Corrosion Behavior and Passive Film Composition of Alloy 825 in High Temperature and High H2S-CO2 Containing Environment

The effect of high temperature on corrosion behavior and passive film composition of Ni-based alloy 825 in H2S-containing environment was investigated by Confocal Laser Microscope (CLM), Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), and X-ray Photoelectron Spectroscopy (XPS). The experiment was carried out at 150 and 230°C in NaCl solution. The partial pressure of H2S was set to 1.2 MPa and CO2 was set to 3.2 MPa. The results showed that Ni-based alloy 825 presented good general corrosion resistance. Pitting corrosion was likely to occur at 230°C because of Cr depleted in the passive film. NiS appeared at high temperature and is damaging to protectiveness of passive film.


INTRODUCTION
With the increase in energy demand and development in oil field exploitation methods, ultra-deep oil and gas fields have gradually increased. During cooperation with colleagues in field, we found the environment in working condition is more severe than expected. As depth increases, downhole equipment is facing temperatures above 100°C. Because of the high H 2 S, high Cl − , and high CO 2 that exist in the downhole environment, materials usually used in production suffer from decreasing corrosion resistance. Carbon steel will face high corrosion risk in the harsh corrosive environment (Javidi and Bekhrad, 2018). Nickel-based alloys with better corrosion resistance are used in production equipment. There was plenty of research about the corrosion behavior at temperature lower than 200°C. H 2 S partial pressure was stabilized at several KPa. This didn't match to the high temperature and high H 2 S partial pressure in production. There are few reports on the passive film composition of alloy 825 under extreme environments. The relation between passive film composition and corrosion resistance still needs to be studied. Incoloy 825 is the most widely used Ni-based alloy in engineering. Its high content of Cr and Ni makes for great passivation performance and good corrosion resistance. Early research had discovered the passivation ability of Cr (Kirchheim et al., 1989) and it became a common element in various stainless steels. Mo is a typical additive to improve the stability of passive film and prevent occurrence of localized corrosion (Laszczyńska et al., 2017;Hendersona et al., 2018).
However, Ni-based alloys cannot be corrosion-free in extreme environments. There were many researchers in related fields. Elshawesh et al. (2015) had reported a failure case of a 825 joint in CO 2 /H 2 S environment at 60°C which was caused by pitting corrosion in long-term service. Banaś et al. (2007) presented S 2− ion could hinder the formation of oxide film and changed the corrosion mechanism of alloys. Ding et al. (2014) believed H 2 S accelerate the anodic and cathodic reactions of stainless steel. H 2 S could reduce the corrosion resistance of materials by changing the electrochemical properties of passive film. Alexander et al. (2018) compared the difference of corrosion mechanism between duplex stainless steel and nickel-based alloy in H 2 S environment. Both materials showed the susceptibility of selective corrosion. Dong et al. (2011) revealed the increase of austenite phase could increase the corrosion resistance of crevice corrosion. Cheng et al. (2000) proved the presence of nickel sulfide and H 2 S accelerated the corrosion rate of Alloy 825 by using an electrochemistry method. Temperature can influence the corrosion resistance of materials. Zhao et al. (2011) tested Ni-based alloy using a solution containing H 2 S/CO 2 and found the increase in temperature resulted in a higher corrosion rate while presence of elemental sulfur led to localized corrosion. Elizabeth et al. (2014) used optical measurement to observe corrosion behavior 825 in 3 M NaCl with CO 2 at 250°C. However, the downhole environment is much more severe. The Cl − concentration is nearly saturated under severe working conditions and the H 2 S and CO 2 could reach MPa level. Further research on Ni-based alloy corrosion behavior under severe conditions is still needed.
In this paper, the corrosion resistance of the passive film of Nibased alloys has been further studied by using Confocal Laser Microscope (CLM), Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS). The pitting corrosion behavior of Alloy 825 by changing the temperature in a high Cl − high H 2 S/CO 2 environment is discussed.

Materials and Solutions
All test specimens were cut from an 825 Ni-based Alloy plate. The chemical composition of 825 is listed in Table 1.
The corrosion test specimens were cut to 40 mm × 20 mm × 3 mm. The specimens for surface characterization were cut to 10 mm × 10 mm × 3 mm in size. All samples were cleaned with acetone, alcohol, and deionized water after machining. Corrosion test specimens were polished to 600# and surface characterization specimens were successively polished to 2000# to make it suitable for SEM and XPS test. Specimens were cleaned by deionized water and alcohol, then dried by cool air, and then weighed.
For a more precise simulation of working condition, 350 g L −1 NaCl was applied in an immersion test. The test solution containing approximately 350 g L −1 NaCl were purged with N 2 for 2 h. Before the test, solution was saturated with CO 2 and then H 2 S was introduced and stabilized at 1.2 MPa. The partial pressure of CO 2 is controlled to 3.2 MPa which is similar to working condition. As the gas pressure is fully stabilized, the autoclaves were heated to 150 and 230°C, respectively. The test duration was set to be 720 h. The total pressure of two conditions is about 5 MPa after heating.

Weight Loss
After experiment, specimens were taken out of the autoclave. Deionized water and acetone were used to rinse. The specimens were blow-dried by cool wind. The corrosion product was removed by 15% HCl and then weighed. The corrosion rate was calculated by the following equation: where CR is the corrosion rate (mm/a), W is for total weight loss (g), A is for sample surface area (cm 3 ), t is for experiment time (s), D is for density which is 8.14 g/cm 3 for Alloy 825.

Surface Morphology
CLM and SEM were used to observe the corrosion product on the specimen. After the immersion test, the specimens were rinsed with deionized water and dried by cool air. The surface of the specimens was analyzed using CLM to obtain the dimensional information of pits. A Quanta250 environmental SEM equipped with an EDS was used to analyzed the surface morphology and corrosion products. The depth of corrosion pits was acquired and pitting corrosion rate was calculated by the following equation: where CR P is the pitting corrosion rate (mm/a), D is for depth of the deepest pits (mm), and t is for experiment time (h).

Surface Characterization
The composition of the passive films formed at various temperatures was analyzed by XPS with a PHI5000 VersaProbe III [monochromatic Al Kα (hυ 1,486.6 eV), analysis angle 45°]. The shift change was adjusted by C 1s at 284.8 eV. XPSPEAK 41 was used to analyzed the XPS result.

RESULT Corrosion Rate
We used Eq. 1 to calculate the general corrosion rate. The corrosion rate is 0.0007 ± 0.0001 mm/a for 150°C and 0.0010 ± 0.0001 mm/a for 230°C. The Alloy suffered from higher general corrosion rate at 230°C than 150°C. However, it should be pointed out that corrosion rates were extremely low, indicating Alloy 825 showed good general corrosion resistance under the testing conditions. Figure 1 shows the morphology of Alloy 825 samples after the immersion tests at different temperatures. It was observed that samples lost metallic luster after test. Figure 2 shows the micro-morphology of the 825 sample after the immersion tests at different temperatures. There is no obvious pit on the surface of the sample tested at 150°C. A large number of pits were found at 230°C. Several researches (Banaś et al., 2007;Tomio et al., 2015) indicated that in H 2 S-containing environments the Ni-Cr-Mo alloy would form oxides of Cr and Mo and black sulfides such as FeS, NiS, and MoS 2 on the surface. The composition of passive film is different from that formed in H 2 S-free environment (Wang et al., 2017;Wang et al., 2018;Wang et al., 2020). Figure 3 shows micro-morphology of the 825 sample after removal of the corrosion products. The corrosion product was removed with 15% HCl. The samples tested at 150°C showed no pit on the surface, and no height difference caused by corrosion was observed on the entire surface. A large number of pits could be clearly found on the surface of the specimen and were evenly distributed on the surface at 230°C. The depth of pits ranged from 7 to 10 μm. The pitting corrosion rate was 0.12 mm/a according to Eq. 2. The rising temperature greatly increased localized corrosion susceptibility of Alloy 825 in a sulfur-containing environment. Figure 4 shows the SEM images of Alloy 825 sample after the immersion tests. A layer of film could be observed on the surface of the sample tested in 3.2 MPa CO 2 , 1.2 MPa H 2 S at 150°C. The film was intact and had no obvious corrosion product accumulation. The  material showed good corrosion resistance in this environment. The particles corrosion product appeared at 230°C, indicating the corrosion tendency at 230°C is higher than that at 150°C. Scratches still could be seen over the surface, which suggested the material had strong resistance to general corrosion. This is consistent with the results in Corrosion Rate. EDS was used to analyze the surface corrosion product film. The analysis found corrosion product film contains a small amount of sulfur element and the sulfur content in the particle corrosion product is much greater than corrosion product film.

XPS Result
For corrosion product deposited on the corrosion product film and the corrosion product film, XPS is used for composition analysis. All samples were sputtered for 30 s to remove the outermost product film. The sampling depth of XPS is 0.5-2 nm for metals and about 1-3 nm for inorganic products. Figure 5 shows the result of XPS results at 150°C. It mainly contained Cr, Fe, Ni, Mo, and O and S. The Cr shows obvious split peaks. The peaks were corresponding to Cr 2 O 3 (576.8 eV) and CrO 3 (578.5 eV). Cr oxides were still formed in high-temperature H 2 S-containing environment which was the main reason for Alloy 825 to maintain good corrosion performance under severe conditions. Fe spectrum were mainly composed of Fe 2 O 3 (710.9 eV) and FeS (713.8 eV). It can be determined the corrosion products are Fe 2+ /Fe 3+ oxides and sulfides in the passive film. The content of Mo in the passive film is the least of the four elements. Its spectrum was split into three peaks of Mo, MoS 2 , and MoO 3 . The Ni spectrum mainly consisted of Ni(OH) 2 (861.2 eV) and satellites. O spectrum was divided into O 2− (530.2 eV) and OH − (531.6 eV). The peak of S was mainly S 2− (530.2 eV). Figure 6 shows the surface XPS results at 230°C, 1.2 MPa H 2 S, 3.2 MPa CO 2 . The Cr and Fe spectra could be separated to similar  Ni formed NiS and Ni(OH) 2 at 230°C. The appearance of sulfides in the passive film lead to a decrease in general corrosion resistance and induced pitting corrosion. The O spectrum was composed by O 2− and OH − which is the same as the result at 150°C. The S spectrum could be separated to different peaks of S 2− and S 2 2− . Figure 7 shows the relative proportions of the elements in the passive film under the two conditions. As the temperature increased, the content of Cr in the passive film reduced. At 150°C, the relative proportion of Cr was above 60%, which reduced to 20% after the temperature was increased to 230°C. At the same time, the relative proportions of Fe and Ni increased. At 150°C, the relative ratios of Fe and Ni in passive film were lower than that of Cr which was less than 15%. However, when the temperature raised to 230°C, the relative ratio was increased to about 30% for Fe and 40% for Ni. Mo kept about 10% in both conditions.

DISCUSSION
The increase in temperature influenced the corrosion behavior of nickel-based alloys in H 2 S-containing environments. It increased the corrosion rate and risk of pitting corrosion by affecting the composition of the passive film. From 150°C to 230°C, changes occurred in the composition of the surface passive film of nickelbased Alloy 825. Under sulfur-containing conditions, S appeared in the passive film and sulfide had detrimental effect to isolated corrosive media.
At 150°C, the content of Cr in the passive film was higher than that of Alloy 825 which showing enrichment in passive film. When temperature increased, the Cr in the passive film greatly reduced. This is consistent with other studies at lower temperatures which pointed out an increasing loss of Cr 2 O 3 as temperature increases (Zhang and Shoesmith, 2013).
The oxides and sulfides of Fe were formed in both conditions. The content of Fe in the passive film increases with temperature. Low corrosion resistance of Fe/FeS results in an increase in the dissolution rate at 230°C.
At lower temperatures, Ni only formed the Ni(OH) 2 , while NiS was produced at high temperatures. Monnot (Monnot et al., 2017) found that appearance of sulfide will not only reduce the density of the film but also cause changes in the electrochemical property of passive film. The original ionselective oxide film converted to the sulfide film which was lacking in ion-selective. This promoted the permeation of corrosive ions such as Cl − , S 2− , and CO 3 2 to the base metal, increasing the pitting susceptibility. Therefore, pitting corrosion occurred in 825 at 230°C.
Ni(OH) 2 has good corrosion resistance. At the same time, the relative content of Ni in the passive film was significantly increased, which is close to the proportion of Ni in Alloy 825. Because of the formation of sulfides, the composition of the passive film changes and the corrosion resistance is reduced. The cathodic reaction of H 2 S corrosion is a series of depolarization processes of H 2 S, HS − , and H + . It lowers the pH of the environment as in the following: The dissolution process of Ni at lower temperature is as following (Cheng et al., 2000): The hydrolysis reactions of Ni 2+ lead to a formation of Ni(OH) 2 at 150°C. At 230°C, Ni forms NiS in the sulfur-containing environment. NiS was a common corrosion product of nickel-based alloys after localized corrosion . The NiS is formed as follows (Banaś et al., 2007;Sun et al., 2019;Li et al., 2021): NiS could not form a protective layer at low pH (Davoodi et al., 2011) which caused continuous corrosion. The presence of NiS may be one of the reasons for the decreased protection of passivation film at high temperature. The content of Mo in the Alloy 825 is relatively low. But it plays a critical role in improving performance of the passive film. The sulfide and oxide of Mo have a good insulation of corrosion environment. Previous studies have shown that oxides of Mo have good corrosion resistance (Tomio et al., 2015;Henderson et al., 2019). Its protective performance comes from the ability to form passive film with cation selectivity. But when the temperature reaches 230°C, the following reaction may happen (Li et al., 2020a): The reaction (9) is nearly impossible at room temperature and atmospheric pressure (Natishan et al., 1999). But MoS 2 might formed in a sulfur-containing environment at 230°C. Figure 8 shows the possible mechanism of Alloy 825 in the experimental environment. The corrosion behavior of passive film at lower temperature has been studied extensively (Zhang and Shoesmith, 2013;Henderson et al., 2019;JianqiaoYang et al., 2020;Li et al., 2020b;Nimmervoll et al., 2021). As Figure 8A shows, Mo is enriched in the outer layer and Ni/Cr in the metal border. This is the result of dissolution of passive film and the deposition of Mo oxide at low pH. NiS does not form and amount of FeS is limited. Dissolution/formation of passive film is relatively slow. Therefore, a complete oxide passive film can be formed to keep good corrosion resistance. With the increase of temperature, the dissolution/formation process of the passive film is accelerated. Because of the higher diffusion ability of Ni than Cr, it takes priority to combine with S 2− , so NiS corrosion products are formed on the metal surface. MoO 3 is converted to MoS 2 with less corrosion resistance. The content of Cr is decreased in passive film. The surface was adhered by a loose layer of NiS/FeS and Mo sulfide without precipitating a protective film on the metal. When corrosive solution reaches substrate, pitting corrosion happens.
In summary, the increase in temperature will cause the corrosion of Alloy 825 to accelerate in high H 2 S environment. At 150°C, Cr enriched passive film could still be formed on the surface of Alloy 825. There was only a small amount of FeS in the passive film, which had little effect on the corrosion resistance. The main composition of passive film was still Cr and Ni oxides. Oxides of Mo also provided great corrosion resistance. After the temperature rose to 230°C, the relative proportion of Cr in the passive film reduced while the proportion of Ni and Fe increased. The appearance of sulfide reduced the protectiveness of the passive film. Therefore, the surface of the sample tested at 150°C has only a thin film of corrosion products. No partial corrosion products were attached to the film and no obvious localized corrosion phenomenon was on the surface. On the surface of the specimen at 230°C, the performance of the passive film deteriorated and the corrosion rate increased. A thicker corrosion product layer appeared and partial corrosion product was attached to the surface. With defects in the passive film, pitting corrosion occurred.

CONCLUSION
In the present work, the corrosion behavior of Alloy 825 in high temperature with CO 2 /H 2 S and composition of passive film were studied. Weight loss, CLM, SEM, EDS, and XPS were used to analyze the specimen.
The conclusion is listed as follows: 1) The general corrosion of Alloy 825 is relatively slow in environment with 3.2 MPa CO 2 , 1.2 MPa H 2 S. Corrosion rate is 0.0007 ± 0.0001 mm/a for 150°C and 0.0010 ± 0.0001 mm/a for 230°C. 2) No pitting corrosion occurred at 150°C. When temperature raised to 230°C, pitting corrosion was likely to occur on the Alloy 825 surface.
3) The passive film is mainly composed of Cr 2 O 3 , CrO 3 , Fe 2 O 3 , FeS, Ni(OH) 2 , Mo, MoS at 150°C. At the higher temperature of 230°C, NiS is present in the passive film. 4) Cr suffered from a rapid decline in proportion while Fe and Ni were more enriched in film when temperature raised from 150°C to 230°C. NiS and MoS 2 also occur in film which caused the losses of corrosion resistance. Therefore, Alloy 825 faced risk of pitting corrosion in high temperature.

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 author.