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. 2024 Mar 4;9(11):13067–13080. doi: 10.1021/acsomega.3c09540

Corrosion Behavior of Carbon Steel in Diethylenetriamine Solution for Post-combustion CO2 Capture

Xinyue Chen †,‡, Yongkang Cui †,‡, Shujuan Wang †,‡,§,∥,*
PMCID: PMC10955713  PMID: 38524427

Abstract

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In the realm of postcombustion carbon capture, diethylenetriamine (DETA), recognized for its substantial CO2 absorption capacity, presents a formidable challenge due to its corrosive impact on equipment. This study delves into the corrosion behavior of 20# carbon steel immersed in DETA solutions under varying conditions, employing weight loss and electrochemical methods. The investigation incorporates scanning electron microscopy/energy-dispersive spectroscopy and X-ray diffraction analyses for characterization. Corrosion experiments were also conducted in monoethanolamine (MEA) solutions for a comparative analysis. Results from the corrosion tests in DETA solutions mirror the temperature-dependent corrosion rate (CR) observed in MEA. However, a distinctive trend emerges as the CO2 loading of DETA increases from 0.2 mol CO2/mol amine to 1.2 mol CO2/mol amine, leading to a continuous decrease in the CR of carbon steel—contrary to MEA solutions. This anomaly is attributed to DETA’s robust complexing ability with metal ions and its elevated solubility of Fe2+ in solution. Additionally, an examination of the corrosion mechanism in the presence of oxygen was conducted through characterizing the specimen surface and solution precipitates postexperiment. The absence of a protective FeCO3 layer can be attributed to insufficient concentrations of free Fe2+ and CO32– in the solution, failing to achieve the minimum saturation required for protective film formation. The insights gained from studying the corrosion behavior of carbon steel in DETA solutions lay the groundwork for subsequent developments in corrosion inhibitors.

1. Introduction

The excessive emission of greenhouse gases caused by human activities has caused a serious global warming. CO2 is the most important greenhouse gas and the main cause of climate change.1 Addressing this issue, carbon capture and storage (CCS) technology emerges as a promising avenue for curtailing CO2 and greenhouse gas emissions. Within CCS, postcombustion CO2 capture, particularly employing chemical absorption with aqueous amine-based solvents, stands as the most established and efficacious method.2,3

However, aqueous amine solutions will corrode equipment, which is one of the major issues facing CO2 capture technology.4 Corrosion transpires through electrochemical reactions at the metal–electrolyte interface. The anodic reaction involves the metal-losing electrons, whereas the cathodic reaction entails the oxidant-gaining electrons from the metal.5 While amines exhibit some corrosion inhibition capabilities, they induce severe corrosion of carbon steel equipment under CO2 capture conditions due to the formation of bicarbonate and carbamate species.6 Moreover, the presence of 4–8% O2 in flue gas exacerbates the corrosion extent.7

The widely used 30 wt % monoethanolamine (MEA) solvents serve as a benchmark for CO2 capture, possessing maturity in application. In comparison, diethylenetriamine (DETA) boasts a higher CO2 absorption capacity and a faster absorption rate due to its three amino groups,8 essential components in biophysical solvents for CO2 capture.9,10 Recognized as potential candidates for energy-efficient CO2 capture, biphasic solvents, particularly liquid–liquid biphasic absorbents, have been extensively investigated.11 DETA’s relatively high molecular weight and elevated electron density on the nitrogen functional group enable it to act as an electron donor, forming ligand covalent bonds with metal surface electrons,12 thus rendering it suitable as an organic corrosion inhibitor in weakly alkaline and acidic corrosive media, such as seawater and CO2-saturated NaCl solutions.13−15 Consequently, DETA is anticipated to exhibit a high absorption capacity without inducing significant corrosion, promising excellent application prospects.

While numerous studies have delved into the corrosion behavior of common primary amines (MEA), secondary amines (diethanolamine, DEA), and tertiary amines (methyldiethanolamine, MDEA),16−18 the corrosion behavior of polyamines such as DETA has received limited attention. Rafat et al.19 examined the corrosion behaviors of classic amine solutions and new ionic liquids, revealing that the corrosion rate (CR) was determined by the characteristic CO2 absorption capacity, with MEA > DEA > MDEA under identical conditions. Nainar et al.20 explored the corrosion of carbon steel in aqueous solutions of MEA and piperazine (PZ), investigating the impact of amine concentration, CO2 loading, temperature, and the presence of heat-stable salts on the CR. Fischer21 discussed the influence of Fe2+ solubility in different amine solutions on the formation of protective layers on carbon steel surfaces in his doctoral dissertation, emphasizing the difference in Fe2+ solubility in MEA and PZ solutions. However, limited research has been conducted on the corrosion of polyamines like DETA. Hayfron-Benjamin E22 examined the effects of DETA degradation products and temperature on the corrosion behavior of stainless steel, but the variations under different CO2 loadings were not thoroughly investigated.

Therefore, this study primarily investigates the corrosion behavior and corrosion mechanism of carbon steel in DETA solutions, providing pivotal insights for subsequent research on optimizing working conditions and implementing corrosion inhibition in actual application processes.

2. Experimental Section

2.1. Sample and Solution Preparation

Given the inherent corrosion challenges under the operating conditions of the postcombustion CO2 capture process and the corrosion induced by the absorbent, stainless steel is deemed the optimal material for process equipment.23 However, considering the substantial cost difference—stainless steel being 3–6 times more expensive than carbon steel—carbon steel remains widely used in practical projects to mitigate the total cost of postcombustion CO2 capture.24 This study, focused on unraveling the corrosion behavior of DETA, seeks to establish a foundation for its extensive use in practical engineering. The 20# carbon steel specimen, with its chemical composition (wt %) detailed in Table 1, was chosen as the experimental material. All specimens were sourced from the same batch and obtained from identical pipes.

Table 1. Chemical Composition of 20# Carbon Steel.

component C Si Mn S P Cr Ni Cu Fe
weight (%) 0.17–0.24 0.17–0.37 0.35–0.65 ≤0.035 ≤0.035 ≤0.25 ≤0.25 ≤0.20 Bal
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For both weight loss and electrochemical experiments, type III carbon steel specimens (40 × 13 × 2 mm, surface area 12 cm2) were utilized. Before the experiments, the specimens underwent polishing, cleaning with ethanol and deionized water, and drying in hot air.

MEA and DETA were provided by Macklin (Shanghai, China) (purity ≥99%). Hydrochloric acid and methenamine, which were used to clean the surface products of the corrosion specimens in the weight loss method, were provided by Tongguang Fine Chemicals (Beijing) and Lanyi Chemicals (Beijing). The 1,10-phenanthroline, hydroxylammonium chloride, acetic acid, and sodium acetate used in the analysis process were obtained from Aladdin (Shanghai, China).

The experiments necessitated aqueous solutions of MEA and DETA, with MEA serving as the comparison group. Varied working conditions encompassed amine concentration, temperature, and CO2 loading, as detailed in Table 2. Amine concentration adjustments were achieved by altering the mixing ratio of amine to deionized water. Temperature adjustments were made using a water bath, while the CO2 loading of the amine solution underwent variation through a CO2 absorption device, as illustrated in Figure 1. To attain the required CO2 loading, the amine solution was initially saturated with CO2, and the saturation loading was measured by titration. The titration device, comprising a U-shaped tube and a sealed tank, facilitated a neutralization reaction when excess acid was added. The volume of CO2 produced equated to the volume of CO2 absorbed by the measured solution. CO2 absorption capacity was quantified in units of mol of CO2/mol of amine. Subsequently, a fresh amine solution was added to achieve the desired CO2 loading.

Table 2. Different Conditions of the Amine Systems in Experiments.

amine parameter condition
MEA temperature (°C) 40, 60
  CO2 loading (mol CO2/mol amine) 0.0, 0.4
  amine concentration (%) 30, 40, 50
DETA temperature (°C) 40, 60
  CO2 loading (mol CO2/mol amine) 0.0, 0.4, 0.8, 1.2
  amine concentration (%) 30, 40, 50
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Figure 1.

Figure 1

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Schematic diagram of the experimental setup for the CO2 absorption.

2.2. Methods

2.2.1. Weight Loss Method

The weight loss method, a well-established technique for assessing equipment corrosion in solution, has seen extensive application in prior studies.25,26 Preceding the experiment, all 20# carbon steel specimens underwent washing with deionized water and ethanol, subsequent drying, and initial weighing. Various MEA and DETA solutions, featuring distinct amine concentrations and CO2 loadings, were introduced into 60 mL sample bottles, housing the carbon steel specimens. These sample bottles were immersed in a water bath set at a predetermined temperature for a duration of 30 days. After that, the corroded specimens were extracted, subjected to cleaning procedures conforming to the GB/T16545–2015 standard, and reweighed. It is worth noting that this system cannot eliminate the impact of the presence of oxygen; therefore, the dissolved oxygen has been considered during the analysis and discussion of results. The weight loss method, adept at capturing long-term corrosion behavior in a specific environment, facilitated the computation of the average CR using eq 1

2.2.1. 1

where CR is the corrosion rate (mm/a); m0 and m1, respectively, denote the weight of the specimen before and after corrosion (g); A, ρ, and t, respectively, denote the exposed surface area of the specimen (cm2), sample density (g/cm3), and corrosion time (h).

2.2.2. Electrochemical Testing

While the weight loss method provides actual corrosion rates through extended experiments and characterizes corrosion through various methods, the corrosion process fundamentally involves electrochemical reactions. Electrochemical experiments offer deeper insights into electrochemical corrosion phenomena, delivering a more comprehensive understanding of the reaction process and its influencing factors.27,28

A schematic representation of the experimental setup for electrochemical corrosion is presented in Figure 2. Employing a three-electrode electrolytic cell placed within a thermostatic water bath to ensure a constant temperature, the electrochemical corrosion tests were conducted. The working electrode (W.E.) comprised 20# carbon steel in the test solution. The reference electrode utilized was Ag/AgCl (saturated KCl), and a platinum electrode served as the counter electrode. The electrolytic cell, sealed for precision, had its three electrodes electrically connected to an electrochemical workstation (reference 1000, Gamry, USA).

Figure 2.

Figure 2

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Schematic representation of the experimental setup for the electrochemical corrosion tests.

The experimental procedure commenced with the measurement of the open-circuit potential (OCP) until stabilization (∼0.1 mV). Electrochemical impedance spectroscopy (EIS) followed at the OCP, employing a perturbation amplitude of 10 mV and scanning across a frequency range of 100 kHz–0.01 Hz. Tafel curve measurements spanned from −250 to +250 mV (versus OCP) with a constant scanning rate of 0.167 mV/s. Tafel curves, anode/cathode polarization curves generated by polarizing the specimen in both anode and cathode directions within a defined potential range, were employed. Electrochemical parameters were derived through Tafel extrapolation to ascertain the corrosion current (icorr). The instantaneous corrosion rate was calculated as CR using eq 2

2.2.2. 2

where CR is the corrosion rate (mm/a); icorr is the corrosion current (μA); W is the equivalent weight (g) of the specimen; and A and ρ represent the W.E. area (cm2) and specimen density (g/cm3), respectively. It is noteworthy that the corrosion current serves as the primary determinant of the CR.

2.3. Surface Characterization

In-depth understanding of corrosion and exploration of the corrosion mechanism of 20# carbon steel in DETA solutions necessitated the characterization of corrosion specimens.29,30 Scanning electron microscopy (SEM) was employed for observing surface morphologies, while energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD) were utilized to analyze the components of corrosion products on specimen surfaces. The surface morphology of carbon steel was characterized by using FE-SEM (Merlin, Zeiss, Germany). The EDS detector mounted on the SEM instrument facilitated the determination of the elemental composition of the carbon steel surface. XRD (D/max-2550, Rigaku, Japan) operated at a voltage of 40 kV and a current of 40 mA, equipped with a Cu target, with a measurement range spanning from 10° to 90°. Diffraction patterns were analyzed using MDI Jade 6 software with the ICDD PDF-2 version 2012 database.

Moreover, SEM and XRD were employed for ex situ characterization of precipitates generated in the solution, such as the blue-black precipitate following 30 days of corrosion in MEA solutions loaded with CO2. Postexperiment, the precipitate in the solution underwent vacuum filtration and rinsing with deionized water. The extracted solids were dried and characterized in an oven before final analysis.22

2.4. Solution Analysis

Utilizing the phenanthroline spectrophotometric method, the concentration of Fe ions in test solution samples was measured using a UV–visible spectrophotometer (Agilent, cary100).31 The experiments were conducted at least twice, and average values are reported.

3. Results

3.1. Weight Loss Results

The corrosiveness of fresh amine solutions was negligible; however, upon CO2 absorption, they exhibited corrosion tendencies. This study delved into the corrosion behavior of 20# carbon steel in DETA solutions through weight loss experiments under varying conditions. For comparative reliability, corrosion experiments were conducted on MEA solutions. The long-term corrosion rates, measured using the weight loss method, are depicted in Figure 3. In fresh amine solutions, the corrosion rates of 20# carbon steel were nearly 0 mm/a. Yet, after CO2 absorption, different phenomena emerged in the MEA and DETA solutions. With escalating temperature, the corrosion rates of specimens under identical conditions gradually increased, attributed to accelerated oxidation and reduction reactions at the anode and cathode with rising temperature.32 Interestingly, an increase in CO2 loading resulted in opposing trends in the corrosion rates of 20# carbon steel in MEA and DETA solutions. In MEA solutions, more CO2 absorption led to an increased reduction of ionic substances at the cathode, causing the anode to dissolve and generate more Fe2+5. In contrast, in DETA solutions, as the CO2 loading rose from 0.4 to 1.2 mol/mol, the corrosion rates of 20# carbon steel gradually decreased. This phenomenon was associated with the potent ability of DETA to complex with Fe2+. At low CO2 loading, more DETA molecules in the solution donated electrons to the vacant d-orbitals of iron, promoting the anode dissolution process. A comprehensive analysis is provided in Part 4.

Figure 3.

Figure 3

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Relationship between the CR of carbon steel and the CO2 loading of amine solution after corrosion for 30 days calculated by the weight loss method: (a) MEA solution and (b) DETA solution.

3.2. Electrochemical Corrosion Behavior

Major metal corrosion occurs through electrochemical reactions at the metal–solution interface. Electrochemical measurements, providing information on the metal/electrolyte interface, are an effective technique for studying corrosion behavior and mechanisms.33 The initial step in electrochemical corrosion testing is the measurement of the OCP. Adequate time is allowed for the OCP to stabilize. A stable OCP indicates that the system is in a steady state, signifying a constant speed of various corrosion reactions.

3.2.1. Tafel Curve

The Tafel curve, depicting the reaction rate of the cathode and anode, serves as a rapid and accurate method for measuring corrosion behavior at a given time. In this study, an electrochemical test was conducted for 24 h from the start. The Tafel curves of 20# carbon steel recorded in MEA and DETA solutions under different conditions are illustrated in Figures 4a and 5a. The fitted electrochemical parameters [corrosion potential (Ecorr), corrosion current (icorr), cathode (βc) and anode (βa) Tafel slopes, CR, etc.] are listed in Table 3.

Figure 4.

Figure 4

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Electrochemical results of 20# carbon steel in MEA solutions under different conditions (60 °C): (a) Tafel curve; (b) Nyquist plots; (c) Bode plots of impedance vs frequency; and (d) Bode plots of phase angle.

Figure 5.

Figure 5

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Tafel curves of 20# carbon steel in 30% DETA solutions under different conditions: (a) different CO2 loadings and (b) different temperatures.

Table 3. Fitted Electrochemical Parameters of Tafel Curves for 20# Carbon Steel in Different Solutions.
condition
medium temperature (°C) CO2 loading (mol/mol) EOCP (mV) Ecorr (mV) Icorr (mA) βa (mV·dec–1) -βc (mV·dec–1) CR (mm/a)
30% MEA 60 0 –270.8 –292.26 1.26 × 10–3 323.51 141.31 0.00122
    0.4 –801.6 –807.82 0.232 48.27 55.33 0.225
40% MEA   0 –264.5 –286.80 1.12 × 10–3 358.97 153.58 0.00109
    0.4 –803.3 –803.72 0.463 45.87 62.98 0.448
50% MEA   0 –300.2 –366.76 0.623 × 10–3 203.72 188.84 0.000604
    0.4 –800.1 –799.99 0.763 43.98 83.22 0.740
30% DETA 40 1.2 –848.9 –842.59 0.852 125.09 146.63 0.825
  60 0 –439.1 –438.62 8.71 × 10–2 145.92 154.97 0.0845
    0.2 –990.5 –982.70 –2.54 291.04 283.88 2.764
    0.4 –981.3 –972.39 2.73 186.52 211.72 2.644
    0.8 –935.2 –934.90 1.93 152.35 183.29 1.872
    1.2 –881.1 –871.02 1.18 127.06 143.52 1.146
    sat –812.6 –832.28 0.959 128.924 152.99 0.930
  80 1.2 –912.3 –916.07 2.69 125.11 143.07 2.615
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Figure 4a illustrates that in fresh MEA solutions, the corrosion current was negligible, signifying almost no corrosiveness to carbon steel. This aligns with the weight loss method results. However, when the CO2 loading reached 0.4 mol/mol, the Tafel curve changed significantly. According to the Tafel slopes of the cathode and anode obtained by extrapolation listed in Table 3, the slopes of the cathode curves under the three MEA concentrations were greater than those of the anode, which indicated that the corrosion process was mainly controlled by the cathode reaction. In addition, as the amine concentration increased, the Tafel slope of the cathode increased, and the corrosion current increased, resulting in a higher CR. This is attributed to the fact that more reducing species were contained in the solution with higher amine concentration, accelerating the cathodic reaction and subsequently increasing the corrosion current.

In DETA solutions, the corrosion process exhibited greater complexity. Tafel curves for 30% DETA solutions, after 24 h of corrosion under various CO2 loadings and temperatures, were analyzed. Similar to MEA solutions, fresh DETA solutions showed minimal corrosion currents, indicating low corrosiveness. Yet, as the CO2 loading increased from 0.4 to 1.2 mol/mol, the corrosion current exhibited a declining trend, correlating with a reduced CR. The Tafel curve indicated that the corrosion potential became more negative at 0.4 mol/mol, signifying increased metal dissolution and heightened corrosion activity.19 Both cathode and anode polarization curves shifted upward, indicating intensified reactions at both interfaces. Comparing the Tafel slope of 20# carbon steel in DETA solutions with different CO2 loadings, it is found that it continued to increase with decreasing CO2 loading, indicating a faster kinetic process. Under low loading conditions, the larger Tafel slope explained the higher corrosion current and CR, allowing the anode to dissolve quickly.31 This was related to the strong complexing ability of DETA with metal ions and corresponded to the weight loss results. Temperature effects are evident in Figure 5, where higher temperatures accelerated both anodic and cathodic polarization curves, expediting the reaction rate in MEA solutions.

3.2.2. Electrochemical Impedance Spectroscopy

To delve deeper into the corrosion behavior of 20# carbon steel in CO2-loaded amine solutions and the electrochemical behavior of the metal/solution interface, EIS measurements were conducted.

Figure 4 displays Nyquist and Bode plots illustrating the corrosion of 20# carbon steel in MEA solutions. Fresh solutions exhibited three incomplete semicircles in Nyquist plots (Figure 4b), indicating constant |Z| values in the low-frequency range (Figure 4c). The Bode plots (Figure 4d) showed phase angles decreasing to 0° at high frequencies due to the electrolyte resistance response, with a small peak at low frequencies. For MEA solutions with CO2 loadings, Nyquist plots displayed a single capacitive loop, signifying a charge-transfer process control. The semicircle diameter decreased with rising MEA concentration, revealing reduced charge-transfer resistance and an increased CR. Bode plots showed decreased absolute impedance and phase angle relative to fresh solutions, indicating more active sites available for charge transfer.

Figure 6 presents EIS results for 20# carbon steel in DETA solutions. Fresh solutions (Figure 6a) displayed larger semicircle diameters, representing high impedance and almost noncorrosiveness. Upon CO2 absorption, Nyquist plots exhibited semicircles with the corrosion process still predominantly controlled by charge transfer. Increasing the CO2 loading from 0.4 to 1.2 mol/mol resulted in an increased semicircle diameter, higher charge-transfer resistance, and reduced CR, consistent with weight loss method results. The influence of temperature on EIS data confirmed that higher temperatures enhanced the corrosion reaction.

Figure 6.

Figure 6

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EIS results of 20# carbon steel in 30% DETA solutions under different conditions: (a) Nyquist plots; (b) Bode plots of impedance versus frequency; (c) Bode plots of phase angle versus frequency.

The EEC and EIS data, fitted using Gamry Analysis software, are presented in Figure 7 and Table 4. In the EEC of fresh solutions (Figure 7a), Rs denotes the solution resistance, constant phase element (CPE) represents a CPE associated with the double-layer capacitance, and Rct signifies the charge-transfer resistance. R stands for polarization resistance, and Qf refers to the double-layer capacitance at the interface between 20# carbon steel and the solutions within the film pores. Notably, the Rs values in fresh amine solutions were substantial, suggesting low conductivities. The Rct values exhibited a high order of magnitude, implying that the lower the CR of 20# carbon steel, the more significant the hindrance of charge-transfer resistance.34 Integrating weight loss method, Tafel curve, and EIS results validated the noncorrosive nature of fresh amine solutions to the equipment.

Figure 7.

Figure 7

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Electrochemical equivalent circuit (EEC) of EIS impedance spectrum in the MEA and DETA solutions: (a) fresh solutions and (b) CO2-loaded solutions.

Table 4. Fitted EIS Parameters of Electrical Equivalent Circuit for 20# Carbon Steel in Different Solutions.
condition
   CPE (F·cm–2)
      
medium temperature (°C) CO2 loading (mol/mol) Rs (Ω·cm2) Y0 × 10–5 n Rct (Ω·cm2) Qf×10–5 (F·cm–2) R (Ω·cm2)
30% MEA 60 0 646.3 3.45 0.887 4.07 × 105 78.7 2176
    0.4 14.48 26.8 0.780 887.8    
40% MEA   0 825.3 3.42 0.896 6.07 × 105 48.2 2906
    0.4 17.15 36.5 0.757 638.9    
50% MEA   0 1542 3.43 0.883 5.43 × 105 40.9 6181
    0.4 21.3 55.5 0.761 484.0    
30% DETA 40 1.2 51.7 24.4 0.881 557.2    
  60 0 3367 4.03 0.720 4.06 × 105 1.41 81.9
    0.4 38.98 50.3 0.833 195.7    
    0.8 33.94 36.5 0.875 243.2    
    1.2 36.04 38.7 0.814 547.7    
  80 1.2 21.32 110 0.777 128.2    
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The EEC for corrosion in CO2-loaded amine solutions is depicted in Figure 7b. As no protective film was observed on specimen surfaces in corrosion experiments with the two amine solutions, Rs represents the solution resistance in the EEC, graphically determined from the real (X) axis intercept at a high frequency in the Nyquist plots. Lower Rs values indicated a high conductivity in all solutions. The charge-transfer resistance Rct could be directly determined from the semicircle diameter of the Nyquist plots. To optimize the fit, a CPE was used instead of double-layer capacitance (Cdl) to accommodate deviations from ideal capacitive behavior due to surface roughness, heterogeneity, and adsorption effects.35 In DETA solutions, the change in Rct differed from that in MEA solutions. Rct increased with rising CO2 loading, while the CPE continuously decreased. DETA, being a corrosion inhibitor due to its strong ability to bind metal ions, explained the opposite trend in corrosion rates between DETA and MEA solutions. Figure 5c,d illustrates Bode plots of 20# carbon steel in DETA solutions with varying CO2 loadings. Across a broad frequency range from 0.01 Hz to 100 kHz, only one time constant was observed, suggesting a similar corrosion mechanism in all solutions.36 Absolute impedance at low frequencies in Bode modulus plots increased with higher CO2 loading in DETA solutions and decreasing temperatures, indicating improved protection of 20# carbon steel against corrosion.

3.3. Surface Characterization

The surfaces of 20# carbon steel specimens corroded in 30% MEA and 30% DETA solutions (Figure 8) with different CO2 loadings were analyzed using SEM, EDS, and XRD. No apparent protective layer of corrosion products was observed for #20 carbon steel in the MEA solution. In the EDS results, only Fe, C, and other elements from the original carbon steel were detected on specimen surfaces, with no O detected. XRD patterns showed only iron peaks on the surface of specimens (Figure 9). These results align well with the observed change in the CR of 20# carbon steel in MEA solution.

Figure 8.

Figure 8

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SEM images of carbon steel surface after 30 day corrosion treatment in 30% DETA solutions under different CO2 loadings (60 °C): (a, b) 0 mol/mol; (c, d) 0.4 mol/mol; (e, f) 0.8 mol/mol; and (g, and h) 1.2 mol/mol.

Figure 9.

Figure 9

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XRD images of 20# carbon steel surface after corrosion treatment in MEA solutions for 30 days under different conditions.

In the DETA solution, the corrosion behavior intensified. The microstructure of 20# carbon steel consisted of two phases: pearlite and ferrite. The SEM image of the specimen surface (Figure 8) reveals pearlite (Fe + Fe3C) areas in parts c, e, and g. Given that the corrosion potential of Fe3C is more positive than that of ferrite, a galvanic effect formed between them, leading to selective dissolution of the ferrite phase. Consequently, a significant amount of pearlite residual iron carbide (Fe3C) remained on the carbon steel surface. As ferrite continued to dissolve, more Fe3C was exposed, resulting in increased corrosion attacks.35,37,38 XRD results for the surface in Figure 10 indicate that with an increase in CO2 loading, the relative intensity of peaks for Fe3C residues continuously decreased. A smaller amount of carbide on the surface corresponded to a lower CR,23 consistent with weight loss method results. EDS results (Table 5) for the surface after DETA experiments with three CO2 loadings revealed trace oxygen elements. These findings suggest that in the DETA corrosion experiment, the corrosion behavior of 20# carbon steel was more severe than that in MEA. The surface was predominantly covered by Fe3C residues with a potential formation of a small oxide layer, even though not uniformly spread.

Figure 10.

Figure 10

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XRD images of 20# carbon steel surface after corrosion treatment for 30 days in 30% DETA solutions under different CO2 loadings.

Table 5. Three EDS Analysis of 20# Carbon Steel after Corrosion in DETA Solution.

  element
conditions (mol/mol) Fe (wt %) C (wt %) O (wt %)
30% DETA 0 92.29 7.33 0
30% DETA 0.4 88.60 10.04 1.04
30% DETA 0.8 89.49 9.01 1.10
30% DETA 1.2 90.21 8.22 1.25
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3.4. Precipitate Analysis

In addition to characterizing the corroded 20# carbon steel surface, precipitates were observed in some solutions during experiments. MEA solutions loaded with CO2 became turbid, exhibiting more precipitates under all amine concentrations and temperature conditions. After suction filtration and drying into a powder, characterization tests were conducted. SEM micrographs of precipitates in MEA solutions (Figure 11) displayed variations in surface morphology at different exposure positions. The precipitates were identified as small crystalline globules or flowery plates (γ-FeOOH-lepidocrocite) (Figure 11a), semicrystalline spherical structures (goethite) (Figure 11b), and cotton ball-like structures (akaganeite) (Figure 11c). These SEM findings suggested that the precipitated product of 20# carbon steel in MEA solutions primarily consisted of FeO(OH) in various crystal phases, typical for carbon steel corrosion in MEA solutions under oxygen-containing conditions.38 The XRD pattern in Figure 12 corresponds to this observation, although the baseline was not flat, possibly due to the presence of other components like Fe2O3, Fe3O4, and other oxides in small amounts that could not be clearly identified.

Figure 11.

Figure 11

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SEM image of precipitates after corrosion for 30 days in CO2-loaded MEA solutions: (a) crystal ball or flowery flake (γ-FeOOH lepidocrocite); (b) semicrystalline (goethite); and (c) cotton ball (akaganeite).

Figure 12.

Figure 12

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XRD analysis of precipitations in CO2-loaded MEA solutions after corrosion for 30 days.

In the corrosion process of 20# carbon steel in DETA solutions, a minor amount of precipitates emerged, and the solutions remained clear. The collected precipitate underwent characterization. XRD results in Figure 13, coupled with literature information,39 indicate that the predominant products were magnetite Fe2O3 and Fe3O4. The sphere cluster structure depicted in Figure 14 represented a typical morphology of γ-Fe2O3.40 In the presence of oxygen, fine particles of magnetite Fe3O4 and less-stable γ-Fe2O3 could potentially transform into hematite (α-Fe2O3) to exist in a stable state. Hematite is a primary component of rust resulting from the corrosion of carbon steel.39

Figure 13.

Figure 13

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XRD analysis of precipitations in DETA solutions after corrosion for 30 days.

Figure 14.

Figure 14

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SEM image of precipitations in CO2-loaded DETA solutions after corrosion for 30 days: (a) scale bar is 10 μm and (b) scale bar is 1 μm.

4. Discussion

4.1. Mechanism of Corrosion

In general, amines are not inherently corrosive due to their high pH and low conductivity. However, their corrosive potential increases when they absorb acidic gases such as CO2. The presence of various ions in the solution can act as oxidants or reducing agents in the corrosion reaction, significantly influencing the speed and direction of the corrosion process. Numerous studies have detailed the reaction processes of amine solutions absorbing CO2 [(R1)–(R6)].22,41 The CO2 absorption process primarily involves the reaction of MEA and DETA (primarily primary amino acids) with CO2:

Dissociation of water

4.1. R1

Formation of bicarbonate ions

4.1. R2
4.1. R3

Formation of carbonate ions

4.1. R4

Formation of the carbamate ion

4.1. R5

Hydrolysis of the carbamate ion

4.1. R6

In the Amine-CO2–H2O system, the ions resulting from the reactions described previously create conditions conducive to corrosion. Typically, the corrosion process encompasses the oxidation reaction (R7) of the anode iron and the reduction reactions [(R8)–(R10)] of the cathodic species.42

Dissolution of anode iron

4.1. R7

Cathodic reaction

4.1. R8
4.1. R9
4.1. R10

The anions in the solution included OH– and CO32–, and the formation of the corrosion products was as follows

4.1. R11
4.1. R12

In the presence of oxygen, the cathode also underwent an O2 reduction (R13).

Reduction of O2

4.1. R13

Dissolved oxygen plays a significant role in influencing the corrosion of the system. It directly participates in the reduction process, contributing to an increase in the corrosion current and accelerating the dissolution of 20# carbon steel, despite its low concentration and nonprimary role in the reduction reactions. Furthermore, the presence of dissolved oxygen hinders the formation of a protective layer, potentially a key factor in the pronounced corrosion observed in CO2-loaded amine solutions.43 In the presence of dissolved oxygen, Fe2+ becomes unstable, with some undergoing further oxidation to Fe3+ (R14). Subsequently, Fe3+ can give rise to insoluble corrosion products, such as Fe(OH)3, in contrast to other possible precipitates like FeCO3 and Fe(OH)2 (R15). The precipitation of Fe(OH)3 and its subsequent reactions contribute to the formation of additional corrosion products.39

4.1. R14
4.1. R15

Corrosion can be mitigated through the formation of a protective layer, such as siderite (FeCO3) or magnetite (Fe3O4), on the surface of the carbon steel. However, the development of these protective layers is contingent upon operating conditions, including amine type, temperature, the presence of oxygen, and solution pH. The formation of FeCO3 relies on the supersaturation (δSD) of Fe2+ and CO32–, as outlined in eqs 3 and 4.44 A protective layer in the form of FeCO3 can precipitate or form only when the concentrations of free Fe2+ and/or CO32– reach the minimum saturation (δSD) required for FeCO3 formation.

4.1. 3
4.1. 4

Where Ksp is the solubility constant of FeCO3, and CFe2+ and CCO32–, respectively, are the concentrations of free Fe2+ and CO32–.

Previous studies have identified free H2O, H+, and HCO3– as the primary oxidants involved in the corrosion of MEA-CO2 solutions.43 In this system, the hydrolysis reaction (R6) of carbamate ions can only occur under conditions of high CO2 absorption.45 Localized regions near the carbon steel surface may not achieve the minimum FeCO3 formation saturation degree (δSD) due to favorable carbamate formation and a low equilibrium constant between carbamate and bicarbonate, resulting in low concentrations of free Fe2+ and CO32–. Characterization results further revealed that the corrosion products of CO2-loaded MEA solutions primarily existed as precipitates with no protective film on the surface, leading to elevated corrosion rates. Fe2+ may undergo oxidation by dissolved oxygen in the solution to form the principal final product, Fe2O3, as illustrated in (R16–R18). Fe(OH)2 serves as an intermediate oxidation product in this system. In the presence of oxygen, the more-stable Fe(OH)3 is formed, accompanied by the production of other corrosion products such as FeO(OH) and Fe2O3 (R19–20). Among these, FeO(OH) was identified as the primary component of the precipitates after corrosion of the CO2-loaded MEA solutions, as confirmed by SEM and XRD results.

4.1. R16
4.1. R17
4.1. R18
4.1. R19
4.1. R20

The CO2 absorption mechanism in the DETA solution closely resembles that in the MEA solution. DETA reacts with CO2 to form carbamates. The two primary and secondary amino groups contained in DETA make it have a stronger CO2 absorption capacity than MEA. Hartono et al.46 conducted 13C NMR studies on DETA solutions with varying CO2 loadings at 298.0 K. The results indicated that peaks corresponding to bicarbonate and carbonate (HCO3–/CO32–) appeared only at a loading of 1.39. In systems with CO2 loadings less than 1.0, carbamates were the predominant species formed, with no HCO3–/CO32– observed. Hence, under the investigated CO2 loading conditions, DETA carbamates hardly underwent hydration. Furthermore, Figure 16 illustrates that the concentration of Fe2+ in the DETA solution was significantly higher than that in the MEA solution, suggesting that most of the Fe2+ was dissolved in the solution, explaining the lack of precipitation. DETA, being a long-chain aliphatic amine, possesses inherent corrosion-inhibitory effects, and its strong complexing ability with metal ions may result in a higher solubility of Fe2+ in DETA solutions. Consequently, free Fe2+ and CO32– concentrations do not accumulate sufficiently to form the FeCO3 protective layer (δSD). The small amounts of precipitation observed in the SEM and XRD results can also be explained by the corrosion mechanism of carbon steel in MEA solutions. Additionally, the hydroxide in (R15) undergoes several phase transitions, with the final stable product existing in the phase structure of α-Fe2O3.

Figure 16.

Figure 16

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Concentration of iron ions in solutions after corrosion for 30 days under different conditions: (a) in MEA and (b) DETA solutions.

4.2. Effects of CO2 Loading on CR of 20# Carbon Steel in DETA Solutions

An intriguing observation from the results is the decrease in the CR of 20# carbon steel in DETA solutions with increasing CO2 loadings. This section delves into a detailed analysis of this phenomenon.

4.2.1. With or without CO2 Loading

The three amino groups in DETA endow it with a robust CO2 absorption capacity. Ionic species generated during this process act as oxidants in the corrosion reaction, promoting anode oxidation. Therefore, once the absorption of CO2 occurs, a tendency for corrosion emerges. Notably, fresh DETA solutions without CO2 loading did not corrode the 20# carbon steel specimens.

4.2.2. Complexation of DETA Molecules and Metal Ions

The strong complexation of DETA with metal ions may be the main reason for the different phenomena between the DETA and MEA solutions. The chemisorption of amine derivative molecules on the carbon steel #20 surface is driven by donor–acceptor interactions between the lone pair electrons of nitrogen atoms and the vacant d-orbitals of iron (Figure 15).47 The process is influenced by physicochemical properties such as functional groups, electron density at the donor atoms, and electronic structure of the molecule.48

Figure 15.

Figure 15

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Interaction between DETA and the carbon steel surface through chemical adsorption processes.

Saha et al.49 employed Frontier molecular orbital theory to calculate the interaction between three organic amine corrosion inhibitors and metal atoms. The study highlighted the role of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbitals (LUMOs) in explaining the reactivity of the inhibitor molecules. Higher HOMO values indicate better electron-donating ability, while lower LUMO values facilitate stronger adsorption onto metal surfaces. The energy gap (ΔE = ELUMO–EHOMO) serves as a fundamental quantum chemical parameter, with lower ΔE values correlating with higher reactivity and enhanced adsorption on metal surfaces, ultimately leading to higher corrosion inhibition efficiency. The results showed that the EHOMO value increased with the increasing length of the amine molecular chain and the number of nitrogen atoms, indicating their electron donation ability in the order of PEHA > TETA > DETA. The ELUMO value comparison order was DETA > TETA > PEHA, which meant an increase in electron acceptance ability. In addition, results from MD simulations indicated that DETA molecules possess a strong electron-donating ability. Adsorption of DETA molecules onto a metal surface through relaxed adsorption released a significant interaction energy of −355.30 kJ/mol, emphasizing the molecule’s strong surface adsorption capacity. The high interaction energy suggests that DETA molecules are prone to spontaneous adsorption onto metal surfaces.

In the CO2 capture process, relatively high amine concentrations are required, such as 2.5 M (∼25.56 wt %), as used in the trail pilot plant50 and this work. The DETA contents in solutions with different CO2 loadings used in this experiment were calculated based on the theoretical maximum CO2 absorption capacity of DETA, as shown in Table 6. It can be understood that at low CO2 loading (0.4 mol/mol), there were more free DETA molecules in the solution. Excess DETA facilitated complexation with iron ions, intensifying the anode dissolution reaction, as observed in the Tafel curve in Figure 5. Figure 16 illustrates the concentration of iron ions in amine solutions after corrosion experiments under various conditions, as measured by spectrophotometry. The high solubility of Fe2+ was a contributing factor, preventing the formation of a protective film, thereby intensifying the corrosion behavior. This finding aligns with the results obtained by Fytianos G′s et al.,51 who observed that higher iron solubility in amine solutions led to less FeCO3 formation and a higher CR. Among the five tested amines, DETA exhibited the highest corrosiveness and the highest Fe2+ solubility.

Table 6. DETA Molecule Concentration in Solutions with Different CO2 Loadings.
condition CO2 loading (mol/mol) DETA concentration (ppm)
30% DETA 0 300,000
  0.4 220,000
  0.8 140,000
  1.2 60,000
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In addition, the pH value of the test solution before and after the weight loss method experiment was measured, as shown in Figure 17. The pH of the DETA solution before the experiment showed a downward trend with the increase of CO2 loading due to the different contents of acidic gas CO2 absorbed. After the 30 day soaking experiment, the CO2-loaded samples all experienced varying degrees of pH decline. This was because the metal ion is a Lewis acid, and the pH of the solution decreased the most under low CO2 loading, indicating that there are the most iron ions in the solution under this condition, which corresponded to the results in Figure 16. In MEA solutions, the increase in the CO2 loading reduced the solution pH, and the reducing substances in the solution increased, which increased the CR. In the DETA solution, not only did the absorption of CO2 cause the cathode reaction to accelerate but the complexation of DETA and iron ions caused the anode to quickly dissolve. The combined effect of the two made the impact of CO2 loading on the CR showing a different trend from that of MEA. Despite this, when the DETA solution has the same amino CO2 loading as MEA (0.4 C/N), the CR of the DETA is much greater than that of MEA (1.146 mm/a > 0.225 mm/a).

Figure 17.

Figure 17

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pH values of 30% DETA solution with different CO2 loadings before and after a 30 day weight loss experiment at 60 °C.

4.2.3. Unable to Form Protective Film

Finally, the inability to form a FeCO3 protective film through CO2 absorption resulted in a relatively high CR of 20# carbon steel in DETA solutions. Most iron ions dissolved in DETA solutions, leading to insufficient concentrations of free Fe2+ and CO32– to facilitate the formation of a protective film, as explained in the Mechanism of Corrosion section.

5. Conclusions

This study comprehensively assessed the corrosion behavior of 20# carbon steel in DETA solutions under varying temperatures and CO2 loadings, comparing it with MEA solutions. The evaluation incorporated weight loss analysis, Tafel tests, and EIS, coupled with characterizations via SEM, XRD, and spectrophotometry. The exploration of the corrosion reaction mechanism yielded the following key findings.

  • 1

    From the perspective of CR, the corrosion behavior of 20# carbon steel in DETA solutions exhibited greater severity compared to MEA solutions. As the temperature increased, both the anode and cathode reaction rates accelerated, leading to a continuous rise in the CR. Contrary to MEA solutions, the trend in the CR change differed in DETA solutions with increasing CO2 loading.

  • 2

    The electrochemical results showed that the corrosion process in DETA solutions was exclusively controlled by charge transfer, without diffusion control. Simultaneously, the increase in corrosion current was jointly influenced by cathodic and anodic reactions.

  • 3

    As in MEA solutions, the corrosion process in DETA solutions failed to form a protective film. This lack of film formation was attributed to concentrations of free Fe2+ and CO32– not reaching the required minimum saturation degree for FeCO3 protective film formation. The presence of O2 exacerbated the formation of nonprotective iron oxides.

  • 4

    With DETA CO2 loading increasing from 0.2 mol/mol to 1.2 mol/mol, the CR of 20# carbon steel decreased continuously. DETA’s strong complexing ability with metal ions resulted in higher solubility of Fe2+ in DETA solutions. Under low CO2 loading, excess DETA promoted continuous anode dissolution, intensifying corrosion behavior.

These findings offer valuable insights into the corrosion behavior and mechanisms of carbon steel in DETA solutions, providing pertinent information for large-scale applications in carbon capture processes. Additionally, this study lays a theoretical foundation for the development of corrosion inhibitors in relevant industrial processes.

Acknowledgments

This work was financially supported by National Key Research and Development Program of China (2022YFE0197800) and Common Key Projects of Shanxi Research Institute for Clean Energy Tsinghua University (2023JG0301006).

The authors declare no competing financial interest.

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