Hiroshi YAKUWA*
Manabu NOGUCHI*
*
Technologies, R&D Division
This section focuses on carburization, nitridation, and steam oxidation, which are found in equipment relating to environments and/or energy, and relevant corrosion protection. Carburization, which is found in environments containing hydrocarbons, degrades the corrosion resistance of alloys by combining carbides with the chromium in the alloys. Nitridation, which occurs in environments containing ammonia, causes materials to become embrittle resulting in impaired mechanical characteristics. This problem is more serious than thickness reduction due to corrosion. In the case of steam oxidation, the oxidation rate in the presence of hydrogen is much higher than that in the air. Selection and design of alloys that form protective films on alloy surfaces is important for the reduction of corrosion in the above cases.
Keywords: High temperature corrosion, Carburization, Metal dusting, Steam oxidation, Nitridation, Ammonia, Reducing atmosphere, Protective film, Hydrocarbon, Water vapor
The materials of equipment relating to environments and/or energy are exposed to various environmental conditions (temperature range, gas composition, etc.) depending on the process. Carburization or metal dusting may occur in a hydrocarbon-based gas, which is a main component of fossil fuel, or a gas containing CO or CO2. Nitridation may occur in an environment containing ammonia, or at high temperatures exceeding 1000 ℃ by nitrogen. Unlike oxidation and sulfidation, in carburization and nitridation, when corrosion damage occurs, an altered layer is often formed inside the alloy that impairs oxidation resistance or that degrades the mechanical properties of the materials, but corrosion scales are not dominantly formed on the surface.
In the first to fifth parts of this lecture, we have described in detail the basics of high temperature corrosion and corrosion protection, chloride corrosion in facilities relating to waste treatment, and sulfidation corrosion in equipment relating to oil refineries. In this issue, we outline carburization, metal dusting, nitridation, and steam oxidation as other types of high temperature corrosion that particularly occur in equipment relating to environments and/or energy.
Carburization arises from the differences in carbon activity between the gas phase and in the alloy, which means the condition of a
c (gas) >a
c (alloy) …………(6-1).
Carbon activity in gas is determined by the equilibrium constant of equations (6-2) to (6-4), and the values corresponding to each temperature are shown in Figure 6-11). When the temperature is lower than about 600 ℃, carburizing ability of the gas containing CO is high; when the temperature is higher than about 600 ℃, carburizing ability of the gas containing CH4 (hydrocarbon) is high. For example, in the case of carbon steel, the carbon activity in the alloy can be calculated from the equilibrium constant of formula (6-5).
H2 + CO = C + H2O………………………………… (6-2)
2CO = C + CO2……………………………………… (6-3)
CH4 = C + 2H2 ……………………………………… (6-4)
3Fe + C = Fe3C …………………………………… (6-5)
However, in addition to Fe and C, various elements are added to practical alloys. The added elements change the carbon activity, ac in the alloy. Si and Ni increase carbon activity in the steel; V, Cr and Mo decrease carbon activity2). Thus, steels with a large content of Si or Ni are hardly carburized, while those with a large content of V, Cr, and/or Mo are easily carburized.
Metal dusting is a kind of carburization and occurs on an alloy surface where graphite is not assumed to precipitate, which means a high carbon activity environment where the calculated carbon activity exceeds 1. This phenomenon causes sevior thickness reduction accompanied with graphite precipitation and powdered metal. It is said that the following mechanism causes metal dusting in carbon steels and low alloy steels3), 4).If the carbon activity of the atmosphere is high, C is dissolved in the material, precipitating cementite. Then, graphite is precipitated, whereby cementite decomposes into Fe and C. In contrast, in austenitic stainless steel and Ni-base corrosion-resistant alloys, pitting-like thickness reduction is often observed. In these alloys, carbides (M23C6, M7C3) are formed by C entering through defects of the protective oxide film on the alloy surface. Furthermore, the alloy substrate is in a supersaturated state, precipitating graphite directly5)-7). Metal dusting is observed in hydrogen production and synthesis gas production processes, and is often seen in a temperature range of 400 to 800 ℃ especially in environments containing CO gas5)-7).
Fig. 6-1 Equilibrium constants of reactions related to carbon activity<sup>1）</sup>
Figure 6-28) shows examples of a damage caused by carburization that occurred in a fuel mixing tube (Co-28%Cr-20% Fe alloy (mass%)), which introduced fuel mixed with air into the combustor. These are crosssections of the fuel mixing tube that operated for about 2400 hours with a combustor. Inside the fuel mixing tube, there was a reducing atmosphere rich in propane gas fuel, while outside the tube, there was an oxidizing atmosphere because of the combustion gas. Although the temperatures of the inner and outer wall surfaces of the tube were not measured, it was presumed that the temperatures inside and outside the tube were less than 1000 ℃ and around 1000 ℃ , respectively. This tube has a thickness of 3 mm or more at the thickest point. Assuming that the entire 3 mm thick tube was corroded in 2400 hours when corrosion damaged the entire tube in the thickness direction, the corrosion rate was 10 mm/y or more. Since this value is significantly larger than the thermal cycle oxidation rate of Co-27%Cr-18% Fe alloy9) in the air at 1093 ℃, it is unlikely that the tube was damaged by simple oxidation. As shown in Figure 6-2 (a)8), in the vicinity of the inner wall of the tube, a large amount of Cr-rich carbide precipitated inside the alloy, and the Cr concentration of the alloy substrate decreased to about 10 mass%. In contrast, at several hundred μm inside from the outer wall surface (Figure 6-2 (b)) 8), internal oxidation progressed surrounding grains along the interface between the carbide and the alloy substrate.This indicates that oxidation has spread to the inside of the alloy. In addition, in the vicinity of the outer wall of the tube (Figure 6-2 (c))8), thick oxide grew. Based on the above observation, the damage mechanism of the fuel mixing tube was inferred as shown in Figure 6-3. Since the inner wall side of the tube has a fuel-rich reducing gas atmosphere, carburization occurred due to C dissociated from the fuel. Carburization proceeded to the inside of the alloy substrate and then reached the outer surface of the wall. In contrast, since there was an oxidizing combustion gas atmosphere on the outer wall side, high temperature oxidation proceeded. At that time, the oxidation resistance of the alloy substrate where carbide precipitated was lowered due to the reduced Cr concentration. In addition to this, internal oxidation progressed deeper inside the alloy along the interface between the carbide and the alloy substrate. These were likely to cause sevior corrosion damage to the tube.
As described above, the sevior thickness reduction of the mixing tube was likely to be caused, not by high temperature oxidation of the alloy substrate with a sound composition, but by a reduction in the effective Cr concentration by the formation of carbide due to carburization, in addition to the preferential diffusion pass for the corrosive gas provided by carbide. Therefore, in order to improve the corrosion resistance in this environment, we determined that it is necessary to improve the carburization resistance of the material. In order to improve the carburization resistance of the material, the following are considered effective: forming a highly protective oxide on the surface of the alloy as a continuous layer to prevent carbon from reaching the alloy surface, and selecting an alloy whose diffusion rate of carbon is low. Highly protective oxides at high temperatures are Cr2O3 and Al2O3. Since Al2O3 has an oxide growth rate coefficient that is smaller than that of Cr2O3 by about two digits10), the oxide is considered a more effective protective oxide film. However, it is necessary to use peeling preventive measures together because the oxide scale is easily peeled off. In contrast, since Cr2O3 shows good corrosion resistance in a wide temperature range and is widely used as a heatresistant material for most general industrial devices, it is considered preferable to use this oxide as a protective oxide film.
In order to form a continuous film of Cr2O3 on the alloy surface continuously, the use of an alloy with low concentration of Cr, which is required for forming an external oxide film of Cr2O3, is advantageous. Compared to Co-Cr alloys, Ni-Cr alloys have lower concentration of Cr required for forming an external oxide film of Cr2O3 11), and are advantageous for forming a continuous film of Cr2O3 continuously. In Fe-Ni-Cr alloys, the carburization resistance is improved as the Ni content increases12). Thus, Ni-Cr alloys are considered to be advantageous for reducing carburization. All things considered, it is possible to extend the tube life by changing the tube material from a Co-Cr alloy to an NiCr alloy.
Fig. 6-2 Characteristic X-ray images of cross sections of damaged parts caused by carburization occurring in the fuel mixing<sup>8）</sup>
Fig. 6-3 Schematic diagram of an expected damage process of the fuel mixing tube
Figure 6-48) shows pitting shape corrosion formed on the inner wall of a combustor of the same type as the combustor damaged by high temperature corrosion, which was introduced in section 2.2. The fuel and the operating temperature were different from those introduced in section 2.2. The atmosphere contained CO and the gas temperature was a little lower and estimated to be 800 - 900 ℃. The material of the combustor was an Ni-Cr-Mo-Fe alloy, which has been widely used as a heat resistant material for various high temperature equipment. Figure 6-5 shows the cross section of A–A in Figure 6-4. The pitting has an approximately hemispherical shape. Since there is the deepest thinning in the vicinity of the center of the diameter, it is found that the pitting grew isotropically from its center. Figure 6-6 and Figure 6-7 show distribution maps of elements of the scales and the sections near the alloy surfaces in Figure 6-5 (a) (a partial view of the edge of the pitting) and Figure 6-5 (b) (a view near the bottom of the pitting), respectively. As shown in the element map (Figure 6-6) of the edge of the pitting, any area outside the pitting was covered with a Cr-rich oxide with a thickness of about 5 μm and an Si-rich oxide layer, which was presumably derived from an Sibased component contained in the gas. In contrast, inside the pitting, the oxide film was very thin or did not exist on the alloy surface even in the extreme vicinity of the pitting edge and in the vicinity of the bottom of the pitting (Figure 6-7), where the deepest thinning was produced. In addition, as shown in Figure 6-7, precipitation of C was observed on the surface inside the pitting.
Fig. 6-4 Pitting shape corrosion formed on the inner wall of a combustor <sup>8）</sup>
Fig. 6-5 Cross sections of pitting shape corrosion formed on the inner wall of a combustor (Cross sections of “A-A” shown in Fig. 6-4) <sup>8）</sup>
Fig. 6-6 Characteristic X-ray images of (a) in Fig. 6-5<sup>8）</sup>
Fig. 6-7 Characteristic X-ray images of (b) in Fig. 6-5 <sup> 8）</sup>
Figure 6-88) shows the profile of C taken in the thickness direction from the inside of the alloy inside and outside the pitting toward the inner wall surface of the alloy. Lines 1 and 2 are profiles outside the pitting, and lines 3 to 5 are profiles inside the pitting. Spikeshaped peaks found in the alloy are considered to be carbide. On lines 1 and 2 outside the pitting, no significant difference in characteristic X-ray intensity of C is observed inside the alloy and in the vicinity of the inner wall surface of the alloy. This indicates that if the alloy surface is covered with a protective oxide film, the penetration of C into the alloy is prevented. In contrast, on lines 3 to 5 inside the pitting, the characteristic X-ray intensity of C increases from the alloy surface to a depth of 0.09 to 0.1 mm. It is characteristic that the penetration depth of C is almost the same at any place in the pitting. This time pittinglike thickness reduction was produced. The C concentration increased at a constant depth from the inner wall surface of the alloy in the pitting. C precipitated on the alloy surface in the pitting. For these reasons, it is likely that this pitting is metal dusting derived from a defective part of the oxide film on the alloy surface.
Fig. 6-8 Profiles of C in the vicinity of pitting shape corrosion formed on the inner wall of a combustor<sup> 8）</sup>
Based on the above, the process of pitting shape corrosion on the inner wall of a combustor is schematically described in Figure 6-98). C in the atmosphere enters through the defective part formed on the inner wall surface. When C is supersaturated in the alloy, graphite is precipitated and the alloy substrate is pulverized. Since C penetrates from the defects of the oxide film, it is thought that pitting shape corrosion is produced. Accordingly, the following carburization preventive measures described in section 2.2 are also considered effective: forming a highly protective oxide on the surface of the alloy as a continuous layer and selecting an alloy whose diffusion rate of carbon is low. Since a Co-Cr alloy was used in the mixing tube, the material was changed to an Ni-Cr alloy, which is more likely to form a continuous Cr2O3 film and has a small diffusing capacity of C to extend the tube life. In contrast, an Ni-Cr-Fe-Mo alloy is already used in this combustor. To prolong the life, preliminary oxidation treatment was added to form a strong and highly protective Cr2O3 film in advance.
Like the carburization preventive measures described in section 2.2, forming Al2O3, which is a more stable oxide than Cr2O3, is also considered effective. For example, the use of an Ni-base superalloy, which is an Al2O3 former, is also considered as a method. However, from the viewpoint of costs, forming an aluminumenriched layer on the surface by using an aluminum diffusion coating or the like is also effective. When these methods are used, it is desirable to use measures to prevent peeling of Al2O3 films together.
Fig. 6-9 Estimated pitting shape corrosion formation process by metal dusting <sup>8）</sup>
It is known that steam oxidation occurs when steel materials are exposed to a high temperature environment containing steam or water vapor13). In the case of a stainless steel boiler tube, when steam oxidation generates thick surface scales, they peel off and accumulate in the tube when the boiler is cooled, blocking the steam passage. As a result, when the boiler was restarted, the tube was overheated causing creep damage, which was reported as an accident14). At present, countermeasures are taken from both in respect to materials (use of fine grained materials, cold worked materials, etc.) and in respect to operation and maintenance (prevention of a tube temperature rise, inspection of U-bend sections, etc.).
Nitridation by ammonia has been reported in ammonia synthesis plants15), 16). NH3 gas adsorbs to the surface of the alloy and decomposes. Atomic N diffuses into the alloy and forms a solid solution or a nitride deteriorating the mechanical properties. In the case of Cr-containing alloys such as stainless steel or the like, nitride formation forms a Cr-depleted zone, which may cause deterioration in corrosion resistance. Some steel materials are nitrided in ammonia at about 450 to 700 ℃; in particular, the toughness is impaired. For carbon steels, it has been reported that they embrittle even at a relatively low temperature, e.g., 200 ℃17).
In a mixed gas of ammonia and water vapor, both steam oxidation and nitridation are concerned. Unfortunately, there have been no detailed reports on corrosion behaviors of metallic materials in a mixed gas of ammonia and water vapor, about which much is still unknown. In this chapter, we will describe the results of examining high temperature corrosion behaviors of various alloy materials in a mixed gas of ammonia and water vapor by experiment18).
In the experiment, low-alloy steel (1Cr steel) and stainless steel (12Cr steel) were exposed in a reactor where the temperature was kept at 400 ℃, 500 ℃, or 565 ℃, while a mixed solution whose mixture ratio of NH3 and H2O was 8 to 2 was fed to the reactor at a constant rate. At that time, the decomposition amount of ammonia was about 5 %. After the exposure, the specimens were removed from the reactor and subjected to a tensile test at room temperature to examine the impact of exposure to a mixed gas of ammonia and water vapor, which affects their mechanical properties.
Figure 6-10 shows the fracture surface and cross section of 12Cr steel subjected to the tensile test at room temperature after exposure to the mixed gas of ammonia and water vapor at 565℃ for 1000 hours. On the fracture surface, a flat brittle fracture surface layer about 350 μm deep from the surface was observed. In contrast, the cross section had a black oxide layer about 50 μm deep from the surface. Under the layer, there was a nitrided layer about 300 μm deep different in color from the alloy substrate. In addition, cracks occurred through these layers. Since the brittle fracture surface layer from the surface was observed by a visual check and the cracks occurred through these layers, the oxide layer and nitrided layer were found to be brittle. This fact suggests that the impact resistance of the material is lowered when these layers are thickly formed.
Figure 6-11 shows the difference in tensile properties at room temperature between 1Cr steel and 12Cr steel with and without exposure to the mixed gas of ammonia and water vapor. When exposed at 500 ℃ or less, the 1Cr steel hardly formed a brittle layer, and the changes in mechanical properties were small before and after exposure. Accordingly, for 1Cr steel, exposure to a mixed gas of ammonia and water vapor at 500 ℃ or less for 500 hours is considered to have no significant impact on its mechanical properties. However, to judge whether the steel can be applied as a structural material in this environment, a longer exposure test is required. Although 12Cr steel showed no significant change in tensile strength and yield strength due to exposure at 500 ℃ , degeneration of the ductility (elongation and reduction of area) was observed. It is believed that, because of the high Cr concentration, a nitrided layer tens of μm deep was formed even at 500 ℃, decreasing ductility. It showed no significant change in tensile strength and yield strength due to exposure even at 565 ℃. However, the ductility was dramatically impaired. As shown in Figure 6-10, this is likely caused by the formation of a thick brittle layer about 350 μm deep from the surface.
It was found that when steel material is exposed to a mixed gas whose mixture ratio of ammonia and water vapor is 8 to 2, it forms an oxide scale on the surface and a nitrided layer inside the alloy just beneath the scale. The nitrided layer grew thicker than the oxide scale, but the decrease in tensile strength and yield strength was small. However, the ductility (elongation and reduction of area) of the material decreased as the nitrided layer thickened. At 400 ℃ , the growth rate of the brittle layer was small and the degree of decrease in ductility was small when the steel was exposed for several hundred hours. However, at 500 ℃ or higher, especially at 565 ℃, the growth rate of the brittle layer was large, which indicates that the ductility significantly decreased. It is thought that the main cause of deteriorating material properties in this environment is the nitrided layer, which was formed more thickly than the oxidized layer. However, depending on the mixture ratio of ammonia and water vapor, the temperature, and the material composition, different behaviors are considered. Selection and design of materials according to environment is needed. It is inferred that high protection of the surface oxide layer and the nitriding tendency are the dominant properties of the material. Especially when the operating temperature is high, Cr-containing alloy or stainless steel is often used because of high-temperature strength and oxidation resistance. Although Cr is a protective oxide film forming element, it is also a nitride forming element. Thus, if the Cr concentration in the alloy is low and a protective oxide film cannot be stably formed, N penetrates into the alloy to precipitate Cr nitride. This event promotes embrittling of the material. In contrast, if the Cr concentration is high and a protective oxide film can be stably formed, N penetration into the alloy is prevented and precipitation of a Cr nitride inside the alloy does not occur. Accordingly, it is thought that the material properties can be maintained. Therefore, if a Cr-containing alloy is used in a mixed atmosphere of ammonia and water vapor, it is necessary to use a material having a Cr concentration sufficient to form a protective oxide film in the environment.
Fig. 6-10 Facture surface and cross section of 12Cr steel subjected to the tensile test at room temperature after exposure to the mixed gas of ammonia and water vapor at 565 ℃ for 1000 hours.
Fig. 6-11 Influence of exposure to a mixed gas of ammonia and water vapor on the tensile properties of 1Cr steel and12Cr steel at room temperature (exposure time: 500 h (1000 h for 12Cr steel exposed at 400 ℃))
As described in chapter 3, it is known that steam oxidation occurs when steel materials are exposed to high temperature environments containing water vapor. Generally, in water vapor the oxygen potential is lower than in oxygen and in the air, but the oxidation rate is large19). In addition, the oxidation rate may not comply with the simple parabolic rate law or accelerated oxidation may occur after a certain incubation time has passed, making the oxidation rate extremely large20). Therefore, compared to oxidation by air or oxygen, there is a high risk of damage to the structural components. However, in equipment relating to environments and/or energy, combustion gas is often handled, and equipment components are not often exposed to environments containing water vapor. Therefore, it is important for equipment relating to environments and/or energy to take appropriate corrosion protection measures by understanding the steam oxidation behaviors of equipment components.
It is known that a thick scale containing a large number of voids is formed during steam oxidation of Fe-base alloys. As an explanation that steam oxidation grows considerably faster than oxidation in the air or oxygen, Fujii et al.21), 22) and Rahmel et al.23) suggest a mechanism that would allow an inner scale to grow thickly when oxygen in water vapor moves in scale voids. Furthermore, various researchers24), 25) who study the effects of water vapor and hydrogen on oxidation suggest that these factors are present in voids and affect the oxidation behavior.
When stainless steel is used for high-temperature equipment components, austenitic stainless steel is often used because of high-temperature strength. In this chapter, we will introduce the results of oxidation experiments carried out to research the basic oxidation behaviors of Type304ss and Type310ss, which are widely used as high-temperature equipment components, in environments containing water vapor.
When steam oxidation occurs on high-temperature equipment components, thickly grown scales peel off. This may cause clogging of the equipment or erosion of the post stage equipment. Since the coefficient of thermal expansion of an oxide scale is smaller than that of the alloy substrate, the scale may peel off during cooling. We made an experimental apparatus, as shown in Figure 6-12, which can elevate the electric furnace while measuring the weight of the test specimen with a thermobalance, to examine the delamination behavior of steam oxidation scales. The temperature was raised and kept at 800 ℃ for 14 hours; then, the electric furnace was lowered, and the gas temperature reached 250 ℃ in the vicinity of the test specimen. This series of operations was defined as one cycle, and this cycle was repeated ten times to apply thermal cycle to the test specimen (Figure 6-13). In order to examine the influence of oxygen partial pressure on oxidation behavior, four types of experimental gas, shown in Table 6-1, were used. In the experiment, water vapor was introduced into the reactor using a mixed gas of N2, N2-H2, or N2-O2 as a carrier gas so as to include the saturation vapor pressure of pure water whose temperature was adjusted in a thermostatic chamber so that a predetermined partial water vapor pressure was obtained.
Figure 6-14 shows cross-sectional views of Type304ss exposed to the laboratory air and an environment containing water vapor. In the Type304ss exposed to the air, nodular corrosion was partly observed, but most of the surface was covered with a thin oxide film of several μm thick, and peeling of the film was not observed. Meanwhile, in the Type304ss exposed to an environment containing water vapor, a thick film grew and evidence of peeling of the film was observed. In the Type304ss exposed to gas (a) (N2-3.4%H2-15.5%H2O (PO2 = 9.3 × 10-18 atm)) at low oxygen partial pressure showed peeling from the interface between the inner and outer layer scales. Meanwhile, in the Type304ss exposed to gas (b) (N2-16.9%O2-15.5%H2O (PO2 = 1.7 × 10-1 atm)) at high oxygen partial pressure, there is almost no external layer scale, and peeling from the inside of the inner layer scale was observed.
Figure 6-15 shows mass change curves of Type304ss in water vapor-containing gas (a) (N2-3.4%H2-15.5%H2O (PO2 = 9.3 × 10–18 atm)) and gas (d) (N2-16.9%O2-15.5%H2O (PO2 = 1.7 × 10-1 atm)). In both cases, mass loss occurred when the furnace gas temperature dropped. Particularly for gas (d) at high oxygen partial pressure, mass loss was observed whenever the temperature dropped. The reason why the oxidation rate gradually decreased after the scale peeled off in both gases (a) and (d) is that the oxide scale peeled off not entirely, but partially, as shown in Figure 6-14. Thus, even under conditions with good oxidation resistance in the air, please note that if thermal cycles are applied in an environment containing water vapor, there may not be sufficient oxidation resistance.
Figure 6-16 shows the scale thickness of Type304ss and Type310ss in environments with constant water vapor content and different oxygen partial pressures. In the Type304ss, when the water vapor content was constant, the higher the oxygen partial pressure became, the thicker the oxide scale became. In contrast, in the Type310ss, the oxide scale thickness was 1/10 or less of that of Type304ss at any oxygen partial pressure, and the influence of oxygen partial pressure was not much observed. On the contrary, in an environment where the oxygen partial pressure was high, the scale tended to be thinner. In the Type310ss containing 25 % of Cr, it is presumed that accelerated oxidation was suppressed by forming a Cr-rich protective film in this environment.
| Gas composition | PO2 (atm) at 800 ℃ | |
| （a） | N2-3.4%H2-15.5%H2O | 9.3×10－18 |
| （b） | N2-0.02%H2-15.5%H2O | 2.7×10－13 |
| （c） | N2-15.5%H2O | 1.2×10－7 |
| （d） | N2-16.9%O2-15.5%H2O | 1.7×10－1 |
Fig. 6-12 Steam oxidation test equipment
Fig. 6-13 Thermal cycle pattern in steam oxidation test
Fig. 6-14 Cross sections of Type304ss exposed to the laboratory air and an environment containing water vapor (800 ℃ –250 ℃ cycle, 144 h)
Fig. 6-15 Mass change curves of Type304ss (800 ℃ –250 ℃ cycle)
Fig. 6-16 Influence of oxygen partial pressure on the scale thickness of Type304ss and Type310ss
As described above, in an environment containing water vapor, Type310ss based on Fe-20Ni-25Cr shows better oxidation resistance than Type304ss. However, other than Fe, Ni, and Cr, various elements are added to Type310ssbase practical alloys. We examined the steam oxidation resistance of an Fe-20%Ni-25%Cr-base alloy to which a small amount of several kinds of elements were added.
Figure 6-17 shows cross-sectional views of oxide scales of various alloys when a model alloy, which is an arc-melted Fe-20%Ni-25%Cr alloy with added Ce, Si, Nb and Mo, was oxidized in an environment containing water vapor. The addition of 0.05 % Ce hardly contributed to suppression of steam oxidation growth, but rather promoted oxidation in a high oxygen partial pressure environment. In the 0.5 % Nb and Si-added alloys, there was an inhibitory effect of oxide scale growth, especially at low oxygen partial pressure, but local corrosion, which seemed to be grain boundary corrosion, was observed in the Si added alloy. In contrast, the 1.5 % Moadded alloy showed significant inhibitory effect of oxide scale growth at both high and low oxygen partial pressures compared with the base alloy. Kochi et al.26) have reported that an Fe-30%Ni-20%Cr alloy to which Nb, Mo, W, and Ta were added, was oxidized in a water vapor atmosphere at 700 ℃ , and an alloy to which Nb, Mo, and W were added to precipitate an intermetallic compound with Fe decreases the inner scale thickness. It is thought that this event occurs since the precipitation of an intermetallic compound between the additive elements and Fe decreases the Cr activity in the alloy immediately below the oxide scale and increases the flux of Cr supplied to the alloy surface.
It is not possible to say that the reason why the addition of 1.5 % Mo suppressed the steam oxidation scale growth of an Fe-20%Ni-25%Cr alloy in this experiment was the same as the reason given by Kochi et al., because neither experiment was conducted with various amounts of additives, nor was a detailed study, such as distribution analysis, conducted. However, it is suggested that steam oxidation behavior greatly changes, depending on the types or the trace amounts of additive elements other than Cr and Ni. Thus, in terms of steam oxidation behavior, it is important to select and design materials taking into account the types and the concentrations of trace amounts of additive elements in addition to the main elements.
Fig. 6-17 Cross sections of oxide scales formed on Fe-20Ni-25Cr-X alloys (800 ℃ –250 ℃ cycle, 144 h)
In this issue, we introduced examples and experiments of carburization, metal dusting, nitridation, and steam oxidation as high temperature corrosion that occurs in various equipment relating to environments and/or energy. These phenomena, including some that have not yet been studied in detail, are high temperature corrosion problems, which will become apparent if conditions are met under the presence of C, N, and H2O contained in any high temperature equipment. Accordingly, it is important to pay attention to these phenomena when equipment relating to environments and/or energy is used.
This lecture introduced some cases of research and development and relevant countermeasures based on examples of high temperature corrosion that the authors have studied for over 20 years since joining the company. In real industries, high temperature corrosion is less likely to be observed than wet corrosion (aqueous corrosion), which is also reflected in the number of presentations at corrosion conferences. In the case of high temperature corrosion, research and development investment depends more on economic growth and the economic situation in the world more than is the case for wet corrosion. In contrast, mishandling of high temperature corrosion often leads to major accidents and no countermeasures against many of these phenomena exist. For this reason, it is likely to be an important field of study. In addition, high temperature corrosion not only motivates corrosion protection technology, but is also certainly important for almost all material engineering fields: metallography, diffusion, electrochemistry, thermodynamics, and reaction kinetics. Therefore, it is important in both scientific and engineering fields, and is of great use not only to researchers of high temperature corrosion, but also to engineers engaged in fields other than research and development.
Many of the technologies introduced in this lecture are based on collaborative research with Toshio Narita, emeritus professor at Hokkaido University, who taught the authors over the years. We express our gratitude to him.
In the future, while deterioration of equipment relating to environments and/or energy will increase, development of life-prolonging technologies and new processes of equipment will be dramatically advanced by development of information technologies such as IoT and AI. In these cases, knowledge and technologies involving high temperature corrosion will undoubtedly be important and indispensable. We hope that this lecture will contribute even just a little to the soundness, reliability maintenance, and development of equipment relating to environments and/or energy.
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15) M. Tanimura and T. Nakazawa: On the Corrosion Embrittlement of Commercial Steels Caused by Hydrogen Attack and Nitriding Attack in an Ammonia Converter, Boshoku-Gijutsu, 21 (5), p.209-218 (1971).
16) M. Okubo: Corrosion and its Prevention in the High Pressure Equipments, Atsuryoku-Gijutsu, 18 (1), p.39-46 (1980).
17) K. Tamura, M. Nakahara, and M. Takahashi: Nitriding of carbon steel in low-pressure ammonia atmosphere, Proceedings of Fushoku Boshoku ’91, p.57-60 (1991).
18) R. G. Rowe, H. Yakuwa, H. H. Vakil, A. Whitehead, M. Miyasaka, and R. Kikuta: Environmental effects on steam turbine structural materials in an NH3/H2O atmosphere, Proc. High-Temperature Corrosion and Protection 2000, p407, Rusutsu (2000).
19) T. Moroishi: Steam Oxidation of Boiler Superheater Stainless Steel Tubes, Boshoku-Gijutsu, 25 (2), p.97-104 (1976).
20) S. Jianian, Z. Longjiang, and L. Tiefan: High-temperature oxidation of Fe-Cr alloys in wet oxygen, Oxid. Met.48 (3/4), p.347-356 (1997).
21) C. T. Fujii and R. A. Meussner: Oxide Structures Produced on Iron - Chromium Alloys by a Dissociative Mechanism, J. Electrochem. Soc., 110 (12), p.1195-1204 (1963).
22) C. T. Fujii and R. A. Meussner: The Mechanism of the High - Temperature Oxidation of Iron - Chromium Alloys in Water vapor, J. Electrochem. Soc., 111 (11), p.1215-1221 (1964).
23) A. Rahmel and J. Tobolski: Influence of water vapour and carbon dioxide on the oxidation of iron - silicon alloys in oxygen at temperatures of 750 to 1050 ℃, Mmaterials and Corrosion, vol.16 (8), p.662-676 (1965).
24) A. Yamauchi, K. Kurokawa and H. Takahashi: Evaporation of Cr2O3 in Atmospheres Containing H2O, Oxidation of Metals, 59 (5/6), p.517-527 (2003).
25) K. Nakagawa, Y. Matsunaga and T. Yanagisawa: Corrosion behavior of ferritic steels on the air sides of boiler tubes in a steam/air dual environment, Materials at High Temperatures, Vol.20, p.67-73 (2003).
26) N. Kochi and Y. Nishiyama: Thermodynamic Consideration on Steam Oxidation Resistance of Austenitic Stainless Steels Forming Intermetallic Compound, Journal of the Japan Institute of Metals, Vol. 81 (9), p. 427-434 (2017)
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