The Structure and Composition of Corrosion Product Film and its Relation to Corrosion Rate for Carbon Steels in CO₂ Saturated Solutions at Different Temperatures

Abstract

For carbon steels immersed in CO₂ saturated solutions at different temperatures, the structure and the composition of corrosion product film formed on the steel surface were studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion rate of the steel was evaluated by potentiodynamic polarization, and the relation between the corrosion rate and the film property was discussed. The corrosion rate of the steel was very closely associated with the structure and the composition of corrosion product film, which were affected significantly by the solution temperature. From 30 to 60 ºC, the corrosion product film composed of FeCO₃ was porous and poorly adherent, and the corrosion rate increased with the rise of temperature. At 70 and 80 ºC, the corrosion product film was also composed of FeCO₃ and presented a compact and dense cubic crystal structure, resulting in the decrease on the corrosion rate. The corrosion rate increased once again when the temperature was up to 90 ºC, which was attributed to the negative effect of high temperature water vapor corrosion on the grain coarsening and the part exfoliation for the FeCO₃ film.

Keywords:
carbon steel; corrosion product film; CO₂ saturated solution; temperature; corrosion resistance

Introduction

Carbon dioxide (CO₂) corrosion is one of the most typical and universal material corrosion failure for carbon steels in oil and gas industries.^1-3

At present, studies involving CO₂ corrosion for carbon steels are mainly focused on CO₂ saturated brine in order to simulate actual CO₂ corrosion environments, and the corrosion behavior of carbon steels is mainly dependent on the following factors: temperature, CO₂ partial pressure, aggressive or inhibitive species, pH and flow rate.^4-9 Zhu et al.⁴ studied the corrosion behavior of the N80 carbon steel in oil field formation water containing CO₂ and reported that the temperature variation played a critical role in the corrosion rate of the N80 steel. The film on the N80 steel surface was mainly composed of iron carbonate (FeCO₃). The temperature rise not only accelerated the steel dissolution but also promoted to the formation of FeCO₃ film. Further, the authors reported that the addition of acetic acid promoted the cathodic process and inhibited the anodic process. However, the detailed effect of temperature on the microstructure of FeCO₃ film was not discussed. Gao et al.⁵ studied the mechanical property of CO₂ corrosion film and its relationship to the general and localized corrosion rates for the X65 carbon steel in simulated stratum water containing Ca²⁺, Mg²⁺, Cl^-, SO₄^2- and HCO₃^-. The authors reported that under static conditions from 0.1 to 1.0 MPa CO₂ partial pressure, the film on the X65 steel surface was composed of FeCO₃, MgCO₃ and CaCO₃; under dynamic conditions, the film composition was FeCO₃ and CaCO₃ mainly. At the same time, the presence of chloride anions induced the initiation of localized corrosion, and the corrosion rates of both the general corrosion and the localized corrosion increased with the increase of flow rate and CO₂ partial pressure. Further, Gao et al.⁶ also studied the growth mechanism of CO₂ corrosion film on the X65 steel surface and reported that the crystal growth controlled the formation of FeCO₃ film at the initial stage when the relative supersaturation of FeCO₃ was low. The nucleation process was accelerated at the high FeCO₃ supersaturation, and the high nucleation rate resulted in the formation of relatively compact film. Unfortunately, in the studies of Gao et al.,^5,6 the effect of CO₂ corrosion film on the corrosion rate was not discussed in detail. Cui and co-workers⁷ studied the chemical composition and the microstructure of CO₂ corrosion film for the N80 steel exposed to simulated stratum water containing CaCl₂ 15 g L^-1 and NaHCO₃ 1.1 g L^-1. The authors reported that the film was composed of a complex carbonate FeCO₃ and CaCO₃ and of a limited amount of α-FeOOH mainly. The complex carbonate was unstable and could be partially decomposed to α-FeOOH in dry air. However, the relation between the corrosion rate and the film property was not discussed. Sun et al.⁸ studied the formation mechanism of CO₂ corrosion film and the electrochemical characteristic of the low alloy steel in CO₂ saturated solution containing Mg²⁺, Ca²⁺, Cl^-, SO₄^2- and HCO₃^-. The authors reported that the film on the steel surface was composed of FeCO₃, CaCO₃ and MgCO₃, which presented the different formation mechanism. As the main component, the precipitation of FeCO₃ might be very closely related to cementite Fe₃C. At the initial stage, the steel substrate dissolved preferentially and left Fe₃C behind, resulting in a high concentration of Fe²⁺ between lamellar Fe₃C. This condition promoted the precipitation of FeCO₃ until the whole steel surface was covered by FeCO₃. However, the effect of temperature on the precipitation of FeCO₃ was not discussed. Yang and co-workers⁹ studied the corrosion behavior of the pipeline steel in simulated produced water saturated with supercritical CO₂ and reported that the film on the steel surface, which significantly affected the corrosion behavior of the pipeline steel, was mainly composed of FeCO₃, CaCO₃ and α-FeOOH. The element distribution of Fe, Ca, C and O was inhomogeneous, and the film formed at a low temperature was more stable than that formed at a high temperature. According to the above reports,^4-9 CO₂ corrosion film on the surface of carbon steels is relatively complicated on the chemical composition and composed of two components mainly: one is the corrosion product composed of FeCO₃ attributed to the dissolution of iron element from the carbon steel substrate, and the other is the salt scale composed of CaCO₃ and MgCO₃ mainly due to the combined action of Ca²⁺, Mg²⁺ and CO₃^2- in CO₂ corrosion environments.

However, CO₂ saturated brine containing Ca²⁺, Mg²⁺, Cl^-, HCO₃^- and SO₄^2- is very close to the actual component of CO₂ corrosion environments, but it is difficult to study the property of corrosion product film in CO₂ saturated brine. The main reason can be attributed to the following two aspects: the precipitation of salt scale and the occurrence of localized corrosion.^10-15 In contrast, the chemical component of CO₂ saturated solution is relatively simple compared with that of CO₂ saturated brine, in which the effects of salt scale and localized corrosion can be neglected. In this work, the structure and the composition of corrosion product film formed on the surface of carbon steels immersed in CO₂ saturated solutions at different temperatures are studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and the corrosion rate of the steel is evaluated by potentiodynamic polarization test. At the same time, the relation between the corrosion rate and the film property is also discussed in detail.

Experimental

The studied material was Q235 carbon steel with the following chemical composition (wt.%): C, 0.16; Mn, 0.53; Si, 0.30; S, 0.045; P, 0.015; and Fe, 98.95. Specimens were manually abraded up to 1000 grit with SiC abrasive papers, rinsed with de-ionized water and degreased in alcohol. For the electrochemical measurement, the specimens with the dimension of 1 × 1 × 0.3 cm were coated with epoxy resin, leaving 1.0 cm² exposed to solution as the working electrode. For the immersion test, the specimens with the dimension of 5 × 1 × 0.3 cm were suspended with PTFE rubber belt completely exposed to the solution.

The studied solution was CO₂ saturated solutions at different temperatures. The pH value and the CO₂ concentration for the studied solution were 3.7 and 0.4 mol L^-1 analyzed with short range pH paper and by the MT/T 257-2000 standard,¹⁶ respectively. The solution temperature was controlled with an electro-thermostatic water bath.

The potentiodynamic polarization tests were performed using a CS350 electrochemical workstation (China). A typical three electrode system was applied for all the polarization tests. The system was composed of a saturated calomel electrode (SCE) as reference electrode, a platinum sheet as counter electrode and the carbon steel specimen with the dimension of 1 × 1 × 0.3 cm as working electrode. Before each polarization test, the working electrode was immersed in the tested solution for 60 min to ensure the open circuit potential (OCP) to be stable. The potential scanning rate was 0.5 mV s^-1, and the potential scanning range was from -0.3 to 0.3 V_OCP.

The carbon steel specimens with the dimension of 5 × 1 × 0.3 cm were immersed in CO₂ saturated solutions at different temperatures for 1 h. The cross-sectional and surface morphologies were observed by a SU1510 SEM instrument (Japan). The surface composition was analyzed by a Kevex SuperDry energy dispersion X-ray spectroscopy (EDS) instrument attached on the SEM system, a D8-Advance XRD instrument (Germany) and an ESCALAB-250 XPS instrument (USA).

Results and Discussion

Cross-sectional microstructure

Figure 1 shows the cross-sectional SEM morphologies of the carbon steel specimens immersed in CO₂ saturated solutions at different temperatures for 1 h. There is a continuous and intact film on the surface of each specimen. However, the effect of temperature on the film microstructure is very prominent. Table 1 shows the relative atom abundance of Fe, C and O in the surface film formed on the specimen surface immersed in CO₂ saturated solutions at different temperatures by EDS analysis.

Figure 1
Cross-sectional SEM morphologies of carbon steel specimens immersed in CO₂ saturated solutions for 1 h at different temperatures: (a) 30, (b) 40, (c) 50, (d) 60, (e) 70, (f) 80 and (g) 90 °C.

Table 1
Relative atom abundance of Fe, C and O in surface film formed on specimen surface immersed in CO₂ saturated solutions at different temperatures by EDS analysis

Temperature / ºC	Fe / %	C / %	O / %
30	19.37	18.82	61.81
40	20.17	19.92	59.91
50	20.61	19.73	59.66
60	19.87	20.21	59.92
70	20.73	19.25	60.02
80	19.27	20.15	60.58
90	32.92	10.27	56.81

From Figure 1a, for the specimen immersed in the solution at 30 ºC, a film with the average thickness about 0.78 µm is observed on the specimen surface. EDS analysis revealed that Fe, C and O were present in the film, and the relative atom abundance of Fe, C and O is, respectively, 19.37, 18.82 and 61.81% (shown in Table 1), which is very close to 1:1:3, suggesting that the film formed at 30 ºC is mainly composed of FeCO₃. Nesic et al.^1,17,18 reported that when carbon steels were exposed in CO₂ corrosion environments, the formation of CO₂ corrosion film was dominated by the following two reverse processes: the precipitation process and the undermining process. At low temperature, the kinetics of undermining process was faster than that of precipitation process, resulting in that a porous and non-protective film was obtained on the surface of carbon steels.¹ At the same time, Schmitt¹⁹ and Dugstad et al.²⁰ reported that when the temperature was less than 70 ºC, the corrosion product film was porous as well as poorly adherent and could not provide the effective corrosion resistance for the steel substrate. Similar results are also observed in the present study. From Figure 1a, there are some cracks and pores within the FeCO₃ film, and the presence of gap between the FeCO₃ film and the Q235 substrate is observed, indicating that the FeCO₃ film formed at 30 ºC is poorly compact and adherent. From Figures 1b to 1d, the microstructure of the film formed from 40 to 60 ºC is similar to that of the film formed at 30 ºC: the presence of cracks, pores and gaps is also observed. Further, from the results of EDS shown in Table 1, the film formed from 40 to 60 ºC is also composed of FeCO₃. Besides, Figure 1 also illustrates that the film thickness increases slightly with the rise of temperature from 30 to 60 ºC, which will be discussed in detail later.

From Figure 1e, for the specimen immersed in the solution at 70 ºC, a compact and dense film with the average thickness about 2.84 µm is observed on the specimen surface. EDS analysis revealed that the main composition of the film is also FeCO₃, as shown in Table 1. Compared with the microstructure of the FeCO₃ film formed at the low temperature, for the FeCO₃ film formed at 70 ºC, there is no pore and crack within the FeCO₃ film and no gap between the FeCO₃ film and the Q235 substrate, indicating the good corrosion resistance. From Figure 1f, the film formed at 80 ºC is similar to that formed at 70 ºC, which is also composed of FeCO₃ according to the EDS results shown in Table 1.

For the specimen immersed in the solution at 90 ºC, as shown in Figure 1g, although the microstructure of the film formed at 90 ºC is also similar to that of the film formed at 70 or 80 ºC, EDS analysis revealed that the relative atom ratio of Fe, C and O is far away from 1:1:3 shown in Table 1, suggesting that the composition of the film formed at 90 ºC may be complicated in comparison with that of the film formed at the low temperature, which will be confirmed by XRD and XPS later.

Film thickness

Figure 2 shows the effect of temperature on the thickness for the corrosion product film formed on the specimen surface immersed in CO₂ saturated solutions at different temperatures for 1 h, where each datum is the average value of ten randomly selected sites at the surface film on the cross-sectional SEM morphology, and the error ranges are also shown. From Figure 2, the film thickness increases slightly with the rise of temperature from 30 to 60 ºC. Afterwards, the film thickness shows very sharp increase from 1.64 to 2.84 µm between 60 and 70 ºC. Finally, when the temperature is up to 70 ºC, the film thickness shows slight variation with the rise of temperature.

Figure 2
Effect of temperature on thickness for corrosion product film formed on specimen surface immersed in CO₂ saturated solutions at different temperatures for 1 h.

The thickness variation of corrosion product film may be attributed to the effect of temperature on the solubility product constant (K_sp) and the solubility temperature coefficient.^21-23 Nesic and co-workers²¹ studied the effects of temperature and ionic strength on the solubility limit of iron carbonate. The authors reported that the K_sp value of FeCO₃ could be described as follows:

(1) $\begin{array}{c}\log K_{\mathrm{sp}}=-59.3498-0.041377\times T_k-2.1963/T_k+\hfill \\ 24.5724\times \log T_k+2.518\times I^{0.5}-0.657\times I\hfill \end{array}$

According to the above equation, the K_sp value decreases with the rise of temperature, which is consistent with the experimental results.²¹ Waard et al.²³ reported that the solubility temperature coefficient of FeCO₃ showed a negative value, indicating that the solubility of FeCO₃ decreased with the rise of temperature. Therefore, it can be inferred that the FeCO₃ crystal formed at the high temperature is more stable than that formed at the low temperature. On the other hand, it was reported that the undermining process and the precipitation process owned the different temperature susceptibility.^1,17,18 At high temperature, the kinetics of precipitation process could be accelerated quickly and was more prominent than that of undermining process,¹ which is also favorable for the formation of corrosion product film. The above two aspects can be responsible for the thickness variation of corrosion product film.

Surface microstructure

Figure 3 shows the surface SEM morphologies of the carbon steel specimens immersed in CO₂ saturated solutions at different temperatures for 1 h. Like the results of cross-sectional microstructure shown in Figure 1, Figure 3 illustrates that the effect of temperature on the crystal structure of FeCO₃ is also prominent.

Figure 3
Surface SEM morphologies of carbon steel specimens immersed in CO₂ saturated solutions for 1 h at different temperatures: (a) 30, (b) 40, (c) 50, (d) 60, (e) 70, (f) 80 and (g) 90 °C.

From Figures 3a to 3d, for the specimens immersed in the solutions from 30 to 60 ºC, there is no obvious crystal shape on the specimen surface, whereas some scratches due to mechanical action of abrasive paper are observed. On the other hand, EDS analysis revealed that the relative atom abundance of Fe for the four specimens was more than 60%. The above results indicate that the FeCO₃ film formed at the low temperature is relatively thin, which is in agreement with the results of cross-sectional SEM morphology. It was reported that when the temperature was low, the negative effect of undermining process were prominent compared with the positive effect of precipitation process,^1,17,18 which is adverse to the formation of CO₂ corrosion film.

However, for the specimens immersed in the solutions at 70 and 80 ºC shown in Figures 3e and 3f, the surface microstructure of FeCO₃ film shows an obvious cubic crystal shape. Gao et al.^5,6,24 reported that the corrosion product film consisted of cubic crystals provided good corrosion resistance for the steel substrate against CO₂ corrosion. Zhu et al.⁴ also reported that the cubic structure could effectively resist the diffusion/permeation of aggressive species reaching the film/substrate interface. Further, for the specimen immersed in the solution at 70 ºC, EDS analysis revealed that Fe, C and O were present in the film, and the relative atom abundance of Fe, C and O was 19.63, 20.16 and 60.21%, respectively. The relative atom ratio of Fe, C and O is approximately 1:1:3, suggesting that the thickness of FeCO₃ film formed at 70 ºC increases compared with that of FeCO₃ film formed from 30 to 60 ºC, which is consistent with the results shown in Figure 2.

From Figure 3g, for the specimen immersed in the solution at 90 ºC, the surface film also shows the cubic crystal structure. However, the crystals formed at 90 ºC are obviously more coarse and bulky than those formed at 70 or 80 ºC. It was reported that the surface film consisted of exquisite crystals could provide better corrosion resistance than that consisted of coarse crystals,²⁵ indicating that the corrosion rate of the steel in the solution at 90 ºC is higher than that of the steel in the solution at 70 or 80 ºC. Later, the relation between the film property and the corrosion rate will be discussed.

Film composition

Figure 4 shows the XRD patterns of the carbon steel specimens immersed in CO₂ saturated solutions at different temperatures for 1 h. For the specimens immersed in the solutions from 30 to 60 ºC, there are only three main peaks visible on the patterns at about 45º, 65º and 82º, corresponding to α-Fe. However, the above XRD results seem to contradict the results of SEM and EDS shown in Figure 1 and Table 1: the FeCO₃ film has been formed but cannot be detected by XRD analysis. Because the measuring depth of XRD is beyond 10 µm, the FeCO₃ film formed from 30 to 60 ºC is too thin to be detected. For the specimens immersed in the solutions at 70 and 80 ºC, besides three diffraction peaks at 45º, 65º and 82º, other three diffraction peaks at about 24º, 32º and 53º corresponding to FeCO₃ were detected by XRD analysis, suggesting that the FeCO₃ film formed at the high temperature is thicker than that formed at the low temperature. However, for the specimen immersed in the solution at 90 ºC, the diffraction peaks at 24º and 53º become no obvious, and the diffraction peak at 32º moves a little to the low angle direction, which may be due to the composition of the film formed at 90 ºC is more complicated than that of the film formed from 30 to 80 ºC.

Figure 4
XRD patterns of carbon steel specimens immersed in CO₂ saturated solutions at different temperatures for 1 h.

Figure 5 shows the wide-scan XPS spectrum of the carbon steel specimen immersed in CO₂ saturated solution at 30 ºC for 1 h. There are three main peaks visible on the spectrum at about 711, 531 and 285 eV corresponding, respectively, to Fe 2p, O 1s and C 1s. The XPS result provides evidence for the presence of Fe, C and O, which is in agreement to the results of EDS and XRD. The wide-scan XPS spectra of the specimens immersed in the solutions from 40 to 90 ºC are similar to that of the specimen immersed in the solution at 30 ºC: only Fe 2p, C 1s and O 1s were detected by XPS analysis.

Figure 5
Wide-scan XPS spectrum of carbon steel specimen immersed in CO₂ saturated solution at 30 °C for 1 h.

Figure 6 shows the high-resolution XPS spectra of Fe 2p for the carbon steel specimens immersed in CO₂ saturated solutions at 30 and 90 ºC for 1 h. Table 2 summarizes the reported binding energy values for Fe 2p in various model compounds from literature.

Figure 6
High-resolution XPS spectra of Fe 2p for carbon steel specimens immersed in CO₂ saturated solutions for 1 h at (a) 30 and (b) 90 °C.

Table 2
Reported binding energy values for Fe 2p in various model compounds from literature

Species	Binding energy /eV	Reference
α-FeOOH	711.83	7
	711.80	26 , 27
	711.65	9
	711.38	7
	711.00	28
Fe3O4	710.90	29
	710.80	29
	710.40	30
FeCO3	709. 75	7
	709.67	7
	709.60	31
	709.52	9

According to the reported data in Table 2, for the specimen immersed in the solution at 30 ºC, the Fe 2p spectrum reveals two peaks at 711.70 and 709.70 eV, corresponding to Fe element in α-FeOOH and FeCO₃, respectively. The high-resolution XPS spectra of Fe 2p for the specimens immersed in the solutions from 40 to 80 ºC are similar to that for the specimen immersed in the solution at 30 ºC: only the presence of α-FeOOH and FeCO₃ was detected by XPS analysis. In contrast, for the specimen immersed in the solution at 90 ºC, the high-resolution XPS spectrum of Fe 2p is obvious different, as shown in Figure 6b. The Fe 2p spectrum reveals three peaks at 711.95, 710.90 and 709.80 eV, respectively corresponding to Fe element in α-FeOOH, Fe₃O₄ and FeCO₃.

FeCO₃ is the carbonate corrosion product for carbon steels exposed to CO₂ corrosion environments, and the mechanism for the precipitation of FeCO₃ is as follows:^4,32

(2) ${\mathrm{Fe}}^{2+}+\mathrm{CO}_3{}^{2-}\to {\mathrm{FeCO}}_3$

(3) ${\mathrm{Fe}}^{2+}+2\mathrm{HCO}_3{}^{-}\to \mathrm{Fe}{\left({\mathrm{HCO}}_3\right)}_2$

(4) $\mathrm{Fe}{\left({\mathrm{HCO}}_3\right)}_2\to {\mathrm{FeCO}}_3+{\mathrm{CO}}_2+H_{2}O$

For α-FeOOH, Heuer and Stubbins²⁷ reported that the exposure of FeCO₃ to air could result in hydrolysis and oxidation to FeO/FeOOH. Cui and co-workers⁷ reported that the presence of α-FeOOH was attributed to the temporary storage of the tested specimen in desiccated air before XPS analysis. Similar results are also reported by Zhao and Chen²⁶ and Zhou and Zuo.³³ Fe₃O₄ is the oxide corrosion product for iron-based metal materials exposed to high temperature water vapor environments, and the formation mechanism of Fe₃O₄ is as follows:^34,35

(5) $3\mathrm{Fe}+4H_{2}O\left(g\right)\to {\mathrm{Fe}}_3{O}_4+4H_2\left(g\right)$

Electrochemical behavior

Figure 7 shows the polarization curves of the carbon steel specimens in CO₂ saturated solutions at different temperatures. Regardless of the solution temperature, all of the specimens presented the electrochemical characteristic of active dissolution in the tested electrolyte. However, the effect of temperature on the corrosion current density (i_corr) is very prominent, as shown in Figure 7.

Figure 7
Polarization curves of carbon steel specimens in CO₂ saturated solutions at different temperatures.

From the values of i_corr, the corrosion rate (V_L) can be calculated according to the following equations:

(6) $V_{\mathrm{corr}}=\left(A\times i_{\mathrm{corr}}\times {10}^4\right)/\left(n\times F\right)$

(7) $V_L=\left(8.76\times V_{\mathrm{corr}}\right)/\rho$

In the above two equations, A represents the relative atomic weight (g), A = 56 g; i_corr represents the corrosion current density (A cm^-2), which can be obtained from Figure 7; n represents the chemical valence, n = 2; F represents the Faraday constant (A h), F = 96500 C = 26.8 A h; V_corr represents the weight corrosion rate (g m^-2 h^-1), which can be calculated by equation 6; ρ represents the material density (g cm^-3), ρ = 7.8 g cm^-3; V_L represents the corrosion rate (mm y^-1), which can be calculated by equation 7.

Relation between corrosion rate and corrosion product film

Figure 8 shows the effect of temperature on the corrosion rate for the carbon steel specimens immersed in CO₂ saturated solutions, where each datum is the average value of five parallel polarization tests, and the error ranges are also shown. From Figure 8, the corrosion rate increases gradually with the rise of temperature from 30 to 60 ºC and shows a peak value at 60 ºC. However, when the temperature is up to 70 ºC, the corrosion rate decreases and shows a least value at 80 ºC. Finally, the corrosion rate increases once again when the temperature is 90 ºC.

Figure 8
Effect of temperature on corrosion rate for carbon steel specimens in CO₂ saturated solutions.

From the above results of SEM, XRD and XPS, a corrosion product film with different structure and composition is formed on the steel surface when the studied steel is exposed in CO₂ saturated solution, and the property of corrosion product film is affected significantly by the solution temperature. It is generally accepted that the corrosion behavior of carbon steels in CO₂ corrosion environments is very closely associated with the presence of corrosion product film.^17,18,24 The relation between the corrosion rate and the film property is discussed as follows.

It was reported that the corrosion resistance of CO₂ corrosion film was mainly attributed to its role of mechanical barrier to restrain the diffusion/permeation of aggressive species reaching the substrate surface, so a protective film should have very few film defects.^5,6 Because of the different temperature susceptibility between the precipitation process and the undermining process, at low temperature, the undermining rate was usually faster than the precipitation rate, resulting in that the undermining process controlled the film-formed kinetics on the surface of carbon steels.^17,18 In this case, a porous, poorly adherent and non-protective film would be obtained on the steel surface, and the film was almost no diffusion/permeation resistance to restrain the cathodic species, such as H⁺, H₂O and H₂CO₃, reaching the film/substrate interface.¹ The corrosion rate was dominated by the electrochemical step, and the following cathodic reactions occurred on the steel surface readily:^36-40

(8) $H^{+}+e\to H$

(9) $H_{2}O+e\to H+{\mathrm{OH}}^{-}$

(10) $H_2{\mathrm{CO}}_3+e\to H+\mathrm{HCO}_3{}^{-}$

In this work, from 30 to 60 ºC, although the thickness of FeCO₃ film increases with the rise of temperature as shown in Figure 2, the presence of pores and cracks within the FeCO₃ film and the presence of gap between the FeCO₃ film and the Q235 substrate are observed, as shown in Figure 1, indicating that the FeCO₃ film formed from 30 to 60 ºC cannot provide the good physical barrier to the steel substrate. At the same time, Nesic et al.³⁸ reported that the kinetics of cathodic reactions was accelerated significantly with the rise of temperature from 20 to 80 ºC. Ogundele and White³⁹ reported the similar results on the cathodic kinetics in the temperature range from 25 to 95 ºC. Therefore, in this work, the increased corrosion rate from 30 to 60 ºC is attributed to the combined effects of temperature and non-protective FeCO₃ film.

However, with the rise of temperature, the high temperature accelerated the kinetics of precipitation process, and the precipitation rate would exceed the undermining rate, resulting in a dense, compact and protective film formed on the surface of carbon steels.^17,18 In the present study, similar results are also observed in Figures 1 and 3. From Figures 1e and 1f, there is no pore and crack within the FeCO₃ film and no gap between the FeCO₃ film and the Q235 substrate, indicating the good function of mechanical barrier for the FeCO₃ film. On the other hand, from Figures 3e and 3f, the FeCO₃ film shows the cubic crystal structure, which can effectively restrain the diffusion/permeation of aggressive species to the film/substrate interface. In this case, the reactant supply for electrochemical reactions shown in equations 8 to 10 become more difficult, and the corrosion rate is dominated by the step of mass transfer. Therefore, the decreased corrosion rate at 70 to 80 ºC is due to the protective FeCO₃ film. Besides, the fact that the corrosion rate at 80 ºC is slightly lower than that at 70 ºC may be attributed to that the FeCO₃ film formed at 80 ºC is thicker than that formed at 70 ºC, as shown in Figure 2.

The composition and the structure of corrosion product film formed at 90 ºC are obviously different from those of FeCO₃ film formed from 30 to 80 ºC. Figure 6b illustrates that the signal of Fe₃O₄ was detected by XPS analysis. However, according to the XRD result for the specimen immersed in the solution at 90 ºC, except the diffraction peaks corresponding to α-Fe and FeCO₃, no peak corresponding to Fe₃O₄ was detected by XRD analysis, suggesting the content of Fe₃O₄ on the steel surface is relatively few. The presence of a small amount of Fe₃O₄ can increase the relative atom abundance of Fe and decrease the corresponding values of C and O, which is consistent with the EDS results shown in Table 1. Therefore, it can be inferred that the corrosion product film formed at 90 ºC is composed mainly of FeCO₃ and of a small amount of Fe₃O₄. The presence of Fe₃O₄ is attributed to the occurrence of high temperature water vapor corrosion.^34,35 Further, Han et al.³⁴ reported that a limited amount of Fe₃O₄ was distributed discontinuously between the FeCO₃ film and the steel substance. Figure 9 shows the enlarged surface SEM morphologies of the carbon steel specimens immersed in CO₂ saturated solutions at 80 to 90 ºC for 1 h.

Figure 9
Enlarged surface SEM morphologies of carbon steel specimens immersed in CO₂ saturated solutions for 1 h at (a) 80 and (b) 90 °C.

Comparing Figures 9a and 9b, the cubic crystals formed at 90 ºC are more coarse and bulky than those formed at 80 ºC, indicating that the corrosion resistance of the film formed at 80 ºC is greater than that of the film formed 90 ºC. On the other hand, from Figure 9b, some cubic crystals have been peeled off from the steel surface, which is also attributed to the presence of Fe₃O₄ on the steel surface. According to the report of Han et al.,³⁴ the discontinuous Fe₃O₄ film not only could not provide the corrosion resistance for the steel substrate but also decreased the adhesive force of FeCO₃ film to the steel surface. Therefore, the increased corrosion rate at 90 ºC is attributed to the combined effects of the grain coarsening and the part exfoliation on the FeCO₃ film, which is derived from the occurrence of high temperature water vapor corrosion.

Conclusions

In this work, the relation between the evolution of corrosion rate and the property of corrosion product film for carbon steels immersed in CO₂ saturated solutions at different temperatures was studied and discussed. The corrosion rate of the studied steel was very closely related to the structure and the composition of corrosion product film, which were affected significantly by the solution temperature. From 30 to 60 ºC, the corrosion product film composed of FeCO₃ was porous and poorly adherent, and the corrosion rate increased with the rise of temperature. At 70 and 80 ºC, the corrosion product film was also composed of FeCO₃ and showed a compact and dense cubic crystal structure, resulting in the decrease on the corrosion rate. The corrosion rate increased once again when the temperature was up to 90 ºC, which was attributed to the negative effect of high temperature water vapor corrosion on the grain coarsening and the part exfoliation for the FeCO₃ film.

Acknowledgments

This work is supported by the National Nature Science Foundation of China (contract 51601133, 51210001 and 51401150).

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Publication Dates

• Publication in this collection
  Dec 2017

History

• Received
  14 June 2017
• Accepted
  16 Aug 2017