Oxidation Start Detection on Heating Parts of Thermogravimetry Curves for High Temperature Alloys Based on Nickel, Cobalt or Iron
The heating parts of thermogravimetry curves performed for cast alloys in air at high temperatures were analysed, after correction from the effects of air buoyancy variation, to characterize the transient oxidation before reaching the targeted temperature for the isothermal stage usually applied thereafter. This was done here for three binary M-30Cr alloys, six ternary M-30Cr-0.4 and 0.8%C alloys, and three M-30Cr-0.4C-6Ta quaternary alloys, heated in synthetic air at 20 K min-1 until 1000, 1100 or 1200°C. The cobalt alloys begin to be oxidized with a sufficient mass gain to be detected by thermogravimetry, earlier than the nickel or iron alloys (i.e. at lower temperatures). The temperature of oxidation start is lowered by the presence of tantalum for the three families of alloys. When the carbon content (or the carbides density) increases in the alloy, the temperature of oxidation start decreases for the cobalt alloys and remains almost constant for the nickel or iron alloys, while the total mass gain due to oxidation during heating increases for the nickel and cobalt alloys and decreases for the iron alloys.
Influence de l'élément de base et de la densité des carbures sur l'oxydation à haute température d'alliages M-30Cr-xC (M = Co, Ni, Fe). Partie I : Constantes cinétiques parabolique et de volatilisation de Cr2O3
Les comportements en oxydation a 1000°C, 1100°C et 1200°C de douze alliages simplifies a base de cobalt, fer et nickel chromine-formeurs binaires et ternaires ont ete etudies du point de vue de la cinetique de prise de masse. La cinetique d’oxydation des alliages base nickel et base fer aux trois temperatures, et celle des alliages base cobalt a 1000 voire 1100°C est de type parabolique. L’analyse de l’evolution de d(Δm/S)/dt en fonction de 1/(Δm/S) a permis dans ces cas de determiner simultanement la constante parabolique Kp et la constante de volatilisation de la chromine Kv, respectivement egales a la pente et a l’oppose de l’ordonnee a l’origine. Les alliages base fer s’oxydent generalement plus vite que les alliages base nickel, avec parfois des valeurs de Kv tres differentes. Les alliages base cobalt sont les plus sujets a l’oxydation catastrophique a T > 1000°C mais un reseau interdendritique de carbures de chrome peut ameliorer leur comportement.
Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating
Abstract A new type of Pt + Hf-modified γ′-Ni 3 Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al 2 O 3 scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I ( i.e. 900 °C/1652 °F) and Type II ( i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na 2 SO 4 salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.