D. L. Douglass

The oxidation mechanism of Ni3Al containing yttrium

The high-temperature oxidation behavior of Ni3Al (Ni-13.2 wt.% Al) with and without additions of 0.5 wt.% yttrium has been studied over the range of 900–1200°C in air. None of the commonly accepted rate laws were followed by the kinetics. Although the weight gains of samples containing yttrium were consistently 10–20% greater than those without yttrium, the steady-state scaling rates were identical. A quantitative x-ray diffraction technique was used to determine the kinetics of growth of the protective alpha-alumina layer (one of several oxides formed). The alumina growth followed the parabolic rate law under all conditions studied. The rate-controlling transport process in alumina was the enhanced diffusion of oxygen down grain boundaries. The presence of yttrium as nickel-rich intermetallics promoted the formation of nickel aluminate (spinel). A marked increase in scale adherence was observed for short times. At longer times, however, the outer layer of spinel and unreacted nickel oxide spalled off along with some of the inner alumina layer. Loss of adherence was caused by a complex yttrium-aluminum oxide which formed by the solid-state reaction of yttria and alumina. The poor scale adherence on Ni3Al was due to the formation of voids at the alloy-oxide interface. These voids concentrated the athermal stresses above the oxide-to-metal adherence strength. The voids were produced as a result of the selective oxidation of aluminum resulting from a “Kirkendall” effect in the substrate. During the selective oxidation process, a vacancy flux directed from the matrix to the metaloxide interface resulted in a supersaturation of vacancies. Equilibrium was maintained by the condensation of excess vacancies. The presence of yttrium as either nickel-rich intermetallics or internal oxide prevented the voids from forming. The yttrium-rich particles relieved the matrix of vacancy supersaturation by providing vacancy sinks. The chemical nature of the particles does not seem important. A necessary and sufficient condition for an effective vacancy sink appears to be the presence of an incoherent boundary between particle and matrix.

The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys

The effect of quaternary additions of 0.5% Y and 0.5 and 1.0% Th to a base alloy of Ni-10Cr-5Al on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000–1200°C. The presence of yttrium decreased the oxidation kinetics slightly, whereas the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements. Although a number of oxides formed on yttrium-containing alloys, quantitative x-ray diffraction clearly showed that the rate-controlling step was the diffusion of oxygen through short-circuit paths in a thin layer of alumina that formed parabolically with time. Mixed oxides containing both aluminum and yttrium formed by the reaction of Y2O3 to form YAlOP3 initially, and Y3Al5O12 (YAG) after longer times. Although the scale adherence of the yttrium-containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles that grew in a “mushroom”-like manner, lifting the protective scale off the substrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi9. This intermetallic compound reacted to form Y2O3, liberating metallic nickel that subsequently reacted to form NiO or NiAl2O4 spinel or both. The Y2O3 reacted with aluminum to ultimately form the YAG “mushrooms.” Thorium did not form any mixed oxides; the only oxide involving thorium was ThO2, which existed as small particles at the oxide-metal interface. A highly beneficial effect of the thoria particles in reducing film spalling was observed. Scale spalling in the base alloy was attributed to void formation at the oxide-metal interface, the voids forming by condensation of excess vacancies from the Kirkendall effect associated with fast back-diffusion, of nickel into the substrate as aluminum was preferentially oxidized and diffused slowly outward. The mechanism of improved scale adherence in the quaternary alloys was the elimination of voids by annihilation of the Kirkendall vacancies at vacancy sinks introduced by the noncoherent interfaces between yttrium and thorium-containing intermetallics or oxides or both.

The Effect of Silicon and Manganese on the Oxidation Mechanism of Ni-20 Cr*

Additions of 3% silicon or manganese to Ni-20 Cr reduced the oxidation rate, whereas additions of 1% had little effect. Three percent silicon alloys formed an inner scale of SiO2, and 3% manganese alloys formed an inner spinel layer of essentially pure MnCr2O4. The experimentally determined solid-state growth rate of NiCr2O4 was about 1000 times slower than the growth rate for Cr2O3. It has been established that the protective layer on Ni-20 Cr (Nichrome alloys) is the spinel and not Cr2O3 as previously postulated. The mechanism for scale growth is discussed for Ni-20 Cr alloys.

International Workshop on High-Temperature Corrosion

The results of a third international workshop on “New Knowledge and Open Questions of High-Temperature Corrosion” that took place in August 1994 in Gohrisch, Saxony, Germany, are presented. The workshop was sponsored by Stiftung Volkswagenwerk and the Electric Power Research Institute (EPRI). Twenty-eight leading corrosion scientists from Europe, North America, and Australia participated. The discussion of nine subject areas in the form of key questions and proposed answers is presented.

Effect of Nb on the high-temperature sulfidation behavior of cobalt

The sulfidation behavior of Co-Nb alloys containing up to 30wt.% Nb was studied in sulfur vapor at a pressure of 0.01 atm in the temperature range of 600–700°C. Increasing niobium content decreased the sulfidation rate, following the parabolic rate law. An activation energy of 25.6 kcal/mole was obtained for Co-10Nb, Co-20Nb, and Co-25Nb, while a value of 20.5 kcal/mole was found for Co-30Nb. All were two-phase alloys, consisting of solid solution α-Co and the intermetallic compound, NbCo3. The two-phase alloys formed a rather thick outer layer of cobalt sulfides and a heterophasic inner layer that was complex. The inner layer always contained the mixed sulfide CoNb2S4 which, depending on the alloy composition, coexisted with cobalt sulfide, NbS2, and / or NbCo3 particles. Short-time sulfidations showed that the solid solution initially sulfidized rapidly to form nodules of cobalt sulfide, whereas the NbCo3 phase formed a thin protective layer of NbS2. The nodules grew laterally until they coalesced into the continuous, outer thick layer, while the NbS2 completely or partially reacted with the cobalt sulfide to form CoNb2S4. Platinum markers were always found at the interface between the inner and outer scales, the location of the original metal surface.

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr2O3 and an inner layer of either (or possibly both) SiO2 and Fe2SiO4. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr2O3 to the gaseous phase CrO3. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr2O3, which is a much more protective scale.

High-temperature sulfidation behavior of Ni-Nb alloys

AbstractThe sulfidation properties of Ni-Nb alloys containing additions of niobium up to 40 wt.% have been studied at

High-temperature sulfidation of Fe-Nb alloys

P_{S_2 } = 0.01