Effects of Oxidation and Hot Corrosion in a Nickel Disc Alloy

Abstract

This paper describes work to study oxidation and hot corrosion damage in nickel disc alloy RR1000. Oxidation damage has been characterised via mass change data from thermogravimetric analyses at temperatures from 700 to 800 ºC and cyclic oxidation testing at 700 and 750 ºC. Thin oxide scales rich in Cr and Ti have been found to grow in a parabolic dependence with time. Oxidation rate constants have been correlated with temperature using an Arrhenius equation. Through the use of simplifying assumptions, mass change data have been converted to oxide thickness values. These values have been compared with observed values of oxide scale and the depth of near-surface damage, which have been measured using focused ion beam (FIB) microscopy.Hot corrosion of RR1000 has been studied in the laboratory by 'deposit' recoat experiments at 700ºC. In these tests, samples were coated with deposits of Na 2 SO 4 /NaCl salt in a gas stream of air-300 vpm SO 2 . Corrosion damage has been quantified by dimensional metrology of samples before and after exposure. This enabled metal losses, i.e. the depth of corrosion pits, to be determined. Distributions of metal loss data were then generated to produce median damage values and exceedance cumulative probabilities. Finally, the effect of contaminants on the hot corrosion resistance of RR1000 has also been evaluated.

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

“…[10][11][12][13][14] Energy-dispersive x-ray analyses showed that the overall oxide morphology from the external scale to internal oxides of Cr 2 O 3 /TiO 2 in the outer scale to Al 2 O 3 internally agrees with the expected thermodynamic behavior; however, and in contrast to the thermodynamic predictions, the alloy formed an outer layer of TiO 2 , believed to be at the expense of its oxidation resistance. 14,15 Cationic diffusion is inferred from the known transport mechanisms for Cr 2 O 3 growth 16,17 and from the fact that titanium must diffuse outwards to form above the outer scale.…”

The effect of minimum dwell cycles on the environmental and fatigue response of RR1000

“…The current specimens received test exposures of 650 • C under minimum dwell fatigue for only 22 to 74 hours. Measurements from FIB analyses showed minimum dwell oxidation damage was of a similar magnitude to stress-free isothermal measurements quoted by Encinas-Oropesa et al after a more onerous 200 hour exposure at 700 • C [5].…”

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

“…Given the fact that oxidation kinetics for nickel alloys at 650C is extremely slow [25], there is insufficient time for oxide formation to happen ahead of the crack tip within the order of seconds. Direct observations of internal oxidation and associated damage were only available on smooth specimens, which have been exposed to oxidizing environment for a considerably long time, mostly greater than 20 hours [2,12,13,26]. The current work also showed that crack growth rates in air at high-temperature can be predicted well from the dynamic embrittlement hypothesis, though collecting experimental evidence of crack tip oxygen diffusion process and measurements of oxygen concentration near the crack tip are challenging tasks and yet to be accomplished.…”

“…and κ 0 are the oxygen diffusivity and mobility at 650°C and assumed to be independent of the oxygen concentration. In the present work, no attempt was made to model chemical reaction kinetics, vacancy formation or the transport of other species [26] other than oxygen diffusion near the crack tip.…”

“…Secondly, it was found that the cracking process is extremely sensitive to rapid changes in oxygen pressure, where fracture could be turned on and off in less than 10s by alternately pumping oxygen from the chamber and re-admitting it. Oxidation kinetics for nickel alloys at 650°C is extremely slow [25,26], and oxidation reaction does not have sufficient time to occur within the order of seconds. Dynamic embrittlement was introduced to define a time-dependent intergranular brittle cracking process in which the supply of embrittling elements is due to grain boundary diffusion into an area of increasing tensile stress ahead of a crack tip [19][20][21][22][23][24].…”