Commercial grade superalloys; Hastelloy C-4, alloy 1.4306S, Incoloy-800H, Incoloy-825, UHBA-25L. Sanicro-28 and Inconel-690 are selected on the basis of .their increasing chromium content. Studies have been carried out to evaluate their resistance to oxidation at elevated temperatures. It is understood that the oxidation resistance is derived from the chromium contents and influenced by other reactive elements. Specimens were exposed to air and steam separately from 600 to 1200°C for I to 400 h. Reaction kinetics of oxidation were determined and morphology of oxide scales investigated.
Hastelloy C-4 in air showed adequate resistance to oxidation up to 1000°C. It followed cubic rate law due to formation of uniform, protective and adherent oxide scales. At 1200°C, breakaway oxidation was observed accompanied by excessive metal loss. The XRD analyses showed the formation of different type of oxides as CrMn1.5O, Mn1.5Cr1.5O4, NiCr2O4, Cr1.3Fe0.7O3, NiCr2O4, NiO, NiMnO8, FeO and NiCrMnO4.In steam it exhibited excellent oxidation resistance at all temperatures under study. This behaviour is due to formation of compact and adherent oxide The oxides observed were mainly ofCr2O3, NiCr2O4, NiCrMnO4, FeCr2O4 and (Cr, Fe)2O3.
Cubic rate law dominated in the oxidation of alloy 1.43068 in air at 600 and 800°C and changed to parabolic at 1000 and 1200°C. Duplex oxide layer developed from the numerous oxide nodules that were formed in the initial stages of oxidation. Two major phases as FeCr2O4 and Fe2O3 for 600°C and 800oC exposures where FeSiO3 was also formed below external scale at 800°C. At higher temperatures Cr203 and CrMn1.50.. spinels formed as external scale. The outermost layer Fe203 had spalled.
In steam the alloy generally obeyed parabolic rate law at all temperatures except for exposures greater than 24h at 1200°C, where the alloy suffered breakaway oxidation. The phases such as FeCr2O4, Fe2O3. FeCr2O, Mnl.5Cr1.5O4 and FeO at 600, 800 and 1000°C were observed. Similarly for 1200°C the FC2O3 phase was found dominating.
Incoloy 800H followed parabolic rate law at 1000 and 1200°C in air. The Cr2O3 oxide was protective in most of the cases. except at 1200°C where deep internal oxidation was observed. XRD results showed that at low temperatures. i.e. 600 and 800°C only MnCrO4 was formed. While at 1000 and 1200°C identical multiple phases were detected, which are Mn1.5Cr1.5O4, MnCrO4, FeCr2O4 [FeO: Cr2O3], (Cr,Fe)2O3 and NiCrMnO4
In steam the reaction kinetics followed cubic rate law at 1200°C. Two layered oxide developed during steam oxidation. The layers were compact and adherent and more protective than those formed in air. Fe203 and FeCr203 on the specimen oxidised at 600 and 800oC, at 1000°C Mn1.5Cr1.5O4 and Cr2O3 were observed Similarly at 1200°C, two other phases Mnl 5Cr1.5O4 and NiCr2O4 along with the domination of Cr2O3 were present.
For the oxidation of Incoloy 825, parabolic rate law dominated during air exposures, thereby indicating diffusion controlled oxidation. At 1200oC dense and adherent oxide scale was observed. Extensive internal oxidation and void formation was also found at many places below the external scale. NiCrMnO4 at 600 and 800°C, and (Cr, Fe2)O3, NiCrMnO4, Cr1.3Fe0.7O3 & (FeO0.6CrO0.4)O3 were formed at 1000°C. Similar oxides at 1200°C (specimen was totally oxidized) along with Ni3 TiO5 NiFe2O3 and (FeO0.6CrO0.4)O3 were observed.
In steam oxidation Incoloy 825 obeyed cubic and parabolic rate laws alternatively from 600 to 1200°C. At 600°C two types of oxides NiCrMnO4 and Fe2O) were observed. At 800°C the oxides were changed to Cr2O), NiCrMnO4 and FeCr2O4 whereas at 1000 and 1200°C the main phases distinguished were NiCrMnO4, Cr2O), FeCr2O4 (FeOCr2O) and TiO2, CrTiO3 and Cr2TiO3.
During the oxidation of alloy UHBA 25L, generally parabolic rate law was observed at temperatures greater than 600°C. The oxide scale grew at the metal-oxide interface forming warts of the oxide which later on joined each other to form regular scale, like in the case of alloy 1.4306S. Breakaway was also observed at 1200°C during longer exposures. At low temperature Cr2O3, (Cr, Fe)2O3, Cr1.3Fe0.7O3 and (Fe0.6Cr0.4)O3 phases were identified. Whereas at 1000°C FeCr2O4 and Fe3O4 and at 1200°C NiCrMnO4, Cr1.3Fe0.7O3, NiCr2O3 and Fe2O3 were found.
Exposure kinetics in steam had tendency to remain parabolic or slower between 600-1 200°C. The oxide scale was adherent at 1000 and 1200°C. At 600 and 800°C exposures two major oxides Fe2O3 and CrMn1.5O4 were resolved. While at 1000 and 1200°C three identical phases were found which are Cr2O)., FeCr2O) and FeOOH.
Sanicro 28 followed parabolic rate law at 800 and 1000°C which changed to linear rate law at 1200°C due to severe oxidation. Extensive spalling exhibited early breakaway at I 200°C. At 800 and 1000°C, two identical phases Cr2O3 and Mn1.5Cr1.5O4 and at 1200°C NiCr2O3, FeCr2O4 and Fe3O4 in addition to Cr2O3 were detected. Steam oxidation of Sanicro 28 produced relatively better results than in air because the total weight gain per unit area of specimens were quite low at 1200°C exposures. The reaction kinetics were generally governed by parabolic rate law at higher temperatures (1000 & 1200°C. Up to 800°C Cr2O3 and Mn1.5Cr1.5O4, at 1000oC Cr2O3, and at 1200°C Cr2O3, FeOOH and FeCr2O4 phases were observed.
The oxidation behaviour of Inconel-690 remained almost the same in both air and steam atmospheres. Parabolic and cubic rate laws dominated their reaction kinetics. Oxide scale was also dense and adherent most of the time. Spalling and internal oxidation increased with the increase in exposure time at 1200°C .At all the temperatures under investigations, Cr2O3 was confirmed by XRD in Inconel-690 samples, exposed in air and steam. At higher temperature oxide also contain NiCrMn04 and FeOOH in both atmospheres.
Another commercial grade superalloy, Inconel-625, was modified by adding small quantities of AI by melting the alloy under vacuum and argon to find out any improvement in resistance to oxidation at elevated temperatures. During oxidation studies a minor improvement was recorded.
Commercial grade Inconel 625 was also aluminised by pack cementation, at 900°C for 4h in argon atmosphere, to obtain an inexpensive protective coating for high temperature oxidation resistant applications. The coated and un-coated alloy specimen were comparatively evaluated by collateral exposures in air at 1000, 1100 and 1200°C. The aluminising resulted in appreciable improvement in the high temperature oxidation of the alloy.