Mott–Schottky analysis of passive films on Cu containing Fe–20Cr–xCu (x = 0, 4) alloys

Effect of copper on the defect density of Fe–20Cr–xCu (x?=?0, 4) stainless steel alloys was investigated in deaerated pH 8·5 borate buffer solution at room temperature using Mott–Schottky analysis. Mott–Schottky analysis revealed that the addition of copper increased the acceptor density (N_A, V_Cr^?3), i.e. decreased the Cr⁺³ content of the passive film. Also the donor densities, shallow donor (N_D1, V_O⁺²) and deep donor (N_D2, V_Cr⁺⁶), of the passive films formed were increased. XPS analysis confirmed the decrease in Cr content and enrichment of copper in the passive film of Cu containing alloys, which ultimately dictated their lower corrosion resistance, i.e. decreased film protectiveness and stability.     

Oxidation of ternary Co-Cr-W alloys

Oxidation rates in air at 1000–1250°C are reported for a series of Co-Cr-W alloys with 34–40 wt. % Cr and up to 10 wt. % W. Alloys with larger W contents exhibited slower oxidation rates and their parabolic rate constants agreed well with those for binary and ternary, Cr₂O₃ protected, Ni-base and Co-base alloys in the Co-Cr and Ni-Cr-W systems. The resulting scales were characterized by optical and scanning electron metallography, and electron microprobe analysis. The favorable effect of W additions to a Cr₂O₃-forming Co-Cr base alloy was the opposite of that reported for Ni-Cr-W alloys. The resupply of Cr to a Cr-depleted matrix beneath a protective CrO₃ scale is achieved by the dissolution (denuding) of Cr-rich second phases in the Co-Cr-W alloys. Thus, the internal oxidation of Cr beneath the Cr₂O₃ scale is avoided for high W alloys. No catastrophic failure by liquid phase formation was observed for high-W alloys oxidized 20 hr at 1250°C.     

Effect of Cr on the evolution of microstructures in as-cast ternary niobium-silicide-based composites

The aim of this report is to present the effect of chromium on the detailed microstructural evolutions in Nb-Si-Cr ternary alloys. The microstructural characteristics of as-cast Nb-Si-Cr ternary alloys with five different compositions have been studied using SEM and XRD. In addition, the lattice parameter and the microhardness of the constituent phases were also estimated. The microstructures of the investigated alloys primarily reveal the presence of three phases: (Nb, Cr, Si)_ss, Nb₅(Si_x, Cr_y)₃ and Nb(Cr_x, Si_y)₂. The nature, morphology and the amount of the phases, as well as that of the eutectic domains, are found to vary with the composition of the investigated alloys. The lattice parameter of the (Nb, Cr, Si)_ss phase decreases, whereas that of the silicide phase first increases and then decreases with increasing concentration of Cr in the investigated alloys. The effect of Cr on the microhardness of (Nb, Cr, Si)_ss is more significant compared to that of the adjacent silicide, or the laves phase or that of the eutectic domains.     

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 °C in 1 atm O₂. The 60 wt% Cr alloy was prepared by powder metallurgy (PM) and had a phase particle size of 50-150 μm. 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.     

Oxidation Behavior of Cr2N,CrNbN, and CrTaN Phase Mixtures Formed on Nitrided Cr and Laves-Reinforced Cr Alloys

A series of single-phase Cr(X), single phase Cr ₂X Laves phase, and two-phase Cr(X) + Cr₂ X alloys (X = Nb or Ta) were thermally nitrided for 24 hr at 1100°C in N₂--4H₂ and then oxidized for 2 hr at 1100°C in air. The Cr(X) phase nitrided to form Cr₂N while the Cr₂X phases nitrided to form a complex local mixture of Cr₂N/Cr and CrNbN/CrTaN, Cr₃Nb₃N/Cr₃Ta₃N phases depending on the depth in the nitrided zone. The Ta only slightly increased the isothermal oxidation rate of nitrided Cr(Ta) and Cr₂Ta-reinforced Cr alloys, compared with nitrided, unalloyed Cr. Further, the nitrided two-phase alloys Cr--9.5Ta and Cr--20Ta exhibited improved Cr₂O₃ scale adherence relative to nitrided unalloyed Cr and Cr--1Ta. In contrast, Nb was detrimental to the oxidation resistance of the nitrided Cr(Nb) and Cr₂Nb-reinforced Cr alloys, resulting in the formation of nonprotective Cr--Nb oxides rather than continuous Cr₂O₃. A phenomenological explanation for these effects based on phase chemistry and microstructural distribution is presented. Implications of these results for understanding the oxidation behavior of developmental high-temperature, Laves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.     

The oxidation of cobalt—tungsten alloys

The oxidation behaviour of CoCrW alloys containing from 0–25%Cr and up to 30%W in oxygen at 900–1100°C has been studied. In CoW alloys there is a slight reduction in the oxidation rate as the tungsten content is increased, hwoever this is much mor emarked in Co15CrW alloys. Tungsten has little effect in Co25CrW alloys. On the binary alloys and CoCrW alloys which do not form Cr₂O₃, the scale has two layers: an outer, tungsten-free layer of columnar-grained CoO, and an inner layer of CoO containing CoWO₄ precipitates together with CoCr₂O₄ particles in the ternary alloys. The relative thicknesses of the two layers and the distribution of the constituents in the inner layer depends in temperature and alloy composition. The CoWO₄ and CoCr₂O₄ particles appear to be responsible for the reduction in oxidation rate by a blocking mechanism in the inner layer. There is some evidence to suggest that tungsten additions to Co?25%Cr alloys assist the exclusive formation of Cr₂O₃.     

Effects of cerium and manganese on corrosion of Fe–Cr and Fe–Cr–Ni alloys in Ar–20CO2 gas at 818 °C

Model alloys Fe–9Cr, Fe–20Cr and Fe–20Cr–20Ni (wt.%) with Ce (0.05%, 0.1%) or Mn (1%, 2%) were exposed to Ar–20CO₂ gas at 818 °C. Scales on Fe–9Cr alloys consisted of FeO and FeCr₂O₄, Fe–20Cr–(Ce) alloys formed only Cr₂O₃, and Fe–20Cr–(Mn) alloys formed Cr₂O₃ and MnCr₂O₄. All Fe–20Cr–20Ni alloys formed Fe₃O₄, FeCr₂O₄ and FeNi₃. Cerium additions had little effects, but additions of 2% Mn significantly improved oxidation resistance of Fe–20Cr and Fe–20Cr–20Ni alloys. Most alloys also carburized. All alloys developed protective chromium-rich oxide scales in air. Different behavior in the two gases is attributed to faster Cr₂O₃ scaling rates induced by CO₂.     

Carbon stabilized A15 Cr3Re precipitates and ductility enhancement of Cr-based alloys

A new metastable A15 phase in the Cr–Re system is predicted based on full-potential linear muffin-tin orbital (FLMTO) calculations of its electronic structure and cohesive properties. While A15 Cr₃Re is unstable as a bulk compound, we demonstrate that nonstoichiometry and carbon impurities play a dominant role in stabilizing this phase and may lead to the occurrence of A15 type particles in Cr-based alloys. Thus, these A15 precipitates may scavenge carbon impurities and prevent the formation of brittle carbides and, additionally, provide an obstacle for dislocation motion. The formation of small close-packed particles appear to explain the improvement of both ductility and strength and is most likely the mechanism of the “rhenium effect” in Cr (W, Mo) based alloys.     

The corrosion of ferritic stainless steels and high-purity Fe-Cr alloys in basaltic lava and simulated magmatic gas

Three ferritic stainless steels, types 410, 430, and 446, containing 12, 17, and 26% Cr, respectively, and two high-purity binary alloys, Fe-19Cr and Fe-24Cr, were subjected to molten tholeiitic basaltic lava with a cover gas simulating magmatic gas at 1150°C for periods up to 400 hr. The oxygen and sulfur partial pressures were 9.8×10^?10 and 7.0×10^?3, respectively. All alloys formed Cr₂O₃ scales. Internal sulfidation occurred in the commercial alloys resulting in the formation of chromium and manganese sulfides. Internal oxidation of silicon also occurred. The extent of internal sulfidation decreased with increasing chromium content. There was a “critical” chromium content between 12 and 17%, above which internal sulfidation did not occur in 96 hr. However, the “critical” chromium level increased with exposure time to nearly 26% for 400 hr. Little internal sulfidation was observed in the high-purity alloys. The different behavior between the commercial and high-purity alloys may be attributed to (i) the formation of more perfect scales on the latter, which inhibited the inward migration of sulfur, and (ii) changes in the sulfur activity gradient across the scale caused by the presence of silicon and manganese.     

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.     

Physical metallurgy and mechanical properties of transition-metal Laves phase alloys

This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB₂ Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard XCr₂ second phases (where X=Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr₂, NbCo₂, and NbFe₂, while they were observed only on the Co-rich side of ZrCo₂. Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr₂, Cr–TaCr₂, and Cr–ZrCr₂ were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr₂ alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.     

The High temperature oxidation of iron-chromium alloys containing 0.1 wt. % cerium or cerium oxide

The effects of 0.1 wt% Ce in both metal and oxide form on the high temperature oxidation of Fe-Cr (Cr = 10, 12, 14, 16, 18, 20) have been investigated at 1000°C in a 0.1315 bar O₂/He mixture at a total pressure of 1 bar. The presence of Ce and CeO₂ markedly affects the oxidation characteristics of the tested alloys, causing rapid initial coverage with protective oxide, decreased overall rates of oxidation, modifications in scale morphologies and enhanced scale adhesion. There is a possibility of a “trade-off” between the amount of Cr in the Fe-Cr materials and the presence of Ce of CeO₂, since such additions have been shown to decrease the amount of Cr necessary to form a protective Cr₂O₃ scale on the alloy surface. The mechanisms by which the Ce or CeO₂ improve the oxidation behaviour of Fe-Cr alloys are discussed.     

Oxidation of Nickel and Cobalt-Based Alloys in the Presence of Condensed Sodium Sulphate

The hot corrosion behaviour of a number of nickel and cobalt-based superalloys has been examined by exposing samples to a high temperature oxidizing environment supersaturated with sodium sulphate vapour. This test seems more able to reproduce typical service behaviour than other laboratory tests. Pure cobalt is unaffected by the presence of the condensed sulphate, whereas CoW and low-chromium, CoCrW alloys undergo acidic fluxing. However, the major change produced by the continuous supply of Na₂SO₄, as opposed to the limited amount of salt available in the coating test is in the behaviour of the high chromium alloys, when the protective Cr₂O₃ layers are removed due to the formation of a Na₂CrO₄ species. Thus, the normally resistant Co25Cr7.5W alloy suffers acidic fluxing. Similarly, the Cr₂O₃ layer on the binary Co25Cr alloys is rendered ineffective; considerable ingress of sulphur into the alloy occurs. Aluminium and manganese additions seem to reduce this effect slightly by stabilising the protective oxide layer. Both these alloying additions have a higher affinity for sulphur than chromium, and this could be important. Binary Ni20Cr seems less susceptible to accelerated attack than the Co25Cr alloys, presumably due to its greater ability to maintain a protective Cr₂O₃ layer. However, addition of 3 vol.-% Y₂O₃ virtually prevents any attack by the Na₂SO₄, preventing sulphur penetration into the alloy and promoting the formation of a protective Cr₂O₃ layer. Under non-condensing conditions, all of the alloys tested oxidize in an unaccelerated manner, supporting the view that condensation of sodium sulphate is necessary for hot corrosion.     

Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

The effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine-containing atmosphere using air mixed with the salt-vapor mixture of NaCl–KCl–CaCl₂ at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo-free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr₂O₃ scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr₂O₃ scale, resulting in internal chromium chloride precipitates and a Cr-depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high-temperature chlorination of Ni–Cr alloys in salt-vapor-containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.     

Comparison of Oxidation Behavior and Electrical Properties of Doped NiO- and Cr2O3-Forming Alloys for Solid-Oxide, Fuel-Cell Metallic Interconnects

The goal of this paper was to determine if NiO-forming alloys are a viable alternative to Cr₂O₃-forming alloys for solid-oxide fuel-cell (SOFC) metallic interconnects. The oxide-scale growth kinetics and electrical properties of a series of Li- and Y₂O₃-alloyed, NiO-forming Ni-base alloys and La-, Mn-, and Ti-alloyed Fe–18Cr–9W and Fe–25Cr base ferritic Cr₂O₃-forming alloys were evaluated. The addition of Y₂O₃ and Li reduced the NiO scale growth rate and increased its electrical conductivity. The area-specific-resistance (ASR) values were comparable to those of the best (lowest ASR) ferritic alloys examined. Oxidation of the ferritic alloys at 800°C in air and air+10% H₂O (water vapor) indicated that Mn additions resulted in faster oxidation kinetics/thicker oxide scales, but also lower oxide scale ASRs. Relative in-cell performance in model SOFC stacks operated at 850°C indicated a 60–80% reduction in ASR by Ni+Y₂O₃, Ni+Y₂O₃, Li, and Fe–25Cr+La,Mn,Ti interconnects over those made from a baseline, commercial Cr₂O₃-forming alloy. Collectively, these results indicate that NiO-forming alloys show potential for use as metallic interconnects.     

Galvanic Corrosion in Molten Salts: A Discussion of the Corrosion Mechanism of Two-Phase Ni–20Cr–20/30Cu Alloys in Eutectic (Li,K)2CO3 at 650°C

The corrosion of a Ni–20Cr and three ternary Ni–20Cr–Cu alloys containing 10, 20 and 30 wt.% Cu, respectively, in a eutectic (Li, K)₂CO₃ melt was studied at 650°C under air. Ni–20Cr and Ni–20Cr–10Cu are solid-solution alloys, while both Ni–20Cr–20Cu and Ni–20Cr–30Cu are two-phase alloys composed of a Cu-depleted matrix together with a small amount of a Cu-rich phase. The results indicate that Ni–20Cr and Ni–20Cr–10Cu have similar corrosion rates, forming a scale of external NiO and inner Cr-rich oxides. The two-phase Ni–20Cr–20/30 Cu alloys corrode rapidly, producing a mixture of Ni–Cr–rich oxides and a Ni–Cu metallic phase, which may be ascribed to the accelerated corrosion of the Cu-depleted matrix coupled with the cathodic Cu-rich phase of the alloys. As compared to Ni–20Cr–30Cu, Ni–20Cr–20Cu suffers from more-severe corrosion, due to a smaller area fraction of the cathodic Cu-rich phase in the alloy. The galvanic corrosion mechanism is also discussed.     

High-temperature phase equilibria in Cr-Cr3Si two-phase alloys

Phase equilibria in two-phase Cr-Cr₃Si alloys have been investigated using metallography, XRD, and electron microprobe analysis. Cr-Si alloys with compositions of up to 15.0% Si were directionally solidified using cold crucible Czochralski crystal growth and equilibrated chemically at either 1200,1400, or 1600 °C. Measured Si concentrations of the Cr and the Cr₃Si phases were found to be significantly lower than those given by the most recent assessment of the Cr-Si phase diagram, particularly at temperatures above 1200 °C. This explains why the volume fraction of Cr₃Si in the directionally solidified eutectic is lower than that predicted by the assessed phase diagram. X-ray lattice parameter data for both Cr and Cr₃Si are also reported.     

High-temperature phase equilibria in Cr-Cr3Si two-phase alloys

Phase equilibria in two-phase Cr-Cr₃Si alloys have been investigated using metallography, XRD, and electron microprobe analysis. Cr-Si alloys with compositions of up to 15.0% Si were directionally solidified using cold crucible Czochralski crystal growth and equilibrated chemically at either 1200,1400, or 1600 °C. Measured Si concentrations of the Cr and the Cr₃Si phases were found to be significantly lower than those given by the most recent assessment of the Cr-Si phase diagram, particularly at temperatures above 1200 °C. This explains why the volume fraction of Cr₃Si in the directionally solidified eutectic is lower than that predicted by the assessed phase diagram. X-ray lattice parameter data for both Cr and Cr₃Si are also reported.