Y. Niu

The air oxidation of two-phase Cu-Cr alloys at 700-900°C

The oxidation in air of three two phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700–900°C. The alloys corroded nearly parabolically, except at 900°C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free from chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu2Cr2O4, often containing particles of chromium metal surrounded by chromia and then by the double oxide. Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.

The criteria for the transitions between the various oxidation modes of binary solid-solution alloys forming immiscible oxides at high oxidant pressures

The possible high-temperature corrosion modes ofbinary solid-solution alloys forming two immisciblecompounds by a single oxidant include (1) the exclusivegrowth of external scales of the most-noble component, which may or may not be associated with theinternal oxidation of the most-reactive component, (2)the formation of composite external scales containing amixture of the two compounds, or finally (3) the exclusive growth of the most-stable compound asan external scale. The conditions for the stability ofeach scale structure depend on a number of thermodynamicand kinetics parameters, whose effects are examined quantitatively in this paper. Theconditions for the stability of the various structuresand the criteria for the transitions among them are alsoexamined. The maximum number of possible scale structures is four, but it can reduce to threeand, in some cases, only to two. In particular, theinternal oxidation of the most-reactive component maynot occur if the stabilities of the two oxides are not sufficiently different from eachother.

The internal oxidation of two-phase binary alloys beneath an external scale of the less-stable oxide

The internal oxidation of two phase binary A-B alloys by a single oxidant at high temperatures, under partial pressures sufficient to also form external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of single-phase alloys. It is shown that, depending on various factors, particularly on the solubility and diffusivity of the most-reactive component B in the most-noble component A, this process may or may not involve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an increase of the rate of internal oxidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to single-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.

Analysis of Pore Formation at Oxide-Alloy Interfaces—I: Experimental Results on FeAl

Pores, or voids, at oxide–alloy interfaces are commonly observed after high temperature oxidation when the alloy does not contain a reactive element. In order to understand the pore-nucleation and growth processes, the density, size and depth of interfacial pores on Fe–40 at.%Al as a function of oxidation time at 1000°C were examined. Scanning-electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the pores after removal of the surface Al2O3 scale. The nucleation of pores was most rapid during the initial stage of oxidation where cation-transported-alumina growth dominates. Pore growth involves widening as well as deepening, where the deepening rate is slower for larger pores. Growth is accomplished by aluminum evaporation after ∼20 min or by surface diffusion before that time. Pore shape within an alloy grain stays constant and is dictated by the balance of surface and interface energies.

Microstructural effects on the high temperature oxidation of two-phase Cu–Cr alloys in 1 atm O2

The oxidation of a Cu-Cr alloy containing about 60 wt% Cr and of two Cu-Cr alloys containing about 40 wt% Cr was studied at 700 and 800 degreesC in I atm. O-2. The 60 wt% Cr alloy was prepared by powder metallurgy (PM) and had a phase particle size of 50-150 mum. One of the two alloys containing about 40 wt% Cr was prepared by mechanical alloying (MA) and had a phase grain size ranging from 10-50 nm to 200-300 nm, depending on the location, while the other was prepared by magnetron sputtering (MS) and had a phase grain size around 5-10 nm. The most important difference between the oxidation behavior of the three alloys is the formation of an exclusive chromia scale on the surface of the Cu-40 wt% Cr alloy prepared by magnetron sputtering and of a continuous chromia layer beneath an outermost layer of copper oxides on the corresponding alloy prepared by mechanical alloying, while the Cu-60 wt% Cr alloy prepared by powder metallurgy formed complex scales composed mostly of CuO, Cu2O with Some Cu2Cr2O4 and Cr2O3. Thus, the microstructure of two-phase binary alloys has a strong effect of their oxidation behavior. In particular, a decrease of the alloy grain size favors the exclusive external oxidation of the most reactive component, reducing the corresponding critical content in the alloy. This effect is attributed to the presence of larger concentrations of rapid diffusion paths for the migration of the components in the alloy as well as to a faster dissolution of the particles of the Cr-rich phase in the copper matrix

The internal oxidation of ternary alloys. I: The single oxidation of the most-reactive component under low oxidant pressures

The internal oxidation of the most-reactive component C of ternary A–B–C alloys by a single oxidant is examined assuming a gas-phase oxidant pressure below the stability of the oxides of the other two components. The precipitation of the most-stable oxide leaves behind a matrix composed of a binary alloy of the two less-reactive components, whose composition affects the solubility and diffusivity of the oxidant within the region of internal oxidation, with an effect on the reaction kinetics. Approximate relations between these properties are proposed and used to predict the kinetics of internal oxidation of C under the assumption of parabolic rate law. The results obtained for the ternary alloys are compared with the behavior of binary A–C and B–C alloys with the same C content. A new important factor in establishing the difference between the internal oxidation in ternary A–B–C alloys and in binary A–C and B–C alloys under a fixed gas-phase oxygen pressure and C content is the ratio between the concentrations of A and B in the bulk ternary alloy.

The high temperature oxidation of a two-phase Ag–Y alloy under 1 and 10−20 atm O2

A silver-based alloy containing about 8 wt% Y (Ag-8Y) has been oxidized under 1 atm and 10(-20) atm O-2 at 700 degreesC. The alloy contains a mixture of a solid solution of Y in Ag (alpha phase) with an eutectic mixture of the alpha phase with the intermetallic compound Ag51Y14 (beta phase). The oxidation rate in 1 atm O-2 is very fast and follows the parabolic rate law to a good approximation. Oxidation under low P(O-2) is much slower and quite- irregular with large deviations from an average parabolic behavior. Under 1 atm O-2 the alloy undergoes only an internal oxidation of Y without any significant outward Y diffusion. as a result of a fast oxygen penetration. Under low P(O-2) the alloy forms an almost continuous Y2O3 layer containing Ag particles, plus a region of internal oxidation of Y much thinner than in 1 atm O-2. In bath cases the alloy forms also an external layer of pure Ag metal, more continuous for the oxidation under 1 atm O-2. The Y2O3 particles within the alloy change shape with depth, being small and globular close to the alloy surface but larger and acicular, elongated nearly perpendicularly to the alloy surface, ai larger depths. For samples oxidized under 10(-20) atm O-2 the volume fraction of internal oxide increases with depth in the alloy, tending to form an almost continuous oxide layer close to the front of internal oxidation, except over regions very rich in Ag

Oxidation of Two Ternary Cu–Ni–20 wt.% Cr Alloys at 700–800°C in 1 atm O2

Two ternary Cu–Ni–Cr alloys containing approximately 20 wt.% chromium, but with a different Cu and Ni content, have been oxidized in 1 atm of pure oxygen at 700–800°C. The alloy containing about 60 wt.% nickel (Cu–60Ni–20Cr) was composed of a single solid-solution phase and formed external scales of chromium ocide with an outermost layer containing a mixture of copper and nickel oxides. The alloy comprised of about 40 wt.% nickel (Cu–40Ni–20Cr) contained a mixture of two metal phases and formed complex external scales, containing copper oxide and a nickel–chromium spinel plus a region where islands of the metallic phase richer in chromium surrounded by a thin chromia layer were mixed with oxidized islands rich in copper and nickel, producing a situation out of equilibrium. With time, a very irregular and thin but essentially continuous layer of chromia formed at the base of the mixed internal region for this alloy, producing a gradual decrease of the corrosion rate down to very low values. The oxidation behavior of the two alloys is interpreted in terms of their different microstructure. In particular, the fast initial oxidation of Cu–40Ni–20Cr, associated with the formation of large amounts of copper oxides, is attributed to restrictions in chromium diffusion in the alloy due to the simultaneous presence of two metal phases.

High-Temperature Oxidation of Two-Phase Nanocrystalline Ag–Cr Alloys in 1 atm O2

Two nanocystalline two-phase Ag–Cr alloys prepared by mechanical alloying and containing approximately 30 and 50 wt.% Cr were oxidized in 1 atm O2 at 700 and 800°C. Under all conditions, a continuous layer of chromia formed at the surface of the alloys, in spite of the very low solubility of Cr in Ag. A layer of AgCrO2 also formed externally to the chromia layer. In the case of the Ag–30Cr alloy, some Ag particles were also present on the scale, directly in contact with the gas phase. Moreover, Cr particles dissolved in the subsurface region of the alloy, while internal oxidation of Cr was absent. Ag–Cr alloys prepared by powder metallurgy with coarse grain sizes were able to form an irregular thin chromia layer only at a Cr content of 69 wt.%, while an alloy containing 35 wt.% Cr corroded much more rapidly than the nanocrystalline Ag–30Cr alloy. This difference in the scaling behavior is attributed to the large reduction in the alloy grain size, which favors the dissolution of the Cr-rich particles in a Cr-depleted silver matrix and thus provides a faster supply of chromium from the alloy to the scale.

The corrosion of NiNb alloys in H2H2S mixtures at 600–800°C

Abstract The sulfidation of two NiNb alloys containing 15 and 30 wt% Nb in H2H2S mixtures providing a sulfur pressure of 10−8 atm was studied at 600–800°C. The corrosion kinetics were nearly parabolic initially and then linear at 600°C but practically linear from the start at 700 and 800°C. At 600°C the scales presented an outer layer of Ni sulfide and an inner complex region containing both Ni and Nb. At 700 and 800°C only a single complex region was observed, due to the formation of a liquid NiS solution which allowed a very fast transport of reactants through the scales. The Nb addition was able to reduce the corrosion rate at 600°C due to a gradual replacement of nickel sulfide by the more protective double NiNb sulfide but was practically inefficient at the higher temperatures due to the appearance of the liquid phase.