Formation and Evolution of the Corrosion Scales on Super 13Cr Stainless Steel in a Formate Completion Fluid With Aggressive Substances

The formation and evolution of the corrosion scales on the super 13Cr stainless steel (SS) surface after exposure in a formate completion fluid with the presence of various aggressive substances was investigated. The results indicate that the formation of Fe₃O₄ covered the surface of super 13Cr SS as the inner layer accompanied with outer scattered FeS. The corrosion rate was below 0.07 mm/year after 120 h of exposure in the formate fluid at 180°C under N₂ environments; the presence of aggressive substances such as sulfide and CO₂ in the formate fluid promoted the proceeding of anodic dissolution in the early period, and the ingress of CO₂ progressively increased the general corrosion rate to 1.7 mm/year. For CO₂-containing conditions, the formation of FeCr₂O₄ and Cr(OH)₃ was detected in the inner corrosion product layers, and the precipitation of “sheet”-shaped iron carbonate (FeCO₃) was detected as the outer layer. The accumulation rate of corrosion products increases by two orders of magnitude with the ingress of CO₂, corresponding to thicker corrosion products, but the dissolution rate is still three orders of magnitude higher than when CO₂ was absent.

Introduction

The exploitation and production of ultra-deep reservoirs for future energy supply have been highlighted as a practical strategy to achieve the effective application of fossil energy. The Tarim Basin, where exists abundant natural gas in the depth between 6,500–8,000 m, has been constructed with 52 wells in Kuqa from 2012 (Liu et al., 2019a). Due to the high pressure (115–140 MPa), high temperature (170°C–190°C), and complex corrosive medium, the corrosion issues of highly dense brine-based completion fluids on tubing and casing materials in ultra-deep high-temperature wells have become a research hot spot in the course of oil and gas exploitation (Yue et al., 2020a; Li et al., 2020; Zhao et al., 2020). The considered competition fluids are mainly the bromine salt completion fluids (KBr, ZnBr₂, and CaBr₂) (Liu et al., 2014; Liu et al., 2019b) and/or formate brines (NaCOOH, KCOOH, and CsCOOH) (Bungert et al., 2000; Leth-Olsen, 2004; Zhu et al., 2011b). In the past decades, one of the major issues is the localized corrosion by using bromine salts due to the presence of Br⁻ (Hoar and Jacob, 1967; Refaey et al., 2005). Recently, formate brines with filtrate viscosity and low-water activities are more potential to be a safe option for the deep wells (the average depth >6,000 m), and also the operating temperature can be over 150°C (Howard and others, 1995; Bungert et al., 2000). Clarke states that the formate brines have many advantages during drilling and completion processes at a temperature up to 160°C, such as preventing the thermal degradation for the conventional drilling and completion fluid polymers (Clarke-Sturman and Sturla, 1988). Among formate brines, KCOOH solution has been widely used as the alternative option for such arduous conditions based on its large solubility and low corrosivity (Leth-Olsen, 2004).

Tubing materials experienced low-corrosion risk in formate brines at a high temperature of 180°C (Li et al., 2018), compared with that of immersing in the CO₂-saturated formation water (Zhu et al., 2011a, 2015a; Hua et al., 2019a). However, corrosion control of the tubing material during the completion period remains an obstinate issue for corrosion engineers due to the intrinsically complicated corrosion processes, especially in the presence of sulfide impurity or CO₂ ingress under such harsh environments. The sulfides were introduced as an impurity by the synthesis of KCOOH where it is inevitable to exit sulfides in the industrial-grade purification process of CO from the exhaust gas (Teng et al., 2016). The CO₂ influx in annulus protection fluids will react with formate brines and form trace amounts of formic acid, which acts as a catalyst in the reactivity of formate (Bungert et al., 2000) and promotes the corrosion process on the material surface (Yang et al., 2019). Sekine et al. (Sekine and Chinda, 1984) stated that the formic acid can be corrosive with an increasing content of KCOOH in the solution due to the dissolved KCOOH increased the solution electrical conductivity. Recently, materials such as super 13%Cr martensite stainless steel (S13Cr) has been considered as a tubing and casing material for such hash conditions based on its good corrosion resistance, low cost, and excellent mechanical properties. Along with the growing understanding of the influence of aggressive substances on the corrosive of formate brines (Bungert et al., 2000; Howard et al., 2009), further study of the corrosion behavior of S13Cr immersed in the formate brines as well as the evolution of the corrosion scales on the surface is urgently pressed. It has been reported that both Fe and Cr elements in the matrix significantly contributed to the corrosion-resistant stainless steel (SS) in an acidized KCOOH solution by comparing the polarization curves between pure metal and 304 SS; Ni promotes the stability of the passive film on the surface as well as the mechanical property (Sekine and Chinda, 1984). Hakon (Leth-Olsen, 2004) found that the formation of the corrosion product scales on 13Cr SS in formate fluids did not act as an effective barrier against corrosion, showing high-corrosion rates after several weeks of exposure, and the surface was covered by nonuniformed crystalline FeCO₃, especially at 180°C (Leth-Olsen and others, 2005). Although the formation of the FeCO₃ layer plays a dominant role in retarding the corrosion rate, the corrosion resistance of S13Cr SS corresponds to a complicated corrosion product scales formation and evolution process, and that the amorphous/nanocrystalline inner layer provides better corrosion protection than outer FeCO₃ precipitation^21–25 under the ultra-deep well conditions [temperatures beyond 180°C (Zhao et al., 2018; Yue et al., 2020a, 2020b, 2020c; Zhao et al., 2021a)]. The systematic study on the formation and evolution of the corrosion product scales on the S13Cr SS surface immersed in a formate fluid under HTHP condition, especially with the presence of aggressive substances such as sulfide or CO₂ is rare.

It has been reported that the formation of the corrosion products under a CO₂–H₂S coexistence environment was related to H₂S and CO₂ concentration (Banaś et al., 2007; Pessu et al., 2017; Li et al., 2021). Having acidic media such as CO₂ and H₂S can promote the ionic conductivity in the solution (Vedage et al., 1993; Zhao et al., 2021b). Ding et al. (2013) discovered that the anodic dissolution was accelerated by high-H₂S/CO₂ partial pressures where both H₂S and CO₂ reduced the stability of the corrosion product scales (He et al., 2009). The composition of corrosion products may vary as H₂S concentration increases. The corrosion products have been reported to become more complicated (generating several FeS compounds in addition to FeCO₃), indicating porous in feature and results in less corrosion protection (Vedage et al., 1993). If the p_CO2/p_H2S ratio is between 20 and 500, there is a possibility that the iron sulfide layer covers the surface locally, and pitting corrosion dominates degradation due to the galvanic corrosion between covered and bare surfaces, which is usually improved by adding inhibitors (Xu et al., 2018; Askari et al., 2021; Hamza et al., 2021).

This article complements the literature gaps by systematically investigating the corrosion behavior of S13Cr immersed in a formate fluid via studying the influence of sulfide or CO₂ on the corrosion product kinetics. The weight-loss method is implemented to review the extent of general corrosion, and the nature and morphology of the corrosion products are identified using a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Raman spectroscopy in order to identify the role CO₂ plays on the corrosion product scale composition and morphology. This article mainly discussed the corrosion risk caused by potential aggressive substances (sulfide and CO₂ ingress) that may be introduced in the formate fluid as the application in the oil and gas industry and revealed the surface interaction mechanisms on the corrosion product scale of S13Cr SS at severe annulus environments.

Experimental Procedure

Material Preparation

The material used was an API-P110 grade S13Cr (UNS S41425) SS with heat-treated through normalization and tempering at 980°C and 590°C, respectively. The chemical composition (wt%) of the material is listed in Table 1. Coupon specimens for immersion tests were machined to the dimension of 30 mm × 13 mm × 3 mm. Before each test, the specimens were ground to 1,200 grit using silicon carbide paper, then rinsed with deionized water and acetone, and dried.

TABLE 1

TABLE 1. Elemental composition of super 13Cr SS (wt%).

Test Methods

The test solutions simulated the composition of the completion fluid from the Tarim field (1.5 g/ml in density) containing potassium formate (>97.1%) and deionized water. The chemical composition is listed in Table 2. The formate completion fluid usually uses CO purified from industrial tail gas as a synthetic raw material, containing sulfide due to the existence of a certain amount of K₂S in KCOOH. The solution was de-aerated by saturating with N₂ (gas in 1–1) or CO₂ (gas in 1–2) in a separate container overnight before testing. The dissolved oxygen was detected using optical oxygen luminescent sensors (HACH, K1100) to ensure the O₂ content within the solution was below 5 ppb. All lines to the autoclave were purged with high pressure N₂/CO₂ and evacuated by lines 2 and 4 to remove the O₂ content within the system. The prepared, de-aerated brine was carefully transferred from the sparging vessel into the closed autoclave containing the test specimens at an ambient pressure and temperature. Then, the autoclave was pressurized and stabilized at 10 bar N₂ or CO₂ at 25°C, respectively. The solution was heated to 180°C, and the final pressure was 26 bar under an N₂ condition and 34 bar under a CO₂ condition, as shown in Table 3. The immersion time for corrosion tests began once the solution temperature reached 180°C in the autoclave (Supplementary Figure S1).

TABLE 2

TABLE 2. Chemical composition of formation water.

TABLE 3

TABLE 3. Test matrix for corrosion tests under different conditions.

A 316L SS static autoclave was utilized to conduct all the weight loss tests as schematically shown in Figure 1. For mass loss experiments, three coupons were mounted on a designed PEEK holder and fully immersed in the test solution. All tests were performed at 180°C with an N₂ or CO₂ partial pressure of 10 bar at 25°C, and the volume to surface ratio was kept at approximately 30 ml/cm² (three samples within the autoclave).

FIGURE 1

FIGURE 1. Schematic diagram of the high-temperature and high-pressure autoclave and the location of mass loss samples.

The original weight (m₀) of coupons was measured before the test, while the final weight (m₁) was obtained after the removal of corrosion products (ASTM G1-03 standard) (Metals, 2011). The corrosion rate (C_R) was reported in mm/year according to the obtained weight loss via Eq. 1:

$C_{\mathrm{R}}=\frac{87600\left(m_0-m_1\right)}{t\rho A},(1)$

where t represents the immersion time in hours, $\rho$ is steel density in g/cm³, and A is the exposed surface area in cm².

The thermodynamically stable states for S13Cr SS were evaluated by the Pourbaix diagram using OLI software⁽⁴⁾. The equilibrium of the aqueous and vapor phase was calculated using Stream Analyzer. Custom was selected as the type of calculation, and the volume of the total inflow was fixed. For thermodynamic calculation, the calculated stream was used in the Corrosion Analyzer. An alloy containing 12% Cr and 88% Fe was selected as the contact surface. In order to determine the total metal cation concentration, metal activities were changed, and the thermodynamics after the mass loss was calculated by iterating over Stream Analyzer.

Surface Analyses

The coupons after immersion tests were scanned using a scanning electron microscope equipped with an energy-dispersive X-ray detector (SEM/EDS), and X-ray diffraction (XRD) and Raman spectroscopy were performed on the surface to determine the morphology and chemical composition of the crystalline and amorphous corrosion products formed on the surface. The crystalline corrosion product was scanned over a 2-theta range from 10–90^o at 0.033°/s on an area of 10 mm × 10 mm using a D8 Advance instrument (Bruker) with Cu Kα radiation; while the surface heterogeneities at the microscopic scale of the corrosion product was detected by confocal Raman microscopy using a LabRAM HR Evolution instrument (HORIBA) with a wavelength of 473 nm combined with a confocal imaging mode to focus on the injection area using the spot size of 1 μm access to frequencies from 200 to 1,200 cm⁻¹. The pit depth was measured by laser scanning confocal microscope (LSCM) measurements after removing the corrosion products according to the standard G46-94 (Metals, 2011, 46). An average pit depth from the 10 deepest pits was used to evaluate the localized corrosion rate (LCR) by the following formula:

$LCR=\frac{8.76h}{t}+C_R,(2)$

where h is the pitting depth in μm after removing corrosion products.

Results

Figure 2 compares the general corrosion rates of S13Cr SS exposed to the formate fluid saturated with N₂ or CO₂ at 180°C and various immersion times. The highest corrosion rates were observed at the first 5 h under both N₂ and CO₂ saturated conditions at 180°C. It can be seen that the corrosion rate of S13Cr SS was high (21.94 mm/year) when CO₂ was introduced into the formate fluid, in comparison with the value of 1.177 mm/year under the N₂ condition. The average mass loss tended to increase with time prolonged; however the slope of the increasing curve decreased with time, which indicates the corrosion rates of S13Cr SS decreased with immersion time, showing a sharply reduction from 5 to 48 h. For a long immersion time of 120 h, the corrosion rate for S13Cr SS was maintained at 0.063 mm/year under the N₂ condition, whereas a higher corrosion rate of 1.76 mm/year was recorded after the introduction of CO₂.

FIGURE 2

FIGURE 2. Total mass loss and corrosion rates of S13Cr SS under the 180°C formate condition with N₂/CO₂ saturated after 5, 48, and 120 h.

The SEM morphology of the corrosion product scales formed on the S13Cr SS surface at various immersion times is given in Figure 3. For the condition with N₂, the polishing marks were still visible on the surface after 5 h of exposure (Figure 3A). It is interesting to note that the corrosion products with a “spherical” shape were observed after 48 h of exposure (Figure 3B) and increased in number after 120 h (Figure 3C). The EDS results in Supplementary Figure S2 indicate that the scattered corrosion products were sulfur-rich. Figures 3D–F show the microscopic morphology of S13Cr SS in the formate solution saturated with CO₂ at various immersion times and 180°C, indicating the corrosion product scales covered the entire surface with time prolonged. It is noted that the spherical corrosion products rapidly precipitated and are visible on a cracked corrosion product layer after 5 h of exposure, as shown in Figure 3D. The higher magnification showed that the spherical products present a spherical-shaped structure in morphology. After 48 h of exposure, the presence of “sheet”-shaped corrosion products randomly covered the inner layer, as shown in Figure 3E. At 120 h, the sheet-shaped corrosion products gathered into clusters and fully covered the surface, as shown in Figure 3F.

FIGURE 3

FIGURE 3. SEM morphology of the corrosion products formed on S13Cr SS exposed to the 180°C formate fluid with (A–C) N₂ and (D–F) CO₂ saturated.

Figure 4 illustrates the XRD patterns of the corrosion product scales formed on the surface of S13Cr SS at various immersion times. In the absence of CO_2, as shown in Figure 4A, no crystalline corrosion products were detected after 5 h of immersion time at 180°C. The peaks of FeS at (100), (101), and (111) were detected for the samples immersed in the solution after 48 h which corresponded to a disordered tetragonal mackinawite structure as spherical MkB (Wolthers et al., 2003) and agreed with the spherical feature observed through SEM/EDS in Figure 3E.

FIGURE 4

FIGURE 4. XRD patterns of the corrosion scale formed on S13Cr SS under the 180°C formate condition with (A) N₂ or (B) CO₂ saturated after 5, 48, and 120 h.

Figure 4B illustrates the overall XRD patterns of the corrosion product scales of S13Cr SS in CO₂-saturated formate fluids at various immersion times. The corrosion products are mainly FeCO₃ when the samples were exposed to the CO₂-saturated formate solution after 5, 48, and 120 h. However, both peaks for MkA (disordered sheet-like mackinawite) and MkB were detected in the first 5 h. Unlike spherical MkB, MkA has been reported to display sheet-like precipitated aqueous FeS clusters (Wolthers et al., 2003), which display a similar morphology with FeCO₃ in the context of this study. The proportion of these two end-member phases was reported to highly depend on the pH value, and thus the formation of MkA at (101) instead of MkB at (101) can be related to the pH drop caused by CO₂ infusion. It is also noted that an enhanced austenite peak located at 43.58° was detected, which suggests that the preferential dissolution of martensite took place on the surface in the early stages. However, neither the peaks of austenite nor martensite was detected after 120 h of exposure.

To identify the inner corrosion product layer, Raman spectroscopy was used to supplement XRD measurements. The Raman spectra in Figure 5 relate to three local scans conducted on the S13Cr SS surface at various immersion times. For 5 h (Figure 5A), there was no corrosion product scale detected on the surface. The spectrum in Figure 5B indicates that the peak at around 662 cm⁻¹ is representative of Fe₃O₄, and the scan in Figure 5C indicates that the peaks corresponding to Fe(II)-S(-II) are located at 213, 282, 384, 485, and 595 cm⁻¹ (Genchev and Erbe, 2016).

FIGURE 5

FIGURE 5. Raman spectra of the corrosion scale formed on S13Cr SS in the formate fluid at 180°C after (A) 5 h, (B) 48 h, and (C) 120 h immersion time.

The Raman spectra in Figure 6 relate to six local scans conducted on the corroded S13Cr SS surface exposed to the CO₂-saturated formate fluids at various immersion times. The selected points were scanned on the outer layer and inner layer after 5, 48, and 120 h of immersion time. The spectrum in Figure 6A indicates that the peak at around 695 cm⁻¹ is representative of FeCr₂O₄ (Anthony et al., 1990), and the scan in Figure 6B suggests the peaks corresponding to FeCr₂O₄ and Cr(OH)₃. For the sample exposed to 120 h (Figure 6C), the corroded sample surface was covered with Cr(OH)₃ which is located at 707 cm⁻¹ (Zhu et al., 2015b). The composition of the inner layer cannot be detected by XRD from 5 to 120 h, indicating the amorphous feature of this layer.

FIGURE 6

FIGURE 6. Raman spectra of the corrosion scale formed on S13Cr SS after (A,D) 5 h, (B,E) 48 h, and (C,F) 120 h immersion time under CO₂-saturated formate condition at 180°C.

Figures 6D,E,F illustrate the conducted Raman spectra on the outer crystalline corrosion product layer. The results indicate that the “spherical”-like and “sheet”-like (scattered/cluster) corrosion products correspond to the mixture of FeCO₃ and FeS, respectively; the strong peaks of FeCO₃ are located at 187, 292, 730, and 1,087 cm⁻¹ (Ondrus et al., 2003), and the weak peaks of FeS are located at 213 and 384 cm⁻¹ (Genchev and Erbe, 2016). It should be noted that fewer peaks of FeS were detected on the outer surface of S13Cr SS, which could be related to the domination of forming FeCO₃ or the transformation from MkB to MkA.

Figure 7 illustrated the cross-sectional of corrosion product scales formed on the surface of S13Cr SS after 120 h under an N₂/CO₂-saturated formate condition at 180°C. As shown in Figure 7A, the thickness of the Fe₃O₄ layer was approximately 2.7 μm after 120 h of immersion in the formate fluid. The enrichment of S can be detected in the outer part of the corrosion product scale, which indicates the precipitation of FeS with time, consistent with the observation from top view. Figure 7B showed the corrosion product scale thickened to a thickness of 20 μm for CO₂-saturated condition at 120 h. The enrichment of Cr within the corrosion product scale can be related to the formation of FeCr₂O₄ and Cr(OH)₃, which is in agreement with the inner layer feature observed through Raman spectra in Figure 6. Besides, the enrichment of S was detected in the inner layer of the corrosion product, which can be resulted in the preferential precipitation of FeS in the early period. It should be noted that the EDS detection cannot separate S and Mo during line scan; therefore, the distribution of S within a film can be interfered with by Mo in the matrix.

FIGURE 7

FIGURE 7. Cross section of the corrosion scale formed on S13Cr SS exposed to the formate fluid in (A) N₂ and (B) saturated CO₂ after 120 h at 180°C.

The SEM images of corroded S13Cr SS samples after removing the corrosion products were observed in Figure 8. Compared to the condition with the infusion of CO₂, S13Cr suffered slight corrosion in the N₂-saturated formate fluid from 5 to 120 h. The polish marks were still visible; however, they became shallow as the immersion time prolonged. In the CO₂-saturated condition, the corrosion product scale can be detected at 5 h (Figure 4 and Figure 6), and the surface of S13Cr SS was significantly corroded as shown in Figure 8B. As shown in Figure 8D, the localized attack can be observed on the surface of S13Cr after 48 h of immersion in the CO₂-saturated formate fluid. After 120 h, the few localized corrosion developed to a more severe attack (Figure 8F).

FIGURE 8

FIGURE 8. SEM images of corroded S13Cr SS samples exposed under the 180°C formate condition with (A–C) N₂ and (D–F) CO₂ saturated after removing the corrosion products.

The localized corrosion rate of S13Cr SS immersed in CO₂-saturated formate fluids was calculated based on the analysis of LSCM measurements on the surface. The highest localized corrosion rate of 0.8 mm/year was determined in the first 48 h. From 48 to 120 h, the development of the pit grew slowly, and the localized corrosion rate declined to 0.45 mm/year after 120 h (note that the error bars were derived from the depth difference of the top ten pits, and the higher value at 48 and 120 h indicates the inhomogeneous development of pitting corrosion), which can be related to the formation of Cr(OH)₃ and FeCO₃ covering the surface and blocking the aggressive ions such as S (Figure 9).

FIGURE 9

FIGURE 9. Localized corrosion rate and pitting profilometry of corroded S13Cr SS samples after removing the corrosion products.

Discussion

The Formation of the Corrosion Product Scales in Thermodynamics

The formation of the thermodynamically stable corrosion scales on the surface can be predicted via Pourbaix diagrams by Corrosion Analyzer in the OLI system. Figure 10 shows the conducting Pourbaix diagram for Cr and Fe in simulated completion fluids (formate saturated with N₂); a total metal cation concentration of 10^–5 mol/l was considered (12% Cr cations and 88% Fe cations) at 180°C (Zhao et al., 2019; Yue et al., 2020c). It is acknowledged that the cathodic process contains the following reactions (Yue et al., 2017) and is provided as the navy dash line [a] in Figure 13:

$2H^{+}\left(aq\right)+2e^{-}\to H_2\left(g\right),(3)$

$2HCOOH\left(aq\right)+2e^{-}\to H_2\left(g\right)+2COO{H}^{-}\left(aq\right),(4)$

$2H_{2}O+2e^{-}\to H_2\left(g\right)+2O{H}^{-}\left(aq\right),(5)$

Where reaction (Eq. 3) can be negligible under the pH beyond 10.5, and concentration of HCOOH is negligible as it can be deduced from its pK_a [4.40 in diluted solutions at 180°C (HwaáKim et al., 1996)]. Therefore, reaction (Eq. 5) dominates the cathodic reactions in this case.

FIGURE 10

FIGURE 10. Pourbaix diagram for S13Cr SS under the formate condition with the presence of sulfide at 180°C.

The initial corrosion rate was relatively high under such high-temperature conditions, which can be attributed to the fast metal dissolution in the absence of the protective layers (solid/insoluble products), and the results are in agreement with our previous research (Yue et al., 2020a; Yue et al., 2020b; Yue et al., 2020c):

$Fe+2H_{2}O\leftrightarrow HFe{O}_2^{-}+3H^{+}+2e^{-}.(6)$

The corrosion rates were reduced sharply after 48 h, accompanied by the detection of the inner Fe₃O₄ layer, suggesting that the inner Fe₃O₄ layer plays a major role in blocking the ion transportation and retarding the corrosion process at the material interface. According to the Pourbaix diagram, the formation of Fe₃O₄ becomes thermodynamically favorable in formate fluids at 180°C and pH 10.55, suggesting the oxidation of the anion HFeO^2- (Eq. 7):

$3HFe{O}_2^{-}\leftrightarrow F{e}_3{O}_4+H_{2}O+O{H}^{-}+2e^{-}.(7)$

For S13Cr SS, the formation of Cr₂O₃ is expected by reaction (Eq. 8) which coexists with Cr(OH)₃ (reaction Eq. 9) in aqueous. However, neither was detected from the Raman analyses and suggests that the amount of Cr₂O₃ and Cr(OH)₃ may be neglected compared to magnetite in the current system so these products may not be detected.

$2\mathit{Cr}+3H_{2}O\leftrightarrow {\mathit{Cr}}_2{O}_3+6H^{+}+6e^{-},(8)$

${\mathit{Cr}}_2{O}_3+3H_{2}O\leftrightarrow 2\mathit{Cr}{\left(\mathit{OH}\right)}_3.(9)$

The dissolution of the substrate in the early period by reaction (Eq. 6) causes a reduction of pH and subsequently drives the formation of FeS by the following reaction when the interface pH value reaches 10.2:

${\mathit{HFeO}}_2^{-}+S^{2-}+H_{2}O\leftrightarrow \mathit{FeS}+3{\mathit{OH}}^{-},(10)$

$\mathit{Fe}+S^{2-}\leftrightarrow \mathit{FeS}+2e^{-},(11)$

where reaction (Eq. 10) is considered as the formation of FeS via precipitation, while the reaction (11) represented the solid reaction for FeS (Rickard, 1995; Liu et al., 2017).

For the environment containing CO₂, as shown in Figure 11, the hydrogen evolution reaction line (navy dash line [a]) indicates a positive shift, which can be related to the change in the ion activity induced by CO₂. The presence of CO₂ can dissolve in the formate solution and hydrate to form H₂CO₃ through the following reactions (Ikeda et al., 1983):

$C{O}_2\left(g\right)\leftrightarrow C{O}_2\left(aq\right);(12)$

$C{O}_2\left(aq\right)+H_{2}O\leftrightarrow H_{2}C{O}_3\left(aq\right).(13)$

FIGURE 11

FIGURE 11. Pourbaix diagram for S13Cr SS under the formate condition with the presence of CO₂ and sulfide at 180°C after (A) 0 and (B) 5 h of mass loss (0.098 g mass loss per sample).

The weak acid H₂CO₃ in the formate solution can be dissociated in two steps, as shown in reactions (Eq. 14) and (Eq. 15) (Nesic et al., 1996b). It is noted that the formation of H₂CO₃ causes a reduced pH to 6.8 and can further react with formate ions to form HCOOH via reaction (Eq. 16) (Bungert et al., 2000).

$H_{2}C{O}_3\left(aq\right)\leftrightarrow HC{O}_3^{-}\left(aq\right)+H^{+}\left(aq\right);(14)$

$HC{O}_3^{-}\left(aq\right)\leftrightarrow C{O}_3^{2-}\left(aq\right)+H^{+}\left(aq\right);(15)$

$H_{2}C{O}_3\left(aq\right)+COO{H}^{-}\left(aq\right)\leftrightarrow HCCOH\left(aq\right)+HC{O}_3^{-}\left(aq\right).(16)$

The cathodic reactions in the CO₂-saturated formate solution include not only the reactions (Eqs 3–5) but also the following reactions (Kahyarian and Nesic, 2020):

$2H_{2}C{O}_3\left(aq\right)+2e^{-}\to H_2\left(g\right)+2HC{O}_3^{-}\left(aq\right);(17)$

$2HC{O}_3^{-}\left(aq\right)+2e^{-}\to H_2\left(g\right)+C{O}_3^{2-}\left(aq\right).(18)$

The reactions (Eq. 4), (Eq. 18), and (Eq. 19) are subsequently accelerated, indicating the fast metal dissolution process at the material interface under the CO₂-saturated condition compared to that of the N₂-balanced condition. The following anodic reaction is considered in the early stage as shown in Figure 11A:

$Fe+2HCO{O}^{-}\leftrightarrow Fe{\left[HCOO\right]}_2\left(aq\right)+2e^{-}.(19)$

It is noted that the surface is covered by scattered FeS, suggesting that FeS becomes thermodynamically stable under the current experimental conditions. According to the constructing Pourbaix diagram, as shown in Figure 11A, the FeS phase becomes thermodynamically stable at pH 7.0, which indicates the formation of FeS at high pH. The following reactions are derived by the pH rise:

$Fe{\left[HCOO\right]}_2\left(aq\right)+H_{2}S\leftrightarrow FeS+2HCOOH,(20)$

$Fe{\left[HCOO\right]}_2\left(aq\right)+H{S}^{-}\leftrightarrow FeS+HCOOH+HCO{O}^{-}.(21)$

Besides, reactions (Eq. 22) and (Eq. 23) can also occur based on the potential drive as the pH increase to 7.

$\mathit{Fe}+H_{2}S\leftrightarrow \mathit{FeS}+2H^{+}+2e^{-},(22)$

$\mathit{Fe}+{\mathit{HS}}^{-}\leftrightarrow \mathit{FeS}+H^{+}+2e^{-}.(23)$

After 5 h of mass loss, the increase in mass loss with immersion times made FeCr₂O₄ thermodynamically stable for Cr in solid-state (the calzo phase in Figure 11B) according to the following reactions (Hua et al., 2019b) in the Pourbaix diagram:

$Fe\left(s\right)+2Cr\left(s\right)+4H_{2}O\to FeC{r}_2{O}_4\left(s\right)+8H^{+}\left(aq\right)+8e^{-};(24)$

$Fe{\left[HCOO\right]}_2\left(aq\right)+2Cr+4H_{2}O\leftrightarrow FeC{r}_2{O}_4\left(s\right)+2HCOOH+6H^{+}+6e^{-},(25)$

Besides, FeCO₃ becomes the thermodynamically stable product after abundant cations in the aqueous, arising via both thermodynamic reactions (Eqs 26–29) (Nesic et al., 1996a) and transformation from FeS and FeCr₂O₄ derived by pH change according to reactions (Eqs 30–33):

$Fe{\left[\text{HCOO}\right]}_2\left(aq\right)+C{O}_3^{2-}\leftrightarrow FeC{O}_3\left(s\right)+2HCO{O}^{-};(26)$

$Fe{\left[\text{HCOO}\right]}_2\left(aq\right)+HC{O}_3^{-}\leftrightarrow FeC{O}_3\left(s\right)+\text{HCOOH}+{\text{HCOO}}^{-};(27)$

$\mathit{Fe}+{\mathit{CO}}_3^{2-}\leftrightarrow {\mathit{FeCO}}_3+2e^{-},(28)$

$\mathit{Fe}+{\mathit{HCO}}_3^{-}\leftrightarrow {\mathit{FeCO}}_3+H^{+}+2e^{-},(29)$

$\mathit{FeS}+{\mathit{CO}}_3^{2-}\leftrightarrow {\mathit{FeCO}}_3+S^{2-},(30)$

$\mathit{FeS}+{\mathit{HCO}}_3^{-}\leftrightarrow {\mathit{FeCO}}_3+H{S}^{-},(31)$

${\mathit{FeCr}}_2{O}_4+{\mathit{CO}}_3^{2-}+2H_{2}O+2H^{+}\leftrightarrow {\mathit{FeCO}}_3+2\mathit{Cr}{\left(\mathit{OH}\right)}_3,(32)$

${\mathit{FeCr}}_2{O}_4+{\mathit{HCO}}_3^{-}+2H_{2}O+H^{+}\leftrightarrow {\mathit{FeCO}}_3+2\mathit{Cr}{\left(\mathit{OH}\right)}_3,(33)$

Although the reactions of corrosion products can be predicted by thermodynamics, it should be noted that the dominant reaction as the growth of corrosion products cannot be defined by thermodynamics and that is the limitation of using the Pourbaix diagram.

The Formation of the Corrosion Product Scales in Kinetics

The corrosion rate (C_R) consists of the reaction rate from corrosion layer accumulation (CLAR) and the dissolution in the aqueous phase (DR) (Sun and Nešic, 2008), which can be calculated by the weight of coupons with the corrosion product (m_a) according to Eq. 34 and Eq. 35 and is shown in Figure 12.

$CLAR=\frac{m_a-m_1}{M\cdot V},(34)$

$DR=\frac{m_0-\left(m_a-m_1\right)}{56\cdot V},(35)$

where M is the mol mass of the main corrosion product, and V is the volume of solution. To simplify the calculation, the S13Cr SS is assumed to be dissolved into iron ions (56 g/mol).

FIGURE 12

FIGURE 12. CLAR and DR of S13Cr SS under the 180°C formate condition with N₂/CO₂ saturated after 5, 48, and 120 h.

According to the observation in Figure 3, fewer outer FeS/FeCO₃ crystals accumulate on the surface at 5 h, which indicates that the CLAR mainly corresponded to the formation of the inner layer consisting of Fe₃O₄/FeCr₂O₄. It is interesting to note that the CLAR of both Fe₃O₄ and FeCr₂O₄ was 0.023 mol/hm², suggesting that more metal dissolution was transferred into Fe [HCOO]₂ in CO₂-saturated formates than the formation of HFeO₂⁻ under N₂ conditions.

Under the N₂ condition, the DR decreased to the level below 1E-4 mol/hm² at 48 h, indicating the protection of the inner layer as a barrier against active metal dissolutions. The reaction rate for the growth of the corrosion products decreased with exposure time and reached to the value of 2.7E-4 mol/hm² after 120 h.

Under the CO₂-saturated condition, the CLAR slightly increased after 48 h of immersion, which can be attributed to the formation of the outer layer consisting of FeCO₃/FeS on the surface. The reduction in DR at 48 h, however, is still higher than CLAR, indicating the continuous releasing of metal ions from the substrate. As the immersion time prolonged, forming the fully covered outer layer to further block the flux of aggressive ions, resulted in a lower level of DR combined with an increased CLAR at 120 h. However, the CLAR in CO₂-saturated conditions was two orders of magnitude higher than that in N₂ conditions at 120 h, consistent with the results in Figure 7B where it shows a thicker corrosion product scale. Besides, the DR was still three orders of magnitude higher than that in the N₂ condition at 120 h. These results suggest that the dissolution of metal into ions occurred under the protection of the double-layered corrosion product scale.

Although the multi-layered corrosion product scales covered and protected the surface of S13Cr SS under the CO₂-containing condition, high DR suggests the presence of defects in the corrosion products consisting of FeS, Cr(OH)₃, and FeCO₃. The previous literature has reported that the formation of the FeCO₃ and Cr(OH)₃ on S13Cr SS was controlled by the dissolution–precipitation process and resulted in relatively high defections (Guo et al., 2012; Li et al., 2019b; Zhao et al., 2019). However, the formation of the compact structures for spinel products such as Fe₃O₄ and FeCr₂O₄ was considered to provide better corrosion protection than that of the precipitation type (Yue et al., 2020a, 2020b, 2020c; Liu et al., 2021). Besides, the outer corrosion products with sheet-shaped clusters characterized as a looser structure which may cause pore connectivity within the outer layer. Wang et al. (Li et al., 2019a; Wang et al., 2021) illustrated that the localized corrosion at CO₂-containing environments was corresponding to the connected pores in the corrosion product scale. The fast kinetics resulted in the small pore sizes as the quicker formation of crystalline FeCO₃. Therefore, the formation of the porous outer layer with high CLAR can induce the localized corrosion for S13Cr under CO₂-saturated conditions. In contrast, the outer FeS scattered on the compact inner layer with low CLAR in the formate fluid without CO₂, showing general corrosion after removal of the corrosion product layer (Figure 8).

The evolution of the double-layered corrosion products in CO₂-saturated formate solutions is illustrated in Figure 13. As shown in Figure 13B, the formation of FeCr₂O₄ acts as the protective layer (in Figure 13A). The FeCr₂O₄ can be destabilized and gradually transferred into Cr(OH)₃. Figure 13C illustrates the precipitation of Cr(OH)₃ in the inner layer due to its extremely low solubility product constant (Xu et al., 2013; Liu et al., 2015). Consequently, a Cr-rich inner layer appears instead of a passive film on the surface when the CO₂-saturated formate fluid was at 180°C. A Cr-rich inner layer can decrease the corrosion rate at low pH; however, it should be noted that the corrosion scale displayed porous with high defects because FeCO₃/FeS grows irregularly and discontinuously, which allows anions to penetrate through the outer layer of corrosion product scales.

FIGURE 13

FIGURE 13. Schematic diagram of (A–D) the sequence of the formation of the outer and inner layers of the double-layered corrosion products and (E–F) the development of localized corrosion in CO₂-saturated formate solution at 180°C.

Note that there were no Cl⁻ ions present in the formate fluid, which indicates a relatively low localized corrosion risk. However, the hydrolysis of Cr³⁺ caused an acidified environment within the inner layer of corrosion products. When the pH dropped below 7.3 (Figure 11B), the oxide layer would dissolve. The hydrolytic action of dissolved Cr³⁺ would subsequently decrease the pH value and result in the development of localized corrosion, as shown in Figures 13D,E.

Figure 14 compares the measured corrosion rate in the formate fluid (tubing outer wall and casing) with those obtained from the literature both in formate fluid and formation water (tubing inner wall) (Leth-Olsen, 2004; Sunaba et al., 2014; Zhao et al., 2018; Qi et al., 2019; Yue et al., 2020b). It reveals that S13Cr shows a lower corrosion rate in the formate fluid than that in formation water. The high corrosion rates were only obtained in the CO₂ infusion formate fluid (showing the same corrosion rate with CO₂-saturated H₂O), which indicates that CO₂ is the governing factor in the ultra-deep downhole condition. It is noted that the highest corrosion rate occurred during the injection of lived acid, and the localized corrosion occurs at the defect of the corrosion product scale (Qi et al., 2019). The CO₂ would ingress into completion fluid once the localized corrosion induced tubing perforation, which would cause a dramatic increase in the corrosion rate for S13Cr in completion fluid.

FIGURE 14

FIGURE 14. Comparison of measured uniform corrosion rates of S13Cr with data obtained from the literature under ultra-deep downhole conditions (180°C).

Conclusion

The research has focused on clarifying the influence of sulfide impurity and CO₂ ingress on the corrosion of S13Cr SS as well as the evolution of the corrosion product scales under both CO₂-free and CO₂-saturated formate fluid conditions. From this work, the following conclusions can be drawn:

• Under N₂-deoxygenated formate fluid conditions, the general corrosion rate of S13Cr SS reached 1.177 mm/year after 5 h and reduced to 0.067 mm/year after 120 h of exposure, compared to high corrosion rates of 21.94 and 1.76 mm/year for samples exposed to the CO₂-saturated formate fluid condition after 5 and 120 h, respectively.

• A thin Fe₃O₄ layer was detected on S13Cr SS under N₂-deoxygenated formate fluid conditions. The presence of sulfide in formate brine can result in the formation of disordered tetragonal mackinawite MkB after 120 h of exposure. With the ingress of CO₂, the formation of the inner corrosion product layer on S13Cr SS was composed of FeCr₂O₄ after 5 h and then transformed to Cr(OH)₃ with time prolonged. The acidification on the surface results in the formation of a combination of disordered mackinawite of MkA and MkB, accompanied by the formation of a unique FeCO₃ morphology which presented as “sheet” crystalline after 48 h of exposure and developed into clusters after 120 h.

• For the system containing sulfide, the formation of FeS, Fe₃O₄, and HFeO₂⁻ became thermodynamically stable. The reaction rate of HFeO₂⁻ was higher in the first 5 h, whereas the accumulation for solid products as a barrier layer became favorable in kinetics after 48 h. Under CO₂-saturated conditions, the formation of Fe [HCOO]₂, FeS, FeCr₂O₄, Cr(OH)₃, and FeCO₃ became thermodynamically stable. Both the reaction rates for dissolution and corrosion layer accumulation are high from 5 to 120 h. The thicker corrosion product did not provide effective protection and induced localized corrosion after 48 h of exposure.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

Author Contributions

XY, YH, and LZ contributed to the conception and design of the study. LH, XY, and ZQ organized the database. XY and ZY performed the statistical analysis. XY wrote the first draft of the manuscript. All authors contributed to manuscript revision and read and approved the submitted version.

Funding

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51871027 and 52101069), the China Postdoctoral Science Foundation (No. 2021M690345), and the Postdoctoral Research Foundation of Shunde Graduate School of University of Science and Technology Beijing (Nos. 2020BH013 and 2020BH010).

Conflict of Interest

Author LH was employed by company State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection Research Institute Co., Ltd.,.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2021.802136/full#supplementary-material

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