The Oxidation of Co-5Si Alloys in 1 atm of Pure O2 at 700 and 800°C

The kinetics and products of oxidation of binary Co-5Si alloys in 1 atm of pure O2 at 700 and 800 oC were investigated. Co-5Si alloys followed a parabolic behavior to a good approximation (n=2). The parabolic rate constants as functions of temperature were 2.789×10-11, 2.240×10-10 g2cm-4s-1 at 700 and 800 oC, respectively. The scales formed on Co-5Si alloys were double-layered, with formation of an outer layer of columnar CoO and an inner region containing a mixture of SiO2 and CoO, coupled to the internal oxidation region of Si.

High-Temperature Sulfidation of Crofer 22APU in Atmospheres Containing H2S and H2O

The work presents analyses of Crofer 22APU steel oxidation in hydrogen atmosphere containing 300 ppm H2S and in hydrogen atmosphere containing 300 ppm and 1% H2O. The oxidation process was conducted on two kinds of samples: pre-oxidized (pre-oxidation process was carried out in the air at a temperature of 800°C during 100 h), and not pre-oxidized at a temperature of 600 and 800°C using periodic method during 500 h. It was established that oxidation rate may be described by means of parabolic law. Determined parabolic rate constants indicate that not pre-oxidized samples undergo sulphurization in H2/300ppm H2S atmosphere, while in H2/300ppm H2S + 1%H2O atmosphere they undergo oxidation 3-4 orders of magnitude slower.

The Oxidation of Co-10Cr Alloys in 1 Atm of Pure O2 at 700 and 800οC

The oxidation of binary Co-10Cr alloys in 1 atm of pure O2 at 700 and 800°C was investigated. It indicated that the oxidation followed approximately the parabolic rate law (n=2). The parabolic rate constants as functions of temperature were 2.03×10-11, 7.05×10-10 g2cm-4s-1, respectively. The scales formed on Co-10Cr alloys were double-layered. The outer layer composed of large columnar CoO grains was relatively compact, while the inner CoO layer was porous, fine-grained and doped by cobaltous oxide CoO and spinel CoCr2O4.

High Temperature Oxidation of Bond Coats in Thermal Barrier Systems

The isothermal oxidation of bond coats composed of vacuum plasma sprayed (VPS) MCrAlY (NiCoCrAlYTa and CoNiCrAlY) or palladium modified nickel aluminides (NiPd + APVS) was studied in several oxygen partial pressures (105, 1 and 10-5 Pa), with two heating rates (20 and 60K/min) at different temperatures (900, 1000 and 1100°C). For MCrAlY coatings, Arrhenius plots of the parabolic rate constants show a kinetic transition below 1000°C. This could be linked to a transition from Al2O3 to Cr2O3 scale growth. Lower oxygen partial pressures induce lower parabolic rate constants at 900°C. This leads to the assumption that scales grown at low oxygen partial pressures are still formed of alumina at 900°C. Nevertheless, these results could not be confirmed by chemical analysis (EDS, XPS). The two tested heating rates show no influence on the oxidation kinetics of both MCrAlY coatings. In the case of aluminide, for low oxygen partial pressures, the parabolic kinetics are reduced of one order of magnitude (for PO2 =105 to 1 Pa) and correspond to a thinner scale of a-alumina. Also, the heating rate modifies the parabolic kinetics (i.e. after the transient stage) and the total weight gains for all oxidation temperatures, with higher parabolic rate constants after heating at slower rate.