# High-Temperature Corrosion Behavior of Incoloy 800H Alloy in the Impure Helium Environment

Authors: Wei Zheng; Huang Zhang; Bin Du; Haoxiang Li; Huaqiang Yin; Xuedong He; Tao Ma
Journal: Science and Technology of Nuclear Installations
 (2022)

---

## Abstract

The helium coolant in the primary circuit of the high-temperature gas-cooled reactor (HTGR) contains traces of impurities, which can induce the corrosion of superalloys when exposed to elevated temperatures. The superalloy damage caused by the corrosion could threaten the safe operation of the reactor. In this work, the corrosion behavior of a representative superalloy (chromium-rich iron base alloy Incoloy 800H) was investigated under the impure helium at different typical temperatures of HTGR. An experimental setup developed for studying the high-temperature corrosion of superalloys was used to investigate the chemical reactions and corrosion behaviors of Incoloy 800H. It was found that CO2 is an important oxygen source in the reaction with chromium, and CO is released as the product. In addition, the observation and computation of the critical temperature (TC) of the reaction between CO2 and carbon in the alloy show that TC is much lower than that (TA) of the microclimate reaction, which indicates that CO2 can protect the scale from destruction. Furthermore, the slight decarbonization of the alloy was found above TC. Also, a model developed by the thermodynamic analysis was proposed to explain the mechanism of slight decarbonization and predict the critical temperature when the CO2-C reaction occurs. This work presents a guideline for protecting the oxide scale of superalloys used in HTGR.


---

## Body

## 1. Introduction

The high-temperature gas-cooled reactor (HTGR) is a Generation IV nuclear reactor featured with a high outlet coolant temperature that could reach 950°C. Since HTGR has a high operating temperature, it can be used for electricity generation, hydrogen production, and other industrial plants requiring high-temperature process heat. Hence, HTGR is economically competitive in the field of Generation IV nuclear energy technology [1–3]. For cooling HTGR and transferring its heat, helium is considered as the coolant since it is an inert gas that hardly reacts with materials [4]. However, the coolant helium could be polluted by impurities, such as H2, CO, CH4, H2O, and CO2 (these partial pressures are usually in the range of 0.1–100 Pa) [5], from different sources. Graham [6, 7] summarized these sources in HTGR as follows: air from fuel element charge/discharge operations, vapor and carbon oxides from the degassing of the graphite and the thermal insulation or from direct leakage, and hydrogen from proton diffusion through heat exchangers, boilers or reformers. On the other side, the content of impurities is controlled at trace levels by the purification plant. Hence, the helium with these trace impurities is called “HTR-He” [6].Although the contents of impurities are very low [5], they can interact with structural materials in the steam generator of HTGR at high temperatures. The interaction can gradually deteriorate the performance of structural materials. Therefore, Ni-based and Fe-based alloys, which have good creep performance and high strength as well as the formation of the continuous compact scale on the alloy surface for resisting deep corrosion, are selected as the candidate materials for intermediate heat exchanger (IHX) of the steam generator of HTGR [8, 9]. Hence, the study on the corrosion behavior of Ni-based and Fe-based alloys is significant for the safe operation of the steam generator. The main corrosion behaviors, including carburization, decarburization, and oxidation, can significantly influence the performance of structural materials [2–13]. Kurata et al. [12, 13] demonstrated that severe carburization may cause a reduction in the creep rupture ductility, and decarburization can lead to a significant loss of creep strength, even though Hastelloy X and the variants XR and XR-II (Ni-based alloys) perform good corrosion resistance in the atmosphere of impure helium at 950°C. The oxidation behavior of alloys can be divided into the formation of scale or internal oxide. Rouillard et al. [8] investigated the corrosion behavior of Haynes 230 alloy and proposed that the dense and adherent surface oxide scale can protect the alloy from further rapid corrosion. However, Tsai et al. [10] thought that internal oxidation might cause a reduction in the creep resistance of the alloy due to the tensile stress in the perpendicular direction. In addition, the corrosion behaviors of other superalloys, such as Inconel 617 and Nimonic 86, have been investigated over the past several decades [6, 7].For most Ni (Fe)-based alloys, a typical corrosion behavior is found that the oxide scale experiences a “destructive reaction” as the temperature increases [8, 9], which results in gradually losing the corrosion resistance of the oxide scale. Brenner and Graham termed this reaction as the “microclimate reaction” [14], which has been observed in experiments [14–16]. This reaction can be written as follows:(1)Cr2O3s+3Csol⟶3COg+2Crs.Previous studies [15] have suggested that the operating temperature should be below the reaction temperature (TA) to avoid the loss of the oxide scale. However, this method that controls the operating temperature might reduce the thermal efficiency and economy of HTGR. In order to avoid the loss of oxide scale while keeping a relatively high efficiency, trace CO2 can be introduced as the oxygen source instead of Cr2O3 in the reaction (1). Meanwhile, CO2 can be used as an oxidant to react with chromium to form an oxide scale on the alloy surface. By controlling CO2 at a proper level in the helium atmosphere, the alloy can be protected against deep corrosion by the continuous oxide layer, while the reactor can be maintained at a high temperature.The Incoloy 800H has been used as a structural material for the steam generator in the Pebble-bed Modular High-Temperature Gas-cooled Reactor (HTR-PM) in China [17]. To the best of our knowledge, although this alloy was also used and tested in the German HTGR program [7], studies on the gas-phase results of impurities and the corrosion behavior of Incoloy 800H are still limited [18]. Hence, we are interested in studying the typical corrosion behaviors that include the oxidation and carbon transfer of Incoloy 800H at high temperatures.Particularly, our work is concentrated on investigating the high-temperature corrosion of the Incoloy 800H in impure helium under the temperatures of 500°C, 800°C, and 950°C. The focus is made on the oxidation of CO2 and the slight decarbonization of the alloy above the critical temperature named TC (“C” stands for “critical”), which is the temperature when CO2 reacts with C and CO is released as the product. Furthermore, the relationship between TC and partial pressures of CO and CO2 can be obtained by thermodynamic calculation. And the prediction model of the CO2-C reaction is established. Finally, other three corrosion experiments with different CO contents are carried out to verify this model which shows a good agreement with the experimental data.


## 2. Materials and Methods

The schematic and actual pictures of the experimental setup are shown in Figures1(a) and 1(b), respectively. The setup consists of a configuring system of impure helium and a high-temperature corrosion system. The configuring system can mix various impurities into helium at a wide level from 0 Pa to 100 Pa. This configuring method could cover almost all the atmospheres of impure helium of HTGR, and it is more convenient and cheaper than the purchased standard gases. As shown in Figure 1, pure helium and impurities, such as hydrogen, carbon monoxide, carbon oxide, and methane, are injected into the high-pressure gasholder to be mixed, which is controlled by the mass flow controllers (MFCs). The impure helium prepared is monitored using a gas chromatograph (GC-6600, Fan Wei, China). The main part of the corrosion system is a high-temperature furnace (ZHONG HUAN ELECTRIC FURNACE, China). The holder and furnace tube are made of quartz to avoid device influence on the atmosphere of impure helium at elevated temperatures. In our experiment, a blank test without any samples was carried out to verify that the device did not react with impurities and that no significant gas/gas reactions occurred.Figure 1
(a) The schematic drawing of the system for high-temperature corrosion under impure helium and (b) the actual picture of the experimental facilities: (1) pure gas cylinder, (2) injection equipment for pure gas consisting of multiple mass flow controllers, (3) vacuum pump, (4) high-pressure gasholder, (5) mass flow controllers, (6) high-temperature furnace, (7) gas chromatography, and (8) water analyzer.
(a)(b)The material used for the corrosion study was a commercial-grade Incoloy 800H plate from WUXI JIAYUN (Jiangsu, China). The chemical composition of Incoloy 800H is shown in Table1 according to analysis certificates by the manufacturer. Also, no thermal treatment was carried out before the corrosion test. The specimens were cut into 20 mm × 8 mm × 1 mm coupons, polished by the abrasive papers, ultrasonically degreased in ethanol, and dried in air. In each corrosion test, two specimens were placed in separate holders and housed in a high-temperature furnace.Table 1
Composition of the Incoloy 800H alloy before the test (wt.%).
AlloyCCrFeNiMnAlSiCuTiIncoloy 800H0.08421.28Base30.960.970.2490.3200.1200.251Before the test, the circuits were evacuated to less than 10−3 Pa and flushed with pure helium until no impurities could be detected. After that, the impure helium was injected into the corrosion system. During the experiment, gas compositions were analyzed both at the inlet and the outlet of the furnace by a gas chromatograph. Table 2 reports the inlet impurity contents in the helium, which is the widely accepted level for a high-temperature corrosion test in HTGR [8–13]. The only difference between this experiment and previous studies is that CO2 was added to the helium. The flow rate of the impure helium was maintained to be 300 mL·min−1 [8] by the mass flow controller. Also, the reaction pressure was set to be the atmospheric pressure. The flow rate, about 0.6 mL·s−1·cm−2, is similar to previous corrosion tests [8, 9, 15, 16]. The samples were heated up at a rate of 2°C·min−1 to the preset temperatures of 500°C, 800°C, and 950°C (three separate tests and two samples for each one), and isothermally reacted with the impurities at the fixed temperature for 20 hours. After that, the samples were cooled down at a rate of 3°C·min−1 to room temperature.Table 2
Impurity contents in helium at atmospheric pressure (101325 Pa).
ImpuritiesH2CH4COCO2H2OPartial pressure (Pa)550.250.586.40.10The specimens were weighed with high-precision electronic balance (accuracy of 0.1 mg, XPR205, METTLER TOLEDO, Switzerland) before and after the corrosion test. Field emission scanning electron microscopy (FESEM, JEOL JSM-7001F, Japan) with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD, D/max-2550, Japan), and carbon sulfur analyzer (CS800, ELTRA, Germany) were used to observe the microstructures and analyze the composition of the specimens.


## 3. Results and Discussion

First, the gas phase of impurities is discussed in Section3.1, which shows the chemical reaction involved in the corrosion process. Second, the corrosion behaviors of Incoloy 800H are shown in Section 3.2 by the observation of SEM, EDS, and XRD. Third, based on the thermodynamic theory, a corrosion prediction model of the CO2-C reaction is established in Section 3.3 and verified by the experimental data. Finally, the CO2-C reaction is compared with the microclimate reaction in Section 3.4, indicating that the introduction of CO2 can protect the oxide scale due to the lower critical temperature.


### 3.1. Gas-Phase Analysis

Since the blank test had proved that there were no significant gas/gas reactions, the gas-phase analysis could indicate the occurrence of substance transfer between the atmosphere and alloys. The partial pressures of the main impurities are shown in Figure2. Since the variation of H2, CH4, and other impurities is very small, the gas-phase data of CO-CO2 would be mainly analyzed here. In addition, the gas phases at 500°C and 800°C are consistent with those at low and medium temperatures in Figure 2. Thus, the gas-phase process of heating up to 950°C is mainly discussed here.Figure 2
Evolution of the CO and CO2 partial pressure and temperature program as a function of time.As shown in Figure2, the variation of CO2 and CO is almost a mirror image, which indicates that they were involved in the same chemical process. The partial pressure of CO2 began to decrease at around 600°C, but the CO increased, illustrating that oxidation of the alloy may occur led by CO2 at this point [19]. It can be written as the following reaction:(2)xMs+yCO2g⟶MxOys+yCOg,with M = Al, Cr, Mn, and Si. Due to the high activity and content of chromium, it is often considered as the representative element which participates in the oxidation reaction. In addition, other mixed oxides could be formed, but chromium oxide is the main component.The gas-phase analysis in Figure2 shows that the consumption of CO2 was around 1.2 Pa and the production of CO was up to 1.5 Pa when the temperature reached 950°C. The phenomenon was inconsistent with the oxidation process, where the ratio of CO2 consumption to CO yield is 1 : 1, showing that there may be another reaction of CO2 and CO as expressed by the following reaction:(3)CO2g+Csol⟶2COg.The C (sol) is the carbon in solution from the alloy, indicating that this reaction leads to the decarburization behavior, which could also cause a potential threat to the alloy if the amount is large. According to the thermodynamic calculation, the Gibbs free energy (ΔG) of reaction (2) is much larger than that of reaction (3), so CO2 should be mainly involved in the oxidation process. It can also be predicted that the amount of decarbonization of the alloy is slight, which can be verified by the measurement of carbon content in the next section. In addition to the oxidation process, the temperature when CO increased may also be the one at which the reaction (3) occurred, which should be verified by the thermodynamic calculation.As compared to the reactants and products, reaction (3) is similar to the microclimate reaction (1) [20, 21], but the oxygen source is CO2 rather than the oxide scale. If this reaction can occur before the microclimate reaction, then the oxide scale can be protected from damage. Obviously, there should be a thermodynamic equilibrium temperature for both reaction (1) and reaction (3), called TA and TC, that determine the priority of these reactions. The mechanism and equilibrium temperature of the reaction will be discussed in later sections.


### 3.2. Corrosion Behaviors of Alloys

The samples were weighed before and after the test, and the mass gain per unit of surface area was obtained according to the following equation:(4)ρA=m2−m1A,where ρA (mg/cm2) is the mass gain per unit surface area, m1 (mg) and m2 (mg) are the weight of samples before and after the test, and A (cm2) is the superficial area. As can be seen from Figure 3, as the temperature increased, the mass gain kept growing, illustrating that there was a significant substance transfer from the atmosphere to the alloy, especially in the high-temperature region. According to the gas-phase analysis (Figure 2), the mass gain is mainly from the oxidation of CO2 in reaction (2), which means that there is no scale reduction and mass loss as in the “microclimate reaction” of Incoloy 800H mentioned by Quadakkers [22]. In addition, Figure 3 also shows the carbon content of alloys at different temperatures, and a slight decrease in carbon content could be observed when the temperature increased above 500°C, which is consistent with reaction (3). As predicted before, the amount of decarbonization is slight, which may be attributed to the blocking effect of the oxide scale.Figure 3
Mass gains and carbon content of the alloy 800H after the corrosion test.Figure4 shows the sample surface of Incoloy 800H exposed at 500°C, 800°C, and 950°C for 20 h, respectively. As the temperature increased, a continuous scale could be gradually observed to form, and the main component was oxide, which can be verified by the oxygen element distribution in EDS results (Figure 5) and the major phase in XRD results (Figure 6). As shown in Figure 4, when the temperature reached 950°C, the oxide layer was about 3 μm and the internal oxidation appeared in the range of 10 μm. This phenomenon indicates that the thinning of the oxide scale led by a “microclimatic reaction” has not occurred, and instead of the oxide scale, CO2 was involved in the carbon transfer reaction. The internal oxide was mainly composed of Al2O3, which was not continuous and was about 10 μm deep inside the alloy below the scale. It is consistent with the corrosion behavior of Haynes 230 tested by Rouillard [8], who thought that some Al oxidized along grain boundaries inside the alloy.Figure 4
FESEM picture at 20 keV with an SEI (secondary electron imaging) contrast; Incoloy 800H under impure helium at 500°C, 800°C, and 950°C.Figure 5
EDS analysis result of oxygen and chromium elements in Incoloy 800H under impure helium at 500°C, 800°C, and 950°C.Figure 6
Grazing XRD analysis of Incoloy 800H surface in impure helium at 500°C, 800°C, and 950°C.Meanwhile, the Cr element distribution in the EDS image is basically similar to that of oxygen, showing that the oxide scale is mainly composed of chromium oxide, which could be verified by XRD (Figure6). The main phases on the sample surface obtained by XRD analysis are listed in Figure 6, including the Fe-Ni matrix, chromium oxide, iron-manganese oxide, and iron-chromium oxide. At 500°C, there were almost no oxide phases on the surface, only the Fe-Ni elements, which is consistent with the results of SEM and EDS. With the increase in temperature, more oxide phases were generated on the alloy surface. The results of XRD proved that Incoloy 800H under impure helium at 950°C formed a mixed (Fe, Cr, Mn) oxide and Cr2O3 on the alloy surface, which is similar to the corrosion result of Alloy 230 [9]. Due to the content and activity of the elements, the Cr2O3 is the main oxide product on the alloy surface in reaction (2).


### 3.3. Reaction Mechanism and Model

According to the measurement results of the carbon content, it can be confirmed that there was slight decarburization of the Incoloy 800H in the impure helium. When the temperature increased, the carbon in the bulk alloy was oxidized by CO2 and it detached from the alloy, leading to the decarbonization phenomenon. It can be described by the following reaction:(5)CO2g+Csol⟶2COg.From the thermodynamic perspective, there is an equilibrium temperature (TC) in reaction (3). When T > TC, the reaction moves to the right side, namely, the decarbonization behavior would occur, so it can be named as “critical temperature.” Then, the equilibrium constant can be expressed as follows:(6)K1TC=PCO2aCPCO2,where K1 is the equilibrium constant of reaction (3), PCO and PCO2 are the partial pressure of CO and CO2, and aC means the carbon activity in the alloy. The equilibrium constant can be obtained by the thermodynamic calculation in Ref. [23, 24] as follows:(7)K1TC=exp−ΔrGR·TC,where ΔrG is the free enthalpy [23] of reaction (3) (kJ/mol) and R is the universal gas constant (J/mol·K). Considering the combination of (6) and (7), the equilibrium temperature can be written as follows:(8)TC=−ΔrGRlnPCO2/aCPCO2.Assuming that the carbon and carbides, or more specifically Cr3C2 or Cr23C6, are in an equilibrium within the substrate [17, 18], then the carbon activity can be obtained by the following reaction:(9)Cr23C6s⟶6Cs+23Crs.Therefore, the carbon activity in the alloy is determined by(10)aC=K2T1/6·aCr23C61/6aCr23/6,where K2 is the equilibrium constant of the reaction (9) and the other three items are the activities of carbon, chromium, and chromium carbide. The method to obtain the equilibrium constant is similar to K1 and the carbide activity (aCr23C6) is taken as the unity [9]. Then, the activity of chromium is the only unknown factor. The accurate data on chromium activity has been a difficult problem for a long time, and the measurement of vapor partial pressure of alloy is a common method to obtain the thermodynamic activity [25]. In this work, the aCr of Incoloy 800H can be obtained according to Ref. [26].The theoretical relationship betweenTC, PCO, and PCO2 can be obtained by combining equations (8) and (10). Then, when the partial pressures of CO and CO2 are measured, the critical temperature (TC) at which the reaction occurs can be calculated. Therefore, a model can be developed to predict the critical temperature (TC) at which the decarbonization behavior of the alloy occurs in the impure helium containing CO2 and CO.According to the thermodynamic analysis, the reaction mechanism and prediction model have been described in detail. Then, it should be verified by gas-phase data in the corrosion test. In this research, whenPCO = 0.58 Pa and PCO2 = 6.4 Pa, the temperature at which CO began to increase was about 600°C, as shown in Figure 3. In order to avoid the bias caused by gas-phase fluctuations, the experimental method for measuring TC is similar to the microclimate reaction [19, 20], namely, it was the temperature for which the increase in the CO partial pressure between the furnace inlet and outlet is equal to 0.1 Pa. Therefore, TC can be identified as about 620°C in this impure helium, and this value is calculated as 550°C by the prediction model.However, the release of CO from the reaction between CO2 and Cr may also occur around this temperature, which may lead to the deviation of the TC measurements. In order to strengthen the accuracy of the prediction model verification, the other three experiments were carried out, in which the CO partial pressures were set as 7.0 Pa, 22.5 Pa, and 51.2 Pa and constant PCO2. The gas-phase data during the heating process is shown in Figure 7, and the critical temperature TC can be obtained by the method of “ΔPCO = 0.1 Pa.”Figure 7
Evolution of the CO as a function of temperature during the heating process.As shown in Figure8, the function curve of alloy 800H obtained through the thermodynamic calculation can express the relationship between CO, CO2, and critical temperature TC. When the CO2 partial pressure is fixed, the higher the CO partial pressure is, the higher the TC is. The experimental results are in good agreement with the prediction model, and they can be used to predict the temperature at which the carbon transfer behavior occurs in a given impure helium atmosphere containing traces of CO and CO2. However, there are still some errors between the experimental data and the model, especially at low temperatures. It can be attributed to the following two possible reasons: (1) the critical temperatures at the test points are not exactly the equilibrium temperature of the reaction (3), which is an approximated value; (2) the activities of carbon and chrome are not experimental data, and the activity coefficients obtained by fitting may differ from the real values. However, this model still has important guiding significance for the experiment.Figure 8
TC = f (PCO) for Incoloy 800H-experimental data and theory.


### 3.4. Comparison with Microclimate Reaction

Compared with the microclimate reaction (1), the introduction of CO2 can protect the oxide scale from damage, which is mainly related to reactions (2) and (3).First, CO2 participated in the oxidation process in reaction (2) and CO was released, leading to an increase in the CO level in the atmosphere. And the equilibrium of reaction (1) is on the left-hand side, so the oxide scale could be inhibited from participating in reaction (1). Secondly, in reaction (3), CO2 directly replaced Cr2O3 to participate in the microclimate reaction, thereby avoiding the reduction of the oxide layer. However, the premise of the occurrence of reaction (3) is TC << TA. According to thermodynamic calculations, the critical temperature models for reaction (2) and reaction (3) are given in Figure 9, indicating that TC << TA at any CO partial pressure, thus verifying the rationality of CO2 instead of oxides in the reaction.Figure 9
f (TC) = P (CO) for Incoloy 800H-experimental data and theory.


## 3.1. Gas-Phase Analysis

Since the blank test had proved that there were no significant gas/gas reactions, the gas-phase analysis could indicate the occurrence of substance transfer between the atmosphere and alloys. The partial pressures of the main impurities are shown in Figure2. Since the variation of H2, CH4, and other impurities is very small, the gas-phase data of CO-CO2 would be mainly analyzed here. In addition, the gas phases at 500°C and 800°C are consistent with those at low and medium temperatures in Figure 2. Thus, the gas-phase process of heating up to 950°C is mainly discussed here.Figure 2
Evolution of the CO and CO2 partial pressure and temperature program as a function of time.As shown in Figure2, the variation of CO2 and CO is almost a mirror image, which indicates that they were involved in the same chemical process. The partial pressure of CO2 began to decrease at around 600°C, but the CO increased, illustrating that oxidation of the alloy may occur led by CO2 at this point [19]. It can be written as the following reaction:(2)xMs+yCO2g⟶MxOys+yCOg,with M = Al, Cr, Mn, and Si. Due to the high activity and content of chromium, it is often considered as the representative element which participates in the oxidation reaction. In addition, other mixed oxides could be formed, but chromium oxide is the main component.The gas-phase analysis in Figure2 shows that the consumption of CO2 was around 1.2 Pa and the production of CO was up to 1.5 Pa when the temperature reached 950°C. The phenomenon was inconsistent with the oxidation process, where the ratio of CO2 consumption to CO yield is 1 : 1, showing that there may be another reaction of CO2 and CO as expressed by the following reaction:(3)CO2g+Csol⟶2COg.The C (sol) is the carbon in solution from the alloy, indicating that this reaction leads to the decarburization behavior, which could also cause a potential threat to the alloy if the amount is large. According to the thermodynamic calculation, the Gibbs free energy (ΔG) of reaction (2) is much larger than that of reaction (3), so CO2 should be mainly involved in the oxidation process. It can also be predicted that the amount of decarbonization of the alloy is slight, which can be verified by the measurement of carbon content in the next section. In addition to the oxidation process, the temperature when CO increased may also be the one at which the reaction (3) occurred, which should be verified by the thermodynamic calculation.As compared to the reactants and products, reaction (3) is similar to the microclimate reaction (1) [20, 21], but the oxygen source is CO2 rather than the oxide scale. If this reaction can occur before the microclimate reaction, then the oxide scale can be protected from damage. Obviously, there should be a thermodynamic equilibrium temperature for both reaction (1) and reaction (3), called TA and TC, that determine the priority of these reactions. The mechanism and equilibrium temperature of the reaction will be discussed in later sections.


## 3.2. Corrosion Behaviors of Alloys

The samples were weighed before and after the test, and the mass gain per unit of surface area was obtained according to the following equation:(4)ρA=m2−m1A,where ρA (mg/cm2) is the mass gain per unit surface area, m1 (mg) and m2 (mg) are the weight of samples before and after the test, and A (cm2) is the superficial area. As can be seen from Figure 3, as the temperature increased, the mass gain kept growing, illustrating that there was a significant substance transfer from the atmosphere to the alloy, especially in the high-temperature region. According to the gas-phase analysis (Figure 2), the mass gain is mainly from the oxidation of CO2 in reaction (2), which means that there is no scale reduction and mass loss as in the “microclimate reaction” of Incoloy 800H mentioned by Quadakkers [22]. In addition, Figure 3 also shows the carbon content of alloys at different temperatures, and a slight decrease in carbon content could be observed when the temperature increased above 500°C, which is consistent with reaction (3). As predicted before, the amount of decarbonization is slight, which may be attributed to the blocking effect of the oxide scale.Figure 3
Mass gains and carbon content of the alloy 800H after the corrosion test.Figure4 shows the sample surface of Incoloy 800H exposed at 500°C, 800°C, and 950°C for 20 h, respectively. As the temperature increased, a continuous scale could be gradually observed to form, and the main component was oxide, which can be verified by the oxygen element distribution in EDS results (Figure 5) and the major phase in XRD results (Figure 6). As shown in Figure 4, when the temperature reached 950°C, the oxide layer was about 3 μm and the internal oxidation appeared in the range of 10 μm. This phenomenon indicates that the thinning of the oxide scale led by a “microclimatic reaction” has not occurred, and instead of the oxide scale, CO2 was involved in the carbon transfer reaction. The internal oxide was mainly composed of Al2O3, which was not continuous and was about 10 μm deep inside the alloy below the scale. It is consistent with the corrosion behavior of Haynes 230 tested by Rouillard [8], who thought that some Al oxidized along grain boundaries inside the alloy.Figure 4
FESEM picture at 20 keV with an SEI (secondary electron imaging) contrast; Incoloy 800H under impure helium at 500°C, 800°C, and 950°C.Figure 5
EDS analysis result of oxygen and chromium elements in Incoloy 800H under impure helium at 500°C, 800°C, and 950°C.Figure 6
Grazing XRD analysis of Incoloy 800H surface in impure helium at 500°C, 800°C, and 950°C.Meanwhile, the Cr element distribution in the EDS image is basically similar to that of oxygen, showing that the oxide scale is mainly composed of chromium oxide, which could be verified by XRD (Figure6). The main phases on the sample surface obtained by XRD analysis are listed in Figure 6, including the Fe-Ni matrix, chromium oxide, iron-manganese oxide, and iron-chromium oxide. At 500°C, there were almost no oxide phases on the surface, only the Fe-Ni elements, which is consistent with the results of SEM and EDS. With the increase in temperature, more oxide phases were generated on the alloy surface. The results of XRD proved that Incoloy 800H under impure helium at 950°C formed a mixed (Fe, Cr, Mn) oxide and Cr2O3 on the alloy surface, which is similar to the corrosion result of Alloy 230 [9]. Due to the content and activity of the elements, the Cr2O3 is the main oxide product on the alloy surface in reaction (2).


## 3.3. Reaction Mechanism and Model

According to the measurement results of the carbon content, it can be confirmed that there was slight decarburization of the Incoloy 800H in the impure helium. When the temperature increased, the carbon in the bulk alloy was oxidized by CO2 and it detached from the alloy, leading to the decarbonization phenomenon. It can be described by the following reaction:(5)CO2g+Csol⟶2COg.From the thermodynamic perspective, there is an equilibrium temperature (TC) in reaction (3). When T > TC, the reaction moves to the right side, namely, the decarbonization behavior would occur, so it can be named as “critical temperature.” Then, the equilibrium constant can be expressed as follows:(6)K1TC=PCO2aCPCO2,where K1 is the equilibrium constant of reaction (3), PCO and PCO2 are the partial pressure of CO and CO2, and aC means the carbon activity in the alloy. The equilibrium constant can be obtained by the thermodynamic calculation in Ref. [23, 24] as follows:(7)K1TC=exp−ΔrGR·TC,where ΔrG is the free enthalpy [23] of reaction (3) (kJ/mol) and R is the universal gas constant (J/mol·K). Considering the combination of (6) and (7), the equilibrium temperature can be written as follows:(8)TC=−ΔrGRlnPCO2/aCPCO2.Assuming that the carbon and carbides, or more specifically Cr3C2 or Cr23C6, are in an equilibrium within the substrate [17, 18], then the carbon activity can be obtained by the following reaction:(9)Cr23C6s⟶6Cs+23Crs.Therefore, the carbon activity in the alloy is determined by(10)aC=K2T1/6·aCr23C61/6aCr23/6,where K2 is the equilibrium constant of the reaction (9) and the other three items are the activities of carbon, chromium, and chromium carbide. The method to obtain the equilibrium constant is similar to K1 and the carbide activity (aCr23C6) is taken as the unity [9]. Then, the activity of chromium is the only unknown factor. The accurate data on chromium activity has been a difficult problem for a long time, and the measurement of vapor partial pressure of alloy is a common method to obtain the thermodynamic activity [25]. In this work, the aCr of Incoloy 800H can be obtained according to Ref. [26].The theoretical relationship betweenTC, PCO, and PCO2 can be obtained by combining equations (8) and (10). Then, when the partial pressures of CO and CO2 are measured, the critical temperature (TC) at which the reaction occurs can be calculated. Therefore, a model can be developed to predict the critical temperature (TC) at which the decarbonization behavior of the alloy occurs in the impure helium containing CO2 and CO.According to the thermodynamic analysis, the reaction mechanism and prediction model have been described in detail. Then, it should be verified by gas-phase data in the corrosion test. In this research, whenPCO = 0.58 Pa and PCO2 = 6.4 Pa, the temperature at which CO began to increase was about 600°C, as shown in Figure 3. In order to avoid the bias caused by gas-phase fluctuations, the experimental method for measuring TC is similar to the microclimate reaction [19, 20], namely, it was the temperature for which the increase in the CO partial pressure between the furnace inlet and outlet is equal to 0.1 Pa. Therefore, TC can be identified as about 620°C in this impure helium, and this value is calculated as 550°C by the prediction model.However, the release of CO from the reaction between CO2 and Cr may also occur around this temperature, which may lead to the deviation of the TC measurements. In order to strengthen the accuracy of the prediction model verification, the other three experiments were carried out, in which the CO partial pressures were set as 7.0 Pa, 22.5 Pa, and 51.2 Pa and constant PCO2. The gas-phase data during the heating process is shown in Figure 7, and the critical temperature TC can be obtained by the method of “ΔPCO = 0.1 Pa.”Figure 7
Evolution of the CO as a function of temperature during the heating process.As shown in Figure8, the function curve of alloy 800H obtained through the thermodynamic calculation can express the relationship between CO, CO2, and critical temperature TC. When the CO2 partial pressure is fixed, the higher the CO partial pressure is, the higher the TC is. The experimental results are in good agreement with the prediction model, and they can be used to predict the temperature at which the carbon transfer behavior occurs in a given impure helium atmosphere containing traces of CO and CO2. However, there are still some errors between the experimental data and the model, especially at low temperatures. It can be attributed to the following two possible reasons: (1) the critical temperatures at the test points are not exactly the equilibrium temperature of the reaction (3), which is an approximated value; (2) the activities of carbon and chrome are not experimental data, and the activity coefficients obtained by fitting may differ from the real values. However, this model still has important guiding significance for the experiment.Figure 8
TC = f (PCO) for Incoloy 800H-experimental data and theory.


## 3.4. Comparison with Microclimate Reaction

Compared with the microclimate reaction (1), the introduction of CO2 can protect the oxide scale from damage, which is mainly related to reactions (2) and (3).First, CO2 participated in the oxidation process in reaction (2) and CO was released, leading to an increase in the CO level in the atmosphere. And the equilibrium of reaction (1) is on the left-hand side, so the oxide scale could be inhibited from participating in reaction (1). Secondly, in reaction (3), CO2 directly replaced Cr2O3 to participate in the microclimate reaction, thereby avoiding the reduction of the oxide layer. However, the premise of the occurrence of reaction (3) is TC << TA. According to thermodynamic calculations, the critical temperature models for reaction (2) and reaction (3) are given in Figure 9, indicating that TC << TA at any CO partial pressure, thus verifying the rationality of CO2 instead of oxides in the reaction.Figure 9
f (TC) = P (CO) for Incoloy 800H-experimental data and theory.


## 4. Conclusions

The isothermal corrosion tests were carried out for Incoloy 800H, which was used in the steam generator in the HTR-PM, in impure helium at 500°C, 800°C, and 950°C. The corrosion behaviors were analyzed and discussed based on the SEM, EDS, XRD, and carbon sulfur analysis. The reaction mechanism was investigated by thermodynamic analysis and gas-phase data, and a model was developed to rationalize the variation of the critical temperature for carbon transferTC as a function of the CO and CO2 partial pressure in the gas phase, which can be used to predict the temperature at which the carbon transfer behavior occurs in a given atmosphere. More specifically, this study presents the following findings:(1)
Incoloy 800H was mainly oxidized by CO2, releasing a large amount of CO. And the oxide scale was mainly composed of mixed (Fe, Cr, and Mn) oxide and Cr2O3 on the alloy surface(2)
As the temperature rose, the thickness of the oxide layer gradually increased and more oxide phases was observed(3)
The carbon inside the alloy would react with CO2 to produce CO at high temperatures, which would lead to the phenomenon of slight decarbonization of the alloy(4)
TC is the critical temperature at which the decarbonization would occur. And when the CO2 partial pressure is constant, the higher the CO partial pressure is, the higher the TC is. Experimental values and the theoretical predictions for TC are in good agreement(5)
Compared to the “microclimate reaction,” it can be found that the oxide layer destruction can be overcome if trace CO2 is introduced as the oxygen source instead of Cr2O3 in the CO production reaction due to TC << TAIn general, due to the long-term service of Incoloy 800H in the reactor, it is particularly necessary to investigate the high-temperature corrosion of the alloy. The alloy resistance relies on the growth of a protective surface scale, and the reduction of the scale represents a major risk to the material integrity. Therefore, a method that traces CO2 introduced as the oxygen source in impure helium was proposed to protect the oxide scale from damage. This method can improve the corrosion resistance of alloys, which is very valuable for the long-term operation of HTGR.



---

*Source: 8098585-2022-12-16.xml*