Temperature Effect on Oxidation Behavior of Ni-Cr Alloys in CO₂Gas Atmosphere

Abstract

Model Ni-Cr alloys containing 5, 10, 15, 20, 25 and 30 wt% Cr were oxidized in Ar-20 vol% CO 2 gas mixtures at temperatures of 650, 700 and 800 • C. In general, multi-layered oxide scales were observed on the surface after reaction. With increased alloy Cr content, the oxide structure changed from external NiO, plus intermediate inner oxides and an internal oxidation zone, to forming a thin chromia band at the base of the oxide scale and, at higher Cr levels, an exclusive chromia scale. Increasing temperature accelerated the oxidation kinetics of low chromium-containing alloys, but significantly reduced the oxidation rate of high chromium alloys by promoting formation of the chromia band/scale. The critical Cr concentration required for chromia scale formation and maintenance decreased with increasing temperature, in accord with diffusion theory. Intergranular carbides were formed in high Cr content alloys, indicating elevated carbon activities beneath the oxide scales. In conventional power plants, fossil fuels are burnt in air to generate electricity. This process is of low cost, but can produce a large amount of CO 2 gas, a principal contributor to global warming. To reduce CO 2 emissions, a new technology called oxy-fuel combustion has been developed. In this process, coal is burnt in a mixture of oxygen and recirculated flue gas, so the finally released flue gas consists mainly of CO 2 and water vapor. After condensation of water vapor, sequestering CO 2 gas from the flue gas is relatively easy.Unfortunately, this technology has the potential to exacerbate the corrosion problems encountered in power plants. The CO 2 gas has been found to be very corrosive to the steels of the critical heat exchanging components in boilers, producing severe oxidation and carburization of ferritic-martensitic steels. Exposed to this atmosphere, the chromia-forming steels which are used successfully in air, undergo breakaway oxidation and carburization. [1][2][3][4] In addition, increased operating temperatures to improve boiler heat efficiency are desirable to meet continuously increasing energy demand, e.g. in advanced ultra-supercritical power generation. As a result, the ferritic/martensitic and even austenitic steels currently used in traditional coal-fired power plants will not survive when the operating temperature increases to values higher than 650• C. 5-7 Under these circumstances, nickel-based alloys are alternative candidate materials, owing to their superior creep strength and corrosion resistance at higher temperatures. However, little is known about the corrosion behavior of Ni-based alloys in CO 2 -rich gases at high temperatures.The oxidation of various model Ni-Cr alloys in air and oxygen has been well investigated at temperatures between 800 and 1200• C, [8][9][10][11][12][13] and the oxidation of these alloys in H 2 O-containing environments has also been reported. [14][15][16][17] In air and oxygen environments, it has been generally accepted that binary Ni-Cr alloys with Cr contents lower than 10 wt% oxid...

Atomistic Origins of Reversible Noncatalytic Gas–Solid Interfacial Reactions

“…However, deviations from the parabolic kinetics during the steady-state oxide scale growth period have frequently been observed. Several interpretations have been used to explain such deviations, including a phase-boundary process as the rate-determining step of the reaction, , built-in electric field driven ionic diffusion, , doping effect in the oxide scale, − and microstructure evolution effect on the oxide scale growth. − The reverse reaction is often ignored to explain the deviations from the parabolic kinetics of the scale oxide growth. As shown from our results of the CO 2 -induced Ni oxidation, the reverse element of the reaction indeed exists and should be taken into account to understand the overall reaction rate.…”

Corrosion Behaviors of Heat-Resisting Alloys in High Temperature Carbon Dioxide

“…This result is the same as that observed on commercial alloys in Figure 9. However, as the temperature increased, the oxidation rates of Fe20Cr, Ni25Cr and Ni30Cr binary model alloys unexpectedly decreased [47,73]. The different result is closely related to the diffusion rate of Cr and the Cr content of the alloy.…”

“…These previous results indicate that chromia-forming nickel-base alloys have higher carbonization resistance than chromia-forming iron-base alloys. Surprisingly, the intergranular carbides were revealed by etching in Ni25Cr and Ni30Cr model alloys after exposure to 650 • C/1 bar Ar-20%CO 2 gas for 500 h but not in Ni20Cr [47]. When the temperature was up to 800 • C, the Ni20Cr model alloy also formed the intergranular carbides.…”

“…(c) Austenitic steels or high-Cr FMs [22,25,29]. (d) Nickel-base alloys [28,29,46,47]. The carbides above are caused by carburizing, and the precipitation of carbides after aging is not considered.…”

Microstructure of Rhenium Doped Ni-Cr Deposits Produced by Laser Cladding

“…The Ni-Cr protective layers provide excellent mechanical properties, high creep resistance, and adequate chemical resistance at elevated operating temperatures—such as high corrosion resistance. The effect of temperature on the oxidation behavior of binary Ni-Cr alloys in the CO 2 gas atmosphere has been studied in [ 7 ] and it was shown that at 650 °C, the oxidation rates were not significantly decreased until the alloy Cr content reached 30% (wt.). At a temperature of 800 °C, the oxidation rates of Ni-5Cr, Ni-10Cr, and Ni-15Cr alloys continued to increase, while those of alloys with 20 wt.% Cr and above reduced due to protective external chromia scale formation.…”