiCO₃ direct reduction, as these are two independent processes. That is the reason why in club to distinguish them, the arguments must be based on the behavior of pure accuse transfer controlled currents so that the electrochemical activity of H_iiCO₃ tin be separated from the chemic equilibria (buffering) associated with this species. This concept was not properly accounted for in the analysis of the surface pH measurements fabricated by Remita, et al.¹⁷ Their results conspicuously showed that in the presence of H_twoCO_iii the surface pH is lower than in a N₂-saturated solution of the same pH. While this ascertainment farther confirms the buffering power of H₂CO_three, information technology provides no insight into the electrochemical action of this species, every bit they claimed. In fact, because the fast kinetics of recombination as compared to the CO₂ hydration,^9-ten the surface pH is expected to be nearly identical irrespective of whether H₂CO₃ is reduced (to H_two and ⁠) or non. In brief, the written report by Remita, et al., is of significance every bit it farther elucidated the possible mechanisms underlying CO_ii corrosion by explicitly focusing on the buffering ability of H_twoCO_three. Withal, the arguments and the experimental results did non provide sufficient prove equally it relates to electrochemical activeness of H_iiCO₃.

In effort to accost the shortcomings of the previous studies, Tran, et al., conducted a series of experiments at elevated pressures upwards to 10 bar (1,000 kPa) CO₂.¹⁸ At such weather condition, the authors were able to investigate the electrochemical activity of H_twoCO₃, every bit the dominant chemical species, with more confidence. Yet, it was noted that even at such high CO_ii partial pressures the accuse transfer controlled currents could not exist observed on X65 mild steel because of the interference of the anodic reaction at low current densities and the mass transfer limitation at the college end. Therefore, the experiments were conducted on a Type 304 stainless steel (UNS S30400 ⁽ⁱ⁾ ) surface. The suppressed anodic current densities on stainless steel surface allowed the cathodic charge transfer controlled currents to be observed clearly. The experimental results showed that the presence of H_iiCO₃, fifty-fifty at loftier levels (when fractional force per unit area of CO₂ [pCO_two] = 10 bar), did not consequence in any significant change of accuse transfer controlled currents as measured on a stainless steel surface. This observation demonstrated that H₂CO_iii is not electrochemically agile, at least not on the surface of stainless steel. While the experimental conditions of this study allowed a proper measurement and discussion of the electrochemical activity of H_twoCO₃, ane must consider the fact that the 2 surfaces (actively corroding balmy steel vs. the passive stainless steel) are very dissimilar. The presence of a significant corporeality of alloying elements (i.e., ∼twenty% Cr, 10% Ni) raises the uncertainty most whether the electrochemical mechanisms identified on stainless steel can be just assumed to be valid for mild steel surfaces. The mechanism and the kinetics of the hydrogen development reaction are known to exist significantly influenced past the composition of the substrate and even by the fine differences in surface preparation procedures.^xix-21 Additionally, the passive oxide layer formed on stainless steel may alter the kinetics and the mechanism of the hydrogen development reaction,²² considering that the hydrogen evolution reaction (from H⁺ or H_iiCO_iii) involves a series of surface dependent chemical/electrochemical adsorption/desorption steps.^23-24

The review of the existing literature clearly shows that despite many decades of research on the mechanism of CO_two corrosion, some important mechanistic aspects have remained unresolved. Amidst them is the electrochemical activity of H_iiCO_three. However, neither of the two competing ideas nigh the electrochemical action of H₂CO₃ appears to take sufficient experimental evidence in their support then far. As identified previously,^18,25-28 the directly experimental evidence for electrochemical activity of a weak acrid (such every bit H_twoCO₃) may be obtained by investigating the behavior of pure charge transfer controlled cathodic currents. If the reduction of H_twoCO_three is meaning, at a fixed pH, the charge transfer controlled currents would increase as pCO₂ increases—as a result of increased H₂CO₃ concentration, and thus, increased rate of H₂CO₃ reduction reaction. On the other paw, if the accuse transfer controlled currents remained unaffected by pCO₂, it can exist deduced that H⁺ reduction is the ascendant cathodic reaction and H₂CO₃ is non significantly electroactive.

It is apparent from the previous attempts on investigating the electrochemical activeness of H₂CO₃ ^16-xviii that the main claiming in verifying these hypothetical behaviors is to create the experimental conditions required to find the charge transfer controlled cathodic currents. In the present report, in addition to the experiments conducted in a conventional three-electrode drinking glass prison cell exam apparatus, a thin channel catamenia geometry, enabling high-catamenia velocities, was used in order to further increase the limiting currents. This was accentuated by lowering the temperature in order to disproportionally decrease the rates of electrochemical reactions, making them the rate determining footstep, and also past increasing the pCO₂ in a higher place atmospheric pressures.

Measurements in the nowadays report were conducted on three different surfaces: Type 316L stainless steel (UNS S31603), 99.9 wt% pure iron, and API 5L X65 mild steel. Mild steel is a typical textile of option for the transmission pipelines in the oil and gas industry, which is at focus in the present study. The stainless steel electrode was selected considering of the same considerations as suggested by Tran, et al.,^xviii and as well to provide an opportunity for the comparison with the previous studies.^xvi,18 The option of pure iron was made because of its close relevance with mild steel (which consists of ∼98 wt% iron), when compared to stainless steel (∼70 wt% iron), in order to provide further insight into the possible outcome of alloying elements.

MATERIAL AND METHODS

The Glass Cell

A series of experiments were conducted in a ane Fifty glass jail cell, using a RDE, iii-electrode test apparatus. The experimental setup is similar to that described for an before report.²⁶ The 0.1 M NaCl supporting electrolyte was purged with N₂ or CO₂ gas, depending on the type of experiment. The outlet gas was monitored with an oxygen sensor (Orbisphere 410 ^† ) to assure sufficient deoxygenation (∼1 ppb_m dissolved oxygen). The solution pH was so adjusted to 4.0 using a small amount of diluted NaOH or HCl solutions. That was followed by further purging of the solution to maintain the minimal amount of dissolved oxygen content. The solution pH was monitored throughout all experiments to define a constant value.

The RDEs with a 5 mm diameter were made of either 99.99 wt% pure atomic number 26 or API X65 5L mild steel (composition in Table i), press fitted into a Teflon^TM† electrode holder (Pino instruments). The electrodes were polished and electrochemically treated according to the procedure discussed elsewhere.²⁵

Tabular array 1

Chemic Composition of Steel Working Electrodes (wt%)

The cathodic polarization measurements were initiated from open up-circuit potential (OCP) toward the more negative values afterward a stable OCP was observed (< ±2 mV drift over 5 min). The steady-state voltammograms were obtained using staircase voltammetry at 0.5 mV/s scan rate and ane southward^−ane sampling menstruum. The reported results are corrected for ohmic drib using the solution resistance obtained from electrochemical impedance spectroscopy (EIS) measurements performed later on the polarization experiments (DC potential at OCP, AC potential ±5 mV, frequency range ten kHz to 0.2 Hz at eight points/decade). The linear polarization resistant (LPR) measurements were conducted in split up tests following the abovementioned preparation procedure. The measurements were done past sweeping the potential from 5 mV_OCP to −5 mV_OCP, using 0.125 mV/s scan rate and ane s⁻¹ sampling flow.

The Sparse Aqueduct Menstruation Cell

The detailed description of the thin channel menstruum cell (TCFC) used in the present written report can exist found in before publications.^29-33 In the present study, the exam department was slightly modified by introducing a saturated Ag/AgCl reference electrode, flush mounted on the lid, directly opposite to the working electrode, every bit shown in Effigy 1. The cell construction was used as the counter electrode.

Figure ane.

The analogy of the three-electrode jail cell arrangement inside the thin channel test section.

Figure 1.

The illustration of the three-electrode cell arrangement inside the sparse aqueduct test section.

Shut modal

The 0.1 Yard NaCl solution (110 L) was made with deionized water and analytical form chemicals. The solution was then purged for ∼3 h, with Due north₂ or CO_ii gas, depending on the desired experimental conditions, while the outlet gas was monitored with an oxygen sensor to ensure proper deoxygenation. Maximum dissolved oxygen content, measured before initiating the experiment, was 3 ppb (typically ∼one ppb). In the high-pressure experiments, after the deoxygenation footstep, the system was pressurized to 5 bar (500 kPa) CO₂ and so maintained at that pressure level until the solution became saturated, afterward at to the lowest degree 3 h. Equally the concluding step, the pH (measured by an OMEGA 5431-10 ^† pH probe) was adjusted to the targeted value by gradual addition of deoxygenated HCl or NaOH solution into the system from a secondary pressurized reservoir.

The experiments were conducted on 3 different substrates: 99.99 wt% pure atomic number 26, Type 316L stainless steel, and API 5L X65 mild steel. The chemical composition of the stainless steel and the mild steel are shown in Tabular array ane. The working electrode assembly was built similarly to that shown in an earlier report,³³ with a unmarried disk working electrode, which was mounted into the test section as shown in Figure 1.

Prior to each experiment, the working electrode was abraded with a 600 grit silicon carbide paper, and so rinsed and sonicated for 5 min using isopropanol. The working electrode was affluent-mounted on the bottom of the thin channel test section, which was and then closed and purged with dry CO₂ or Due north_two. In the case of mild steel and pure iron electrodes, after exposing the electrode to the test solution, the OCP was monitored until a steady value was reached prior to initiating polarization measurements. For the experiments on the stainless steel surface, the polarization measurements were initiated two min after exposing the electrode to the test solution in order to avert any significant passivation of the electrode. The polarization curves were obtained using the aforementioned electrochemical measurement parameters every bit those in the glass cell experiments.

The solution temperature was controlled inside ±0.5°C by using a jacketed immersion heater located in the tank and covered cartridge heaters used to directly heat the test department (for experiments conducted at thirty°C), too every bit a shell and tube heat exchanger connected to a chiller (Air 3000 ^† FLUID CHILLERS, Inc.) for experiments done at ten°C. The period velocity inside the sparse channel examination section was fixed at 13 m/due south throughout the experiments.

All experiments in this study were repeated at least three times. The results shown in the post-obit are the average values of all repeats at sure potential intervals. The fault confined represent the minimum and maximum measured values.

RESULTS AND Give-and-take

The steady-country cathodic polarization curves obtained in glass cell experiments are presented in Figures two and 3. These experiments were conducted in order to examine whether the conditions typical for glass jail cell experiments allow a proper give-and-take of the electrochemical activity of H₂CO_iii. All of the polarization curves reported in the present study (obtained in both drinking glass prison cell and TCFC) demonstrate the same generic trend. A steep increase in current is seen merely below the OCP, followed by the limiting current and a linearly increasing current density range at even lower potentials that are associated with the water reduction reaction.

Effigy 2.

Steady-land cathodic polarization curves obtained at 30°C, pH 4.0, ii,000 rpm RDE on API 5L X65 mild steel, in N₂-saturated and CO_two-saturated solutions.

Figure 2.

Steady-state cathodic polarization curves obtained at 30°C, pH 4.0, two,000 rpm RDE on API 5L X65 mild steel, in N_ii-saturated and CO₂-saturated solutions.

Shut modal

FIGURE 3.

Steady-state cathodic polarization curve obtained at 30°C, pH 4.0, 2,000 rpm RDE in N₂-saturated solution on API 5L X65 mild steel and 99.99 wt% pure iron electrodes.

Figure 3.

Steady-state cathodic polarization curve obtained at thirty°C, pH 4.0, 2,000 rpm RDE in N₂-saturated solution on API 5L X65 mild steel and 99.99 wt% pure atomic number 26 electrodes.

Close modal

Figure two shows the comparison of the cathodic polarization curves obtained in a N₂-saturated solution at pH 4.0 with a CO₂-saturated solution at the same pH. The results prove a clear increase of the limiting current in CO_two-saturated solutions. Equally discussed above, this increment is stemming from the presence of carbonic acrid, and the CO_ii hydration reaction, which can exist readily explained irrespective of whether H_iiCO₃ is electrochemically active or not. The focus in the nowadays study is on the accuse transfer controlled cathodic currents, where the surface concentrations of species are the same as the bulk solution. At such conditions, homogeneous chemic dissociation of H_twoCO₃ has no influence on the current/potential response of the arrangement. Therefore, the surface concentration of H₂CO₃ depends only on pCO_two and temperature. Consider the dissolution and hydration equilibria:⁹

where the brackets denote equilibrium concentration in M, and K_hyd = 1.18 × 10⁻³ and Grand_dis = two.96 × x⁻ⁱⁱ One thousand/bar are the equilibrium constants of the hydration reaction (Reaction [2]) and dissolution reaction (Reaction [1]) at xxx°C, respectively.^34-35 Thus, for pCO₂ = 0.96 bar (96 kPa), the [H₂CO₃] = iii.35 × 10⁻⁵ Thousand. At these conditions, H⁺ is the dominant electroactive species with a 3-fold higher concentration than H₂CO₃. Hence, the theoretical difference expected from the ii proposed mechanisms—i with and the other without the direct reduction of H_twoCO₃—is very minor when compared to the typical experimental error.

Furthermore, the polarization curves shown in Figure 2 exercise non demonstrate a distinguishable accuse transfer controlled current range, in either of the solutions. In fact, the LPR measurement in CO_two-saturated solution estimated the corrosion current to be ane.81 A/m² (based on Stern-Geary equation with B value = thirteen mV). The comparison of this value with the mass transfer limiting current of 6.34 A/m² suggests that even in CO₂-saturated solution the observed cathodic polarization curve is significantly under the influence of the limiting electric current. Remembering that the limiting current density is identical for both mechanisms, a simple comparing tin be made based on:

which suggests that in a mixed charge transfer/mass transfer regime, the theoretical divergence betwixt the ii aforementioned mechanisms is even smaller. Therefore, the results obtained in typical glass cell experiments, which are similar to those reported previously,^16-17 do not let for a proper mechanistic discussion on the electrochemical activity of H_iiCO_iii. In gild to examine the electrochemical activity of H₂CO₃, the experiments were connected in the TCFC exam appliance, where more suitable experimental conditions could be achieved, as described farther beneath.

The influence of substrate limerick on the electrochemical response of the organisation was also examined briefly in the glass jail cell experiments. The cathodic polarization bend obtained on 99.99 wt% pure iron in N_ii-saturated solution is compared with that obtained on API 5L X65 mild steel surface in Figure 3. While the reproducibility of the results obtained on a pure iron surface was slightly lower when compared to steel, the polarization curves showed that the pure iron surface is a significantly weaker goad for the reduction reactions, in agreement with previous reports.^25,36 Because that the pure charge transfer controlled currents for H⁺ reduction were not observed, the true difference in the electrocatalytic outcome of the 2 surfaces cannot exist properly distinguished here. Nevertheless, the observed difference even in this mixed mass transfer/accuse transfer controlled regime signifies the importance of the substrate composition when discussing the electrochemical kinetics and mechanisms.

The post-obit experiments that were conducted in the TCFC test apparatus had 2 chief advantages: the ability to increment the catamenia velocity to significantly higher values in gild to increment the mass transfer limitation and the ability to conduct the experiments at elevated pCO₂ (5 bar maximum operating pressure level), hence, increasing the concentration of H_twoCO₃. Effigy 4 shows the cathodic polarization curves obtained at 30°C in the TCFC.

FIGURE four.

Steady-state cathodic polarization curves at pH 4, 30°C, 13 m/s TCFC, 0.1 One thousand NaCl, 0.5 mV/due south scan rate. (a) Blazon 316L stainless steel, (b) 99.99 wt% pure iron, and (c) API 5L X65 mild steel.

Figure iv.

Steady-country cathodic polarization curves at pH 4, 30°C, 13 m/s TCFC, 0.1 M NaCl, 0.5 mV/south browse charge per unit. (a) Type 316L stainless steel, (b) 99.99 wt% pure atomic number 26, and (c) API 5L X65 mild steel.

Close modal

The cathodic polarization curves obtained on Type 316L stainless steel electrodes are shown in Effigy iv(a). The results conspicuously demonstrate the accuse transfer controlled currents, in a wide potential range. This range of current densities was not affected past increasing the pCO₂ from 0 bar to 5 bar, suggesting that H_twoCO_three reduction on stainless steel is not pregnant at these atmospheric condition. The sole issue of H₂CO₃ was buffering the H⁺ concentration, and hence, increasing of the limiting electric current. These experimental results were establish to exist in agreement with those reported previously.^sixteen,18 The limiting currents in Figure 4(a) testify an increase in presence of CO₂. However, the increment of pCO₂ from 0 bar to 1 bar resulted just in a slight increase in limiting electric current densities. That is a result of the overwhelmingly loftier mass transfer flux of H⁺. As pCO₂ increased farther to 5 bar, the concentration of H₂CO_three increased and the issue of CO₂ hydration reaction became more pronounced, leading to a significantly higher limiting current density.

The cathodic polarization behavior on pure atomic number 26 electrodes is shown in Figure iv(b). The charge transfer controlled currents were also clearly observed over a reasonably extended potential range. On the iron surface, the reproducibility of the results decreased, which is indicated by the larger error bars. The accuse transfer controlled cathodic currents appear to show a slight variation at different pCO₂; however, this is not conclusive because of the magnitude of the error bars. The comparing of the polarization curves, particularly those obtained at five bar pCO₂ (where carbonic acid is the dominant species) with those at 0 bar CO₂, does not indicate any significant electrochemical activity for H_iiCO₃.

The cathodic polarization curves obtained on an API 5L X65 mild steel surface are shown in Figure 4(c). The comparing of the mass transfer limiting currents obtained in N₂-saturated solutions with those obtained in glass cell experiments at similar weather condition (Figure 2) show more than a 5-fold increase in mass transfer limiting electric current (22.ix A/gⁱⁱ vs. 4.two A/m²), notwithstanding the charge transfer controlled current range was still non observed clearly. With increase of pCO_ii to i bar, the accuse transfer controlled range gradually appeared and it is seen conspicuously at pCO_ii = five bar. On the mild steel surface, the pure charge transfer controlled currents were observed in a rather narrow range of potentials, as compared to those on stainless steel and iron surfaces. Nevertheless, the comparing of the results at 1 bar and v bar CO₂ does not indicate significantly higher electric current densities in that range, favoring the arguments that H₂CO₃ is not electrochemically agile on API 5L X65 balmy steel either.

In society to extend the range of charge transfer controlled currents on the API 5L X65 mild steel surface and solidify the mechanistic arguments higher up, similar experiments were conducted at 10°C. Decreasing the temperature was expected to reduce the rates of electrochemical reactions more than the limiting currents. Such a disproportional decrease would allow the accuse transfer controlled currents to be observed in a wider range. That is shown to exist the instance in Figure 5, where the cathodic polarization curves obtained in N₂-saturated solutions at ten°C and 30°C are compared. At 10°C, the rate of H⁺ reduction reaction is decreased almost by an order of magnitude.

FIGURE 5.

The event of temperature on steady-country cathodic polarization bend obtained on API 5L X65 mild steel in Due north₂-saturated solution at pH 4.0, thirteen thousand/s TCFC, 0.one 1000 NaCl, 0.v mV/s.

FIGURE 5.

The effect of temperature on steady-state cathodic polarization curve obtained on API 5L X65 mild steel in N₂-saturated solution at pH 4.0, thirteen m/s TCFC, 0.i Grand NaCl, 0.five mV/s.

Close modal

The effect of pCO₂ on the accuse transfer controlled current densities obtained on API 5L X65 mild steel at x°C is shown in Figure 6. At this condition the charge transfer controlled currents on balmy steel were conspicuously observed and showed no meaning dependence on pCO_two. The results obtained at 10°C were, therefore, constitute to farther support the previous observation that the direct reduction of H_twoCO₃ on a mild steel surface is insignificant.

FIGURE 6.

Steady-state cathodic polarization curves at pH four.0, 10°C, xiii m/s TCFC, 0.1 M NaCl, 0.v mV/southward scan rate on an API 5L X65 mild steel surface.

Figure 6.

Steady-country cathodic polarization curves at pH 4.0, 10°C, 13 m/southward TCFC, 0.1 M NaCl, 0.five mV/southward scan rate on an API 5L X65 balmy steel surface.

Close modal

The polarization curves obtained on the three different surfaces are compared in Figure 7 for solutions at 5 bar CO₂ and xxx°C, where the pure accuse transfer controlled currents were observed on all iii substrates. The results bear witness a significant effect of the surface composition on the observed electrocatalytic action related to H⁺ reduction, in the post-obit order: mild steel > stainless steel > pure iron. Such a large difference in the electrocatalytic behavior of different substrates may result in dissimilar electrochemical mechanisms, especially considering that the investigated reactions are multi-footstep and include dissimilar adsorption/desorption elementary reactions. In that case, fifty-fifty a small change in the adsorption energies of the intermediate species may result in different mechanistic behavior. Hence, while the selection of different substrates may be an appealing arroyo to investigate the electrochemical mechanisms, the complication introduced by different electrocatalytic properties requires a conscientious verification of the results on the substrate of interest.

Figure 7.

The comparison of the steady-state cathodic polarization curves obtained on API 5L X65 balmy steel, Blazon 316L stainless steel, and 99.99 wt% pure fe at pH four, 30°C, thirteen 1000/s TCFC, 0.one M NaCl, 0.five mV/s browse rate.

Effigy 7.

The comparing of the steady-state cathodic polarization curves obtained on API 5L X65 balmy steel, Type 316L stainless steel, and 99.99 wt% pure atomic number 26 at pH four, 30°C, 13 one thousand/s TCFC, 0.1 M NaCl, 0.5 mV/due south scan rate.

Close modal

In the present written report, the cathodic polarization behavior of CO_two-saturated solutions at pH four.0 and pCO₂ upwards to 5 bar was investigated on pure iron, stainless steel, and mild steel surfaces. The experimental results obtained on all three substrates advise that H_iiCO_three reduction was non significant at the atmospheric condition considered here, in support of the contempo mechanistic arguments found in the literature.^16-18 Therefore, the crusade of higher corrosion rates in CO₂-saturated brines has to be sought elsewhere. In a recent study,³⁷ focused on the iron dissolution reaction, the presence of CO₂ was found to significantly increment the observed anodic currents. From these observations, it appears that the underlying mechanism of CO₂ corrosion is nonetheless to be fully established.

CONCLUSIONS

• •

  The cathodic polarization behavior of acidic CO_ii-saturated solutions at pH iv.0 was investigated on 99.99 wt% pure iron, Blazon 316L stainless steel, and API 5L X65 mild steel surfaces, using a conventional three-electrode glass cell and a thin aqueduct flow cell. The charge transfer controlled currents were observed most clearly at high flow rates and lower temperatures achieved in the thin channel flow cell. The cathodic currents obtained on all three substrates showed no indication of direct reduction of carbonic acid upward to pCO₂ = five bar. The comparison of the polarization behavior on the iii substrates showed a significant departure in their electrocatalytic activeness when it comes to H⁺ reduction, with the API 5L X65 mild steel being virtually active, followed by Blazon 316L stainless steel, and with 99.99 wt% pure iron beingness least agile.

^(one)

UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published past the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

ACKNOWLEDGMENTS

The authors would like to give thanks the following companies for their fiscal back up: Anadarko, Baker Hughes, BP, Chevron, CNOOC, ConocoPhillips, DNV GL, ExxonMobil, M-I SWACO (Schlumberger), Multi-Chem (Halliburton), Occidental Oil Visitor, Petrobras, PTT, Saudi Aramco, Shell Global Solutions, SINOPEC (China Petroleum), TransCanada, Total, and Wood Group Kenny.

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© 2018, NACE International

2018

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