Influence of temperature on corrosion behavior of N80 Steel in multiple thermal fluid environment

During offshore oil production, O2 could be introduced into the multiple thermal fluid, and the temperature of the wellbore can reach up to 200°C, which may result severe corrosion of tubing. However, the corrosion mechanism of downhole pipes in such aggressive environment remains unclear in the high-temperature and high-pressure CO2-O2-H2O mixed environment. Herein, we investigated the corrosion behaviour of N80 steel with different temperatures in CO2-O2-H2O environment. The influence of temperature on composition and structure of the corrosion product film of N80 steel was characterized by scanning electron microscope, energy-dispersive spectroscopy and X-ray diffraction. The results show that the corrosion rates of N80 steel performed a downward trend with the increase of the temperature, but it increased slightly at 180°C. The corrosion rate of N80 steel was up to 1.6 mm/y at 60°C, owing to the damage to corrosion product film by O2 and reduced its protection at lower temperature. As the temperature increased, the formation of Fe3O4 enhanced the protection of the inner corrosion product film, thereby greatly reducing the corrosion rate. However, the product of (FeCa)CO3 in the corrosion film completely lost its protection at 180°C, and the corrosion rate increased slightly to 0.84 mm/y. With the temperature increased to 240°C the corrosion rate of N80 steel reduced to 0.24 mm/y on account of the formation of dense and complete Fe3O4.


Introduction
Heavy oil resources are widely distributed around the world, and as conventional oil and gas resources deplete rapidly, the efficient development of heavy oil resources is one of the most effective ways to meet future energy demands [1] .The continuous development and successful implementation of multiple thermal fluid technologies have propelled the development of offshore heavy oil fields in China and achieved significant production enhancements, making a tremendous contribution to the efficient development of offshore oil fields [3] .However, the introduction of O2 into the multiphase thermal fluid system can affect the downhole tubular materials due to the high-temperature and high-pressure CO2-O2-H2O corrosion system, with temperatures reaching over 200 °C.It is generally believed that a protective corrosion product film cannot form on the surface of the base material below 80 °C, leading to severe corrosion.The corrosion rate of carbon steel in CO2 atmosphere increases gradually within the temperature range of 80 °C to 100 °C as a protective FeCO3 film begins to form.Above the temperature, the corrosion rate decreases with increasing temperature [5] .When the temperature exceeds 200°C, the primary corrosion product becomes Fe3O4, and in addition to FeCO3, providing stronger protection and significantly reducing the corrosion rate [7] .Currently, most studies on corrosion in CO2 and O2 coexistence environments focus on the influence of oxygen on the corrosion rate.Some researchers believe that the CO2 and O2 coexistence environment is more corrosive compared to the pure CO2 environment, and the simultaneous occurrence of hydrogen evolution and oxygen consuming corrosion can lead to rapid corrosion failure of oil well tubing within a short period [9] .Studies have shown that even extremely low concentrations of O2 (<20 ppb) can significantly increase the corrosion rate of carbon steel in CO2 environments [13] .Therefore, the reduction reaction of O2 in the CO2 and O2 coexistence environment is often considered the decisive factor affecting carbon steel corrosion [14] .Some studies suggest that in the presence of CO2 and O2, oxygen oxidizes Fe 2+ generated by the dissolution of Fe, making it difficult for a protective FeCO3 film to form [16] .However, most of these studies were conducted at temperatures below 100 °C, and as the temperature changes, the corrosion mechanism may undergo significant variations.In addition to affecting the solubility of CO2 and O2 in water, temperature also has an important influence on the thermodynamics and kinetics of corrosion [17] .Limited research on the influence of temperature on the corrosion behavior in CO2 and O2 coexistence environments indicates an overall decrease with an increase in reaction temperature [18] .This is accompanied by significant changes in the structure and composition of corrosion products [19] .This may be related to changes in the main controlling factors during the corrosion reaction, but no explicit corrosion process or mechanism has been provided.In this work, by simulating the corrosion environment of downhole oil tubing in a multiple thermal fluid extraction system, the influence of temperature on the corrosion behavior of oil well tubing in high-temperature and high-pressure CO2-O2-H2O environments was investigated.

Materials and experiment method
The experimental material used in this study was N80 steel oil tubing produced by BAOSTEEL Company.The chemical composition is shown in Table 1.The heat treatment process involved quenching at 860 °C followed by tempering at 610 °C.The microstructure of the tempered steel is martensite, as shown in Figure 1.The N80 oil tubing was processed into corrosion coupons with dimensions of 40 mm × 13 mm × 3 mm.The surface was sequentially polished using 360#, 800#, and 1000# sandpaper.Then, they were rinsed with deionized water and dried with anhydrous ethanol.Prior to the experiments, the initial weight of each sample was measured using an electronic balance with an accuracy of 0.1 mg.The sample dimensions were measured using a vernier caliper to calculate the corrosion area.Four parallel samples were used for each experimental condition, with three samples used to calculate the average corrosion rate and one sample reserved for corrosion product analysis.The experimental solution was prepared using analytical grade 99.9% pure chemical reagents based on the composition of the produced fluid from a certain heavy oil field, and the drug data for simulating the produced fluid in the oil field are shown in Table 2   The high-temperature and high-pressure corrosion simulation experiment was conducted in a 3 L hightemperature and high-pressure magnetically driven reaction vessel, as shown in Figure 2. At all experimental temperatures, the partial pressures of CO2 and O2 were 0.35 MPa and 0.03 MPa, respectively.Considering the different saturation vapor pressures of water at different temperatures, a certain amount of N2 was introduced to maintain the total system pressure at 5.86 MPa.The specific experimental parameters are shown in Table 3.Before the experiment, high-purity N2 was continuously introduced into the corrosion solution for at least 24 hours to remove oxygen from the solution.First, the fixture with the installed samples was fixed on the stirrer of the reaction vessel.Then, 1.5 L of deoxygenated solution was poured into the reaction vessel, and after sealing the vessel, N2 was again introduced for 2 hours to remove any residual air inside the vessel during the installation process.The temperature was then raised to the predetermined temperature, and the experimental gases were introduced into the reaction vessel until the set pressure reached.The speed of the high-pressure vessel was adjusted to achieve a line speed of 1 m/s for the corrosion coupons.
After the experiment the gases inside the vessel were discharged, and the samples were taken out once the vessel cools down to room temperature.After rinsing the samples with deionized water and then with anhydrous ethanol, dry the samples with compressed air and record the macroscopic appearance of the sample surfaces.The corrosion products were removed from the surfaces of three samples using an acid solution, and the samples were weighed accurately.The corrosion rates of the samples were calculated using the weight loss method, with the following formula: Where V is the corrosion rate, in mm/year; △W represents the weight loss of the sample before and after corrosion, in g; S is the contact area between the sample and the corrosive medium, in cm²; ρ represents the density of the sample, assumed to be 7.85 g/cm³; t represents the corrosion time, in hours.
The corrosion rate reported in this paper is the average value of the corrosion rates of three parallel samples.
Additionally, the surface microstructure of another sample's corrosion product film was observed using the JEOL JSM-7200F scanning electron microscope (SEM).The composition of the corrosion product was analyzed using the OXFORD X-Max50 energy-dispersive X-ray spectrometer (EDS).Partial samples were also taken to observe the cross-sectional morphology and analyze the distribution characteristics of the corrosion product film.The phase composition of the corrosion product film was analyzed using the X'pert PRO MPD X-ray diffractometer (XRD) under the following operating conditions: 40 kV, 150 mA, Cu target.To investigate the chemical properties of corrosion media and their impact on corrosion in hightemperature and high-pressure CO2-O2-H2O environments, the Stream Analyzer module of OLI Analyzer Studio software was used for thermodynamic calculations of water chemistry.As shown in Figure 1, the medium in the 3 L reactor vessel consists of 1.5 L simulated solution and 1.5 L mixed gas phase.The quantities of different gases in the reactor vessel at different temperatures were obtained using the Ideal Gas Law, while the composition of substances in the aqueous phase was determined from Table 2.The data were input into the simulation software to calculate parameters such as pH, dissolved oxygen (DO), and CO2 solubility in the water solution at different temperatures.

Average corrosion rate
Figure 3 shows the average corrosion rate of N80 steel under different temperature conditions in CO2-O2-H2O environment.It can be observed that the average corrosion rate of N80 steel gradually decreased with increasing temperature in the range of 60-150 ℃.From 150 ℃ to 180 ℃, there was a slight increase.Within the range of 180-240 ℃, the average corrosion rate of N80 steel shows a decreasing trend with increasing temperature.The average corrosion rate of carbon steel was closely related to the characteristics of the corrosion product film formed in this environment.The following analysis focuses on the characteristics of the corrosion product film formed on N80 steel under different temperature conditions.

Macroscopic morphology
Figure 4 shows the macroscopic morphology of the corrosion film on the surface of N80 steel after 168 hours of corrosion in a CO2-O2-H2O environment.It can be observed that the macroscopic characteristics of the corrosion products on the surface of N80 steel changed significantly with increasing temperature.At 60 ℃, yellowish corrosion products were locally deposited on the sample surface, while blue corrosion products could be observed in areas not covered by the yellowish products.At 90 ℃, the corrosion film on the surface appeared brick red.With further increase in temperature, the color of the corrosion film gradually changed from brick red to black.At 150 ℃ (Figure 4d), the corrosion products completely transformed into black, with protrusions at the edges of the corrosion film but without detachment.At 180 ℃ (Figure 4e), the surface of the specimen shows a transition from black to gray, and the outer layer of the corrosion film had poor adhesion and was prone to detachment.After detachment, the inner layer of gray-white corrosion products was exposed.At temperatures of 210 ℃ and 240 ℃ (Figures 4f-g), the surface of the specimen appeared gray-white.The surface layer of the corrosion products at 210 ℃ was loosely combined with the inner layer and exhibited localized detachment.However, the corrosion product film on the surface of the specimen at 240 ℃ was more compact, which may be the reason for the rapid decrease in corrosion rate at that temperature.The macroscopic characteristics of the corrosion products, changing with temperature, indicate possible variations in the composition of the corrosion products at different temperatures.

Corrosion product composition and micromorphology
Figure 5 shows the XRD patterns of the corrosion film on the surface of N80 steel after 168 hours of corrosion within the temperature range of 60-240 ℃.It can be observed that the main components of the corrosion film on the surface of N80 steel underwent significant changes with increasing temperature.At 60 ℃, the corrosion film mainly consists of (FeCa)CO3, FeOOH, FeO, and a small amount of Fe2O3.When the temperature increased to 90 ℃, Fe3O4 was detected in addition to the aforementioned products, especially the number of diffraction peaks of brick red Fe2O3 in the corrosion film significantly increased (Figure 5a), indicating an increase in its content, which is consistent with the color change observed in Figure 4b.When the temperature reached 120 ℃, the FeOOH diffraction peak disappeared, and the corrosion film was mainly composed of (FeCa)CO3, Fe3O4, Fe2O3, and a small amount of FeO.Compared to the situation at 90 ℃, the number of diffraction peaks of Fe2O3 and FeO decreased in the XRD pattern at 120 ℃, while the number of diffraction peaks of black Fe3O4 increased.The phenomenon observed in the macroscopic morphology was the gradual color transition of the corrosion film from brick red to black.When the temperature was not lower than 150 ℃, the surface of the corroded specimens appeared completely black, and the XRD pattern mainly detected the peaks of (FeCa)CO3 and Fe3O4, as shown in Figure 5b.Overall, in the lower temperature range (60-120 ℃) the composition of the corrosion film was more complex, consisting of various iron oxides and (FeCa)CO3, while in the temperature range of 150-240 ℃, the main components of the corrosion products are (FeCa)CO3 and Fe3O4.The surface and cross-sectional microstructures of the corrosion products on N80 steel after 168 hours of corrosion in a CO2-O2-H2O environment are shown in Figures 6, 7, and 8. From Figures 6 and 7, it can be observed that the corrosion products exhibited a bilayer film structure between 60 ℃ and 120 ℃.At 60 ℃, the outer layer of the spherical agglomerated structure (yellow product in Figure 4a) was very porous, while the inner layer exhibited relatively compact morphology (blue product in Figure 4a).The outer layer of the spherical agglomerated product (Figure 7a) is mainly composed of Fe and O elements, while the inner layer consists primarily of Fe, Ca, and O (Figure 7a).Combining the XRD results in Figure 4a, it can be inferred that the outer layer consists of iron oxides (FeOOH, FeO, Fe3O2), while the inner layer comprises (FeCa)CO3 crystals.At 90 ℃ and 120 ℃, the structure of the corrosion film on N80 steel was similar, with smaller particles of crystal corrosion products surrounded by spherical corrosion products (Figure 6b).The outer layer of the corrosion products consists of a mixture of (FeCa)CO3 and iron oxides (red and black alternating products in Figure 4b-c), while the inner layer is primarily composed of Fe and O elements (Figure 7b) as iron oxides.At 120 ℃, the particle size of (FeCa)CO3 corrosion products further decreased, and local aggregation phenomena occurred (Figure 6c).When the temperature raised to 150 ℃-210 ℃, the corrosion film on the surface of N80 steel remained a bilayer structure (Figures 7d-f).At 150 ℃, the outer layer of the corrosion film was denser (black product in Figure 4d), composed of fine-grained Fe3O4 corrosion products dispersed within the (FeCa)CO3 crystals (Figure 6d), while the inner layer is Fe3O4(Figure 7d).At 180 ℃, loose fine-grained Fe3O4 corrosion products form the outer layer of the corrosion film (Figure 7e), with (FeCa)CO3 scattered on the surface of the film, unable to effectively protect the internal steel substrate.This is macroscopically manifested as extensive peeling of the surface film in Figure 4e.The inner layer of the corrosion film at 180 ℃ consisted of relatively dense Fe3O4.At 210 ℃, the corroded surface was scattered with polyhedral corrosion product particles (Figure 6f), and the fine corrosion products formed a relatively dense inner layer.Figure 6f indicates that the N80 steel surface after corrosion at 210 ℃ exhibited a relatively thick outer layer of Fe3O4, with island-like (FeCa)CO3 distributed internally.After the temperature raised to 240 ℃, the crystal corrosion product particles were barely visible, and clear machining marks were observed on the sample surface (Figure 8a), with an extremely thin corrosion product film (Figure 8b).From the microstructural results of the corrosion products, it can be seen that

Corrosion mechanism
As the temperature increases, the corrosion rate and the composition and structure of the corrosion film on N80 steel undergo significant changes, indicating a change in the corrosion mechanism.The O2 solubility in the aqueous solution in the CO2-O2-H2O environment was calculated using the thermodynamic calculation tool OLI, and the results are shown in Figure 9.With increasing temperature, the O2 content in the solution shows a trend of initially decreasing and then increasing, similar to the results reported in references [20], [21] .When the temperature reached 210 ℃, the dissolved oxygen concentration in the solution increased to approximately twice that at 60 ℃, and at this point, the corrosion rate exhibits a decreasing trend.Further analysis is carried out to investigate the corrosion mechanism of N80 in the CO2-O2-H2O environment with respect to temperature.The corrosion rate of carbon steel in the CO2-O2-H2O environment gradually decreases with increasing temperature, which is slightly different from the case of carbon steel in the pure CO2-H2O environment, where the corrosion rate initially increases and then decreases [7], [22] .In the CO2-O2-H2O environment, N80 steel is subjected to the combined action of CO2 and O2, and compared to the presence of CO2 or O2 alone, the corrosion rate of the metal is usually higher under conditions where CO2 and O2 coexist.Some studies suggest that this is related to the interaction between CO2 and O2 [7], [19] .Regarding the characteristics of the corrosion product film, at 60 ℃, under the action of Ca 2+ , CO3 2-in the solution combines with Fe 2+ and Ca 2+ to form (FeCa)CO3, which preferentially deposits and forms a defective inner layer of the corrosion film [23] .Fe reacts with O2 (Reaction 2) to form Fe(OH)2 [18] .Additionally, at low temperatures, the solubility of O2 in the solution is high, and the initially formed Fe(OH)2 and (FeCa)CO3 corrosion film undergo oxidative destruction (such as Reaction 4), resulting in the formation of loose and porous Fe(OH)3, greatly reducing the protection of the film [24] .This is an important reason for the severe corrosion of N80 steel at 60 ℃.Fe(OH)3 can dehydrate to form Fe2O3, which is why no diffraction peak of Fe(OH)3 is observed in the XRD spectrum.Fe 2+ +Ca 2+ +CO 3 2-→M(FeCa)CO3 ( 1) Fe(OH)2→FeO+H2O (3) In the CO2-H2O environment, when the temperature rises above 80-100 ℃, the reason for the decrease in corrosion rate is that the protection of the FeCO3 film increases with increasing temperature [22] .However, in the corrosion system of CO2-O2-H2O, at 90 ℃ and 120 ℃, the oxygen transport rate is relatively high, allowing it to penetrate through the defects in (FeCa)CO3 and reach the interface between the substrate and the film, where it reacts with the Fe substrate (Reactions 5 and 6) to form Fe2O3 and Fe3O4 [24] .It also reacts with the corrosion product film to generate Fe(OH)3, which disrupts the integrity of the film.However, within this temperature range, the Fe3O4 formed by Reactions ( 6) and (7)  aggregates with other iron oxides to form an inner layer film with a certain degree of protection [25] .In addition, the decrease in the dissolved oxygen content in the solution at this temperature range results in a significant reduction in the corrosion rate at 90 ℃ (from 1.60 mm/y to 0.90 mm/y).Furthermore, at a temperature of 120 ℃, the Fe3O4 content in the corrosion product film increases, further enhancing the protective properties of the film, and the corrosion rate decreases to 0.64 mm/y.4Fe+3O2→2Fe2O3 Fe(OH)2+ 2Fe(OH)3→Fe3O4+4H2O When the temperature rises to 150 ℃, the dissolved oxygen content in the solution increases to approximately the same level as at 60 ℃.The oxidation action of O2 on the (FeCa)CO3 corrosion film becomes more severe.FeO cannot stably exist in a high-temperature environment and reacts with water (Reaction 8) to generate Fe3O4.The reaction between O2 and Fe is represented by Reaction (6) only.
The thickness of the corrosion film increases, so although the Fe3O4 content in the film continues to rise, the decrease in the corrosion rate is not significant.At an environmental temperature of 180 ℃, the dissolved oxygen content in the solution rises to 0.03 mol/L.At this point, the (FeCa)CO3 film has completely lost its protective nature and is scattered in an isolated manner on the surface of the film.Fe3O4, directly formed by reacting with oxygen, plays a primary role in protecting the substrate.The N80 steel surface, which has lost a layer of barrier, exhibits reduced resistance to corrosion, resulting in an increase in the corrosion rate.3FeO+H2O→Fe3O4+H2 In the CO2-H2O environment, when the temperature rises above 200 ℃, carbon steel can undergo a passivation behavior similar to stainless steel, where the Fe3O4 corrosion product film provides good protection to the substrate [26], [27] .At this point, the Fe3O4 in the corrosion product film not only comes from the oxidation reaction of oxygen with iron, but Fe can also directly generate Fe3O4 with water through Reaction (9) [7] .Although we cannot directly determine which reaction generates Fe3O4 with better protective effects, the continued decrease in the corrosion rate at 210 ℃ indicates that Fe3O4 generated by the reaction with water may be more protective.Furthermore, when the temperature is increased to 240 ℃, the protective effect of the Fe3O4 film generated by the reaction with water dominates for N80 steel.The corrosion film rapidly thins, and the corrosion rate of the specimen decreases to 0.24 mm/y.3Fe+4H2O→Fe3O4+4H2 4 Conclusion 1)Under the high-temperature and high-pressure CO2-O2-H2O corrosion environments, the uniform corrosion rate of N80 steel shows a periodic change with increasing temperature.The corrosion rate decreases continuously from 1.2 mm/y to 0.67 mm/y between 60 ℃ and 120 ℃.In the range of 150-210 ℃, it stabilizes within the range of 0.64-0.84mm/y with some fluctuations.However, when the temperature rises to 240 ℃, the uniform corrosion rate of N80 steel rapidly decreases to 0.24 mm/y.
2) In the CO2-O2-H2O corrosion environment, temperature significantly affects the corrosion mechanism of N80 steel.The oxidative degradation of the (FeCa)CO3 corrosion film by O2 is a major factor causing severe corrosion of N80 steel at low temperatures (60 ℃).When the temperature rises to 90 ℃, the increased oxygen transport rate in the solution allows it to enter the substrate-film interface and react with the substrate to form Fe3O4 film.Together with the outer (FeCa)CO3 product, it forms a corrosion product film with a certain degree of protection, leading to a rapid decrease in the corrosion rate.The protective nature of the film also increases with increasing temperature.However, when the temperature reaches 180 ℃, the protective nature of the (FeCa)CO3 corrosion product is completely lost, despite the continuous increase in oxygen content and oxygen transport rate in the solution.Fe3O4, directly formed by reacting with oxygen, plays a primary role in protecting the substrate, resulting in an increase in the corrosion rate.After the temperature rises above 200 ℃, the thin and dense Fe3O4 film exhibits better protective properties, leading to a further decrease in the corrosion rate.

Figure 2 .
Figure 2. Schematic diagram of the experimental.

Figure 9 .
Figure 9. OLI calculation results of O2 content in solution at different temperatures.

Table 2 .
Chemical composition of formation water extracted from oil field.