Krishan Lal Luthra

Oxidation of carbon/carbon composites—a theoretical analysis

Abstract Theoretical considerations have been used to evaluate some approaches useful for oxidation protection of carbon/carbon composites. Various rate limiting steps have been considered including gas phase diffusion, interface reaction, diffusion through liquid B 2 O 3 , and diffusion through cracks in solid barrier films. The calculated virtual maximum rates for these steps are compared with those acceptable for use to determine the maximum useful temperatures of some of the approaches being considered for carbon protection. Also considered are the conditions in which CO gas bubbles can form beneath a liquid B 2 O 3 film.

Stability of protective oxide films on Ti-base alloys

Thermodynamic calculations are performed to estimate isothermal sections of Ti-Al-O, Ti-Si-O, and Ni-Al-O phase diagrams. Very small aluminum levels (<10−10 at. %) are needed to stabilize alumina on Ni-Al alloys. However, much higher aluminum (≳50%) and silicon (≳40%) levels are needed to stabilize alumina and silica on Ti-Al and Ti-Si alloys, respectively. These calculations suggest that the mechanism of formation of the protective oxide films on titanium-based alloys is radically different from that on nickel-based alloys. The aluminum levels needed to form a continuous film of alumina on nickel-based alloys are dominated by kinetic factors. On the other hand, thermodynamic factors appear to dominate the alloy compositions needed to form protective films of alumina and silica on titanium-based alloys. Further work is needed to evaluate any possible role of kinetic factors.

Mechanism of adhesion of alumina on MCrAlY alloys

X-ray diffraction has been used to measure stains/ stresses generated in oxide films formed under isothermal conditions at 1150–1225° C and cooled to room temperature. High compressive strains, of the order of 1%, were measured in alumina films formed on FeCrAlY. However, little or no strain was measured in oxide films on NiCrAlY and NiCoCrAlY samples, even when there was no scale spallation. Auger Electron Spectroscopy (AES) experiments have also been conducted to evaluate the role of segregation on scale adhesion. Our studies suggest that the adhesion mechanism might depend on the alloy composition. On iron-based alloys, the scale spallation might be prevented by mechanisms that involve strong bonding at the interface. On the other hand, the scale spallation on nickel-based alloys might be prevented by a mechanism that relieves stresses. Yttrium segregation might help in this process.

Simultaneous sulfidation-oxidation of nickel at 603°C in SO2-O2-SO3 atmospheres

The simultaneous oxidation-sulfidation rate of nickel has been measured as a function of SO2 pressure (0.04 to 1 atm) in Ar-SO2 gas mixtures at 603°C. The observed corrosion rates are about 107 times faster than the oxidation rate of nickel in oxygen at 1 atm. The product scale consists of an inner Ni3S2 layer and an outer two-phase layer of NiO and Ni3S2. A linear rate law is observed during an initial time period, and the most probable rate-controlling step is dissociation of SO2. An increase in the scale-gas interfacial area increases the corrosion rate during intermediate time periods. With increasing time, parabolic corrosion rates are measured for SO2 pressures of 0.25 and 1 atm. Values of the nickel diffusivity in Ni3S2 calculated from our measured parabolic-rate constants are in good agreement with recently reported values. This agreement indicates that an interconnected Ni3S2 phase in the outer two-phase layer provides rapid transport paths for nickel diffusion through the scale.

Low Temperature Hot Corrosion of Cobalt-Base Alloys: Part II. Reaction Mechanism

This paper presents a mechanism of low-temperature hot corrosion that is based on rapid dissolution of the more noble metal or metal oxide in liquid salts. It is proposed that the rapid degradation of cobalt-base alloys results from dissolution of cobalt or cobalt oxides on the surface, which prevents the formation of a protective Cr2O3 or A12O3 film. The reaction occurs in two stages: (a) an initial stage, during which an Na2SO4-CoSO4 liquid forms on the surface, and (b) a propagation stage, during which SO3 migrates inward and cobalt outward through the molten salt. At longer times, cobalt dissolves at the scale/salt interface and forms Co3O4 and/or CoSO4(s) in different regions of the reaction product. The mechanisms of transport of various reactants and products through the liquid salt and the effects of their relative transport rates on the reaction product morphology have been considered.

Low Temperature Hot Corrosion of Cobalt-Base Alloys: PartI. Morphology of the Reaction Product

Accelerated oxidation tests have been conducted on a number of Co-Cr, Co-Al, and Co-Cr-Al alloys coated with a thin film of Na2SO4 and exposed at 600 to 750 °C in O2-SO2-SO3 environments containing 0.0095-5 pct (SO2 + SO3). Generally, Co-Cr and Co-Cr-Al alloys reacted nonuniformly, usually in the form of pits, and Co-Al alloys suffered broad frontal attack. The morphology of the reaction product was observed to be dependent on temperature andPSO3 Under all conditions, a thin sulfur-rich band containing sulfides was observed at the alloy/scale interface, and cobalt dissolved near this interface and formed Co3O4 and/or CoSO4(s) in different regions of the reaction product.

Surface segregation in MCrAlY alloys

This paper presents a study of surface segregation in NiCrAlY and FeCrAlY type alloys. The segre-gation was measured using Auger electron spectroscopy. Samples were heated in the spectrometer to temperatures between 800 and 1100 °C, and segregation was measured at temperature as a function of time. The results show that when yttrium and sulfur were present in the alloy, they segregated to the surface where their concentrations were greatly enriched over their bulk concentrations. The presence of yttrium in the alloy did not eliminate sulfur segregation. The surface concentration of aluminum appeared, in most cases, to be greater than its bulk concentration. However, because of uncertainty in sensitivity factors for Auger emission, this point could not be conclusively proven. Cobalt and chromium, in contrast, were depleted from the surface. Sulfur segregation was also ex-amined in Ni-S, Cr-S, Al-S, and Y-S binary alloys. Sulfur was found to segregate extensively in the nickel and chromium alloys, but to a much less extent in aluminum. No sulfur segregation was ob-served in the yttrium alloy.

Low temperature hot corrosion of CoCrAl alloys

Abstract Hot corrosion of CoCrAl alloys occurs rapidly at temperature of 600–750°C because of the formation of the low melting Na 2 SO 4 CoSO 4 eutectic. In this paper the mechanisms and solutions of this low temperature hot corrosion (LTHC) are reviewed. Rapid degradation results from the dissolution of the more noble metal (cobalt) and its oxides (CoO and Co 3 O 4 ) on the surface. On the basis of this mechanism, we have developed CoCr coatings containing about 37.5 wt% Cr or more which are resistant to LTHC and provide adequate resistance against the normal high temperature hot corrosion. Theoretical calculations have also been performed to predict the conditions under which binary K 2 SO 4 CoSO 4 and ternary Na 2 SO 4 K 2 SO 4 CoSO 4 eutectics can form. These calculations suggest that the presence of K 2 SO 4 can aggravate the LTHC caused by Na 2 SO 4 .

High chromium cobalt-base coatings for low temperature hot corrosion☆

Abstract Accelerated oxidation tests have been conducted on a number of cast Co-Cr-Al alloys coated with a thin film of Na 2 SO 4 and exposed in an O 2 -0.15%(SO 2 -SO 3 ) gas mixture at 750°C. These tests show that the corrosion resistance of binary Co-Cr alloys increases with the chromium content of the alloys and that even small concentrations of aluminum have detrimental effects on their corrosion resistance. Tests on Co-Cr coatings on a Rene 80 substrate under the same conditions indicate that there is a transition from a high to a low corrosion rate at a chromium level of about 37.5 wt.%. These results can be explained on the basis of a mechanism that we have already proposed in the literature. Based on our laboratory studies, we have developed new Co-Cr coatings that provide excellent resistance to low temperature hot corrosion and are also acceptable for conventional, high temperature, hot corrosion.

Mechanism of low temperature hot corrosion: Burner rig studies☆

Abstract The unexpectedly rapid corrosion induced by Na 2 SO 4 , which has been observed in recent years on gas turbine components operating in the range 600–730°C, has been studied in burner rig tests. The microstructural features and kinetics of this mode of attack are shown to occur on nickel or cobalt base superalloys and coatings only when the gaseous environment contains sufficient SO 3 to form a liquid eutectic between Na 2 SO 4 and CoSO 4 .