Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 °C

Abstract

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050°C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclicoxidation tests at 1,050°C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050°C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150°C for 100 h and cyclicoxidation tests at 900°C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the b to c 0 transformation leads to the formation of a two-phase b/c 0 layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this b to c 0 transformation occurs gradually by an inward transformation of b leading to the formation of a continuous layer of c 0 phase, parallel to the metal/scale interface.

Substrate Effect on the High-Temperature Oxidation Behavior of a Pt-Modified Aluminide Coating. Part I: Influence of the Initial Chemical Composition of the Coating Surface

“…Long-terms oxidation tests at 1050°C have been performed and reported in the second part of this paper. 43 At this higher temperature, interdiffusion between the coating and the substrate takes a large part in the damaging process 44 and is expected to have a large effect on cyclic-oxidation resistance.…”

Structure and high temperature oxidation of mechanical alloyed Fe-Al coating

“…The mass gain of Fe-Al coatings with and without annealing is relatively stable after 2 cycles. The mass change is relatively constant after the steady-state stages are reached [26]. The obtained result clearly suggests that mechanical alloyed Fe-Al coating can reduce the oxidation rate of low carbon steel.…”

“…It shows that the mass gain decrease in each cycles. Xu et al [25] and Vialas and Monceau [26] revealed that the scale spallation on the oxidized samples was indicated by the mass loss. A different result is shown in the Fe-Al coating on low carbon steels.…”

Numerical Modelling of Diffusion Coupled with Cyclic Oxidation. Application to Alumina-Forming Coatings Used for Industrial Gas Turbine Blades

“…precipitates at the coating/superalloy interface [21] The Net Mass Change (NMC) curves of these oxidation tests have been reported elsewhere [20][21][22][23]. The shape of these curves is characteristic of the competition between oxidation (mass gain) and spallation (mass loss).…”

“…Long term cyclic oxidations (35*300h at 1050°C) have given valuable information on the chemical and microstructural evolution of the studied systems. Degradation due to long term cyclic oxidation tests have been investigated [20][21][22][23]. For the studied systems, it was shown that Al depletion in the coating was due to oxidation and spalling of the protective oxide but also that that the contribution of Al diffusion from the coating toward the substrate was a key element in life assessment.…”