Oxidation of Superalloys in Extreme Environments

Abstract

With the current interest in increased efficiency and reduced emissions, materials in power generation systems are increasingly exposed to more aggressive environments and higher temperatures. It is important to understand how these environments can increase corrosion rates, possibly reducing performance and likely shortening component lifetimes. Superalloys like 718 and its derivatives primarily rely on the formation of an external Cr-rich oxide layer or scale for environmental protection at typical application temperatures (≤700°C). While many laboratory oxidation and creep experiments are conducted in ambient air, the application environment for these alloys often involves combustion products, such as H 2 O and CO-CO 2 . The effect of these environments is not completely understood, especially the impact of long-term exposures, typically required of superalloy components in turbines. Air and fuel impurities, especially S and Na, can result in accelerated degradation such as the well known hot corrosion attack. While the effect of environment on crack growth has been studied, less information is available on the effect of environment on creep and fatigue properties. Initial work is being conducted to better understand the role of environment on creep properties, eventually including in-situ testing. Examples are given from steam and wet air environments and comparisons of different superalloy compositions with cast versus wrought microstructures.

Oxidation behaviour of cast Ni–Cr alloys in steam at 800°C

“…However, the deep internal penetrations in some cases may raise concerns about effects on mechanical properties. No effect of in situ creep testing in steam has been detected for wrought 230,23 nor has ex situ creep testing after exposure to 800°C steam of 25 mm long creep specimens found any effect of steam oxidation on creep properties for wrought 230 or 740 13,24. Similarly, creep testing in reforming gas also had little effect on creep behaviour of chromia forming Ni base alloys 25.…”

“…No effect of in situ creep testing in steam has been detected for wrought 230, 23 nor has ex situ creep testing after exposure to 800uC steam of 25 mm long creep specimens found any effect of steam oxidation on creep properties for wrought 230 or 740. 13,24 Similarly, creep testing in reforming gas also had little effect on creep behaviour of chromia forming Ni base alloys. 25 However, fatigue testing in the presence of steam has not been reported but may reveal important changes.…”

“…exhaust gas applications). [11][12][13][14] However, Ni base alloys are commonly strengthened by Ti and Al additions, which can internally oxidise. 14,15 The internal oxidation could be affected by segregation in the cast microstructure and the steam environment.…”

Isothermal short‐term oxidation behavior of MAR‐M246 nickel‐based superalloy at 800°C and 1000°C

“…[ 7 ] In addition, there is great interest in increasing engine efficiency and reducing gas emissions, since materials for power generation systems, such as superalloys, are increasingly exposed to aggressive environments and higher temperatures. [ 8 ] Under these conditions, degradation by oxidation occurs and plays an important role in determining the lifespan of components. [ 2 ] Moreover, as previously shown, superalloys commonly contain ten or more elements, including given amounts of Cr, Al, Ti, Ta, W, and they may also have different oxidation products, including Cr 2 O 3 , Al 2 O 3 , TiO 2 , Ta 2 O 5 and WO 3 , and spinels such as NiCr 2 O 4 , NiAl 2 O 4 , NiTa 2 O 4 , as well as others.…”