Internal Oxidation Of Aluminum Research Articles

The evaluation of oxide scale formation during hot corrosion of Rene-80 superalloy under thermal shock at different dwell times

ABSTRACT In this study, the oxide scale formation on the Rene-80 superalloy after hot corrosion under thermal shock has been investigated. The X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM) were used for the phase identification and evaluation of oxide scale, respectively. The hot corrosion test under thermal shock was performed by depositing Na2SO4 salt at the beginning of each cycle on the surface of the specimens. The results showed that hot corrosion occurred intermittently with the basic fluxing mechanism of the oxide scale and re-precipitation of discontinuous and porous oxides. The growth rate of the Cr2O3 layer increased with decreasing the dwell time per cycle and the dissolution of titanium. Decreasing the dwell time in each hot corrosion cycle increased the growth rate of the Cr2O3 layer, the chromium depletion from the substrate/oxide scale interface, the internal oxidation of aluminium, and the internal sulfidation of chromium.

High temperature corrosion of alloy 617 OCC at 700 °C in simulated USC power plant environment

High-temperature corrosion of the nickel based superalloy 617 OCC (Optimised Chemical Composition) was studied in the context of proposed application in ultra-supercritical (USC) power plants. Studies were carried out in USC power plant environment simulated in the laboratory. Simulation was performed by coating the alloy with a salt mixture consisting of K2SO4, Na2SO4, Fe2O3, SiO2 and Al2O3 and exposing it to a flowing gas mixture comprising of 15% CO2, 3.5% O2, 0.25% SO2 and the rest N2 (Vol%) at 700 °C. For the sake of comparison, corrosion studies were also carried out under the same conditions, but without the gas mixture. Progress of corrosion was monitored by thermogravimetry, XRD and cross sectional studies on corroded specimens using SEM/EDAX analysis. There was evidence for both external and internal sulfidation with the flowing gas mixture, suggesting that the SO2 present in the gas mixture plays a dominant role. Chromium present in the alloy reacted with the sulfur-containing species in the environment forming chromium sulfide. No internal oxidation of aluminum and titanium was observed in the presence of flowing gas mixture. Internal oxidation was observed, in contrast, without flowing gas mixture at longer exposure times. The paper discusses the findings and possible corrosion mechanisms, coming into play.

Oxidation and Hot Corrosion Behaviors of Service-Exposed and Heat-Treated Gas Turbine Vanes Made of IN939 Alloy

Oxidation and hot corrosion tests were conducted on service-exposed and heat-treated IN939 alloys at 830, 930 and 1030 °C for testing times up to 800 h. The degradation behaviors were studied using optical and scanning electron microscopes. The oxidation results showed no tangible weight change in the samples at 830 °C. At 930 °C, after initial weight gain, the oxidation samples showed weight loss; whereas a continuous weight loss was observed at the higher temperature of 1030 °C. In the hot corrosion tests, however, a large weight loss occurred in the samples even at 830 °C, indicating an effect of fuel impurities on the high-temperature behavior of the alloy. SEM observations revealed that the main features of oxidation and hot corrosion of the alloy were internal oxidation of aluminum and depletion of chromium in the regions beneath the surface scales.

Surface preparation effect on oxidation kinetics of Ni-base superalloy

In the present study an oxidation behavior of commercially available Ni-base superalloy IN 713C with different surface finishing, namely polished up to 1 μm diamond paste (resulting in Ra = 0.023) and ground by SiC paper with 80 grit (Ra = 1.23), were investigated at 950°C in air. It was found that ground sample developed very thin, protective α-Al2O3 scale. Moreover, locally formation of θ-Al2O3 in the forms of spikes was observed. For the sample with polished surface formation of outer chromia scale accompanied with internal oxidation of aluminum was observed. Moreover, for specimen which does not form continuous alumina scale or sub-scale, the GD-OES depth profiles showed an enrichment of boron at the oxide scale/alloy interface. It was proposed that the formation of protective alumina scale on ground samples is caused by introducing the defects into the near-surface region of the material, which are an easy diffusion paths for the elements forming a protective oxide scales.

Scale Formation of Alloy 602 CA During Isothermal Oxidation at 800–1100 °C in Different Types of Water Vapor Containing Atmospheres

The oxidation behavior of the nickel base Alloy 602 CA in atmospheres, relevant to gas separation units in fossil fired power plants, was investigated in the temperature range 800 to 1100 °C. Isothermal oxidation kinetics were determined in Ar–7 %H2O, Ar–4 %H2–7 %H2O, Ar–20 %O2–7 %H2O and, for comparative purposes, in Ar–20 %O2. The alloy formed an external alumina scale during oxidation at 800 °C, regardless of the atmosphere. Increasing the temperature results in gradual replacement of the alumina scale by an external chromia layer and internal alumina precipitates. The chromia growth rate is affected by the gas composition, due to titanium incorporation in the scale and outwardly protruding metallic nodules. The external alumina scale formation was strongly affected by surface cold work during specimen grinding. This effect, which promoted external alumina scale formation, was retained during exposure at 800 °C but was rapidly lost at higher temperatures resulting in external chromia scale formation and internal oxidation of aluminum.

Selective oxidation of Al rich Fe–Mn–Al–C low density steels

The surface quality of steels containing solutes that have a strong affinity for oxygen can be markedly compromised during processing at elevated temperatures. Here, we examine a class of steels that are being developed for low density applications and hence, have relatively large concentrations of aluminium and manganese. These two elements compete for what little oxygen is available in the predominantly hydrogen–nitrogen mixture used to protect the steel during heat treatment. It is found that although the general sequence of oxidation does not seem to vary as the aluminium concentration is increased, the stoichiometry of the oxides changes. Manganese rich oxide is always the first to form for all the dew points examined, but is rapidly overtaken by aluminium oxide when the dew point is kept below −30°C. In the latter case, the eventual formation of an alumina film also halts the internal oxidation of aluminium. The results obtained using a variety of high resolution microscopy and analytical techniques have been analysed by calculating phase diagrams and by creating a finite difference model to reveal aspects of the internal oxidation problem.

Cyclic oxidation of alumina-forming Ni–Al nanocomposites with and without CeO 2 addition

CeO 2 nanoparticles were co-electrodeposited into an alumina-forming Ni–Al nanocomposite that was composed of nanocrystalline Ni matrix and dispersed Al nanoparticles. Cyclic oxidation in air at 1000 °C for 126 h showed that the alumina-forming nanocomposite scaled faster than its counterpart which contained a CeO 2 addition, due to the formation of numerous surface NiO nodules and internal oxidation of aluminum as a result of the composite intergranular cracking during the cycling. The intergranular cracking was profoundly prevented when the CeO 2 was present.

A non-classical type of third-element effect in the oxidation of Cu–xCr–2Al alloys at 1173K

Abstract Oxidation of a Cu–2.2 at.%Al alloy (Cu–2Al) produces external scales containing Cu-oxides plus some alumina with almost no internal oxidation of aluminum. The addition of 3.9 at.%Cr to Cu–2Al has little effect on the oxidation behavior, while addition of 8.1 at.%Cr promotes a fast formation of an alumina layer in contact with the alloy with a simultaneous large reduction in the oxidation rate. This form of third-element effect, not related to the transition from internal to external oxidation of aluminum but to the inhibition of the copper oxidation, is attributed to the complete solubility between alumina and chromia.

High temperature scaling of Ni-15Cu-5Al alloy in 1×105 Pa pure oxygen

The oxidation of the ternary alloy Ni-15Cu-5Al in 1×105 Pa pure oxygen at 700 °C and 800 °C was studied. The results show that the behavior of the Ni-rich alloy is similar to that of the binary Ni-Al alloy with the same Al content in the form of an external NiO layer coupled to the internal oxidation of aluminium. The presence of 15% (mole fraction) Cu cannot modify substantially the values of relevant parameters affecting the transition from the internal to the external oxidation of aluminium. The presence of 5% Al reduces the oxidation rate of the corresponding Ni-Cu alloy during the whole oxidation stages, though 5% Al is still insufficient to form protective external alumina scales.

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5 at.% aluminum has been studied at 800 and 900°C under 1 atm O2. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.

Oxidation behaviour of an austenitic Fe–30Mn–5Al–0.5C alloy

The oxidation behaviour of an austenitic Fe–30Mn–5Al (wt.%) alloy was studied in air in the 600–900°C temperature range. The alloy exhibits good oxidation resistance below 700°C. The low oxidation resistance above 700°C is related to the occurrence of internal oxidation. The susceptibility to suffer a deep attack by internal oxidation increases with increasing the temperature. A mechanism involving oxygen dissolution in the austenitic matrix, internal oxidation of aluminium, destabilisation of the austenitic matrix and further formation of a ferrite zone is proposed to explain the high mass gains presented by these kind of alloys.

The oxidation of a directionally solidified Ni-Al-Cr3C2 alloy at 1100 and 1200°C in oxygen

The oxidation behavior of a directionallysolidified Ni-Al-Cr3C2 alloy ofover all composition Ni-12.3Cr-6.9Al-1.8C (wt.%) hasbeen investigated at 1100 and 1200°C under 1 atmoxygen. Experiments have also been carried out on specimens having the samecomposition but with a nonaligned structure. At1100°C, in both cases and unlike conventionalnickel-base superalloys with the same chromium andaluminum contents, aluminium was found to oxidize internallybeneath an external Cr2O3 scale.Although the volume fraction of the internalprecipitates was significant, they showed no tendency tocoalesce into a compact subsurface layer, but formed randomly distributed clustersin the alloy matrix. The kinetics of oxidation andmorphologies of the oxide scales were not substantiallyaffected either by thermal cycling or by the alloy microstructure. At the higher temperature,continuous Al2O3 scales formedbeneath thick layers of transient nickel andnickel-chromium-rich oxides; no internal precipitationof aluminum-rich oxides was observed. However, internal degradation of thedirectionally solidified specimens at 1200°C wasquite significant, due to in situ oxidation of primarycarbides. The multilayered scales formed at 1200°C spalled extensively on cooling as a consequenceof loss of contact, starting preferentially at theintersections of the Cr2C3 fiberswith the alloy-scale interface. The observed behaviorcan be attributed to a reduction in the availability of chromiumbecause of the multiphase structure of the alloy; this,in turn, resulted in an increase in the flux of oxygeninward, leading to internal oxidation of aluminum at 1100°C. The almost exclusive externaloxidation of aluminum becomes possible at 1200°C,probably because of an increase in the diffusivity ofaluminum in the alloy matrix.

Nb-19at%Al合金の酸素中における1000℃までの定速昇温酸化

Nb-19at%Al alloy was heated in oxygen up to 1000°C at a rate of 2°C/min. Variation during the heating was observed by a thermomicroscope and by a thermobalance, respectively. Process of the oxidation was discussed based on the data obtained by SEM, X-ray diffractometry and EPMA techniques. Obtained pieces of information were: 1) While weight change was observed only above 800°C, surface appeared to be obviously changing already at 350°C under the thermomicroscope. 2) A lamellar structure, being discerned faintly already at room temperature, was increasingly better defined with increase in temperature and was preferentially oxidized above 800°C. 3) Scale over the lamellar structure was thick and composed of a unique textile fabric pattern across its cross-section. 4) The lamellar structure was considered to appear because that part of the alloy in as-cast condition comprised a cell structure of A15-phase enmeshed in a network of A2-phase, or because that part of the alloy transformed to such a cell structure by dissolution of oxygen into the alloy, followed by internal oxidation of aluminium which resulted in supersaturation of niobium and, finally, precipitation of a network of A2-phase from A15-phase. The unique structure of the scale was considered to be caused by oxidation of a cell-structure containing internally oxidized alumina, together with gradual expansion of the scale due to a series of stepwise oxidation of various niobium oxides. 5) Scale was composed of alumina and niobium oxides.

The Regularities of Selective Oxidation of Copper-Aluminium Solid Solutions.

ABSTRACTThe regularities of selective oxidation of copper solid solutions with 2–12 at.% aluminium have been studied by TEM, XPS and AES in combination with argon ion sputtering. It has been obtained that the increase of the aluminium concentrations in the solid solutions leads to the change of the oxidation mechanism. The internal oxidation of aluminium in Cu+(2–4at.%)Al results in the formation of the microincluslons of Al2O3 In the alloy. It has been demonstrated that the selective oxidation of aluminium in Cu+(12 at.%)Al results in the formation of uniform thin (<10 nm) aluminium oxide at the alloy surface. The kinetics of this alloy oxidation obeys the parabolic law.

Ｎｉ‐２０Ｃｒ‐０．５Ａｌ合金の高温酸化に及ぼすＣｅ添加の影響

The isothermal oxidation bahavior of Ni-20Cr-0.5Al alloys with cerium contents of 0.01, 0.07 and 0.43% has been studied in air at 1, 200°C for up to 200 hours using weight-change measurements, X-ray diffraction, scanning electron microscopy, and electron probe microanalysis.Oxides externally formed during all the period of oxidation are composed of the continuous layers of NiO, NiCr2O4 and Cr2O8. The cerium additions reduce the oxidation rate by promoting the formation of a protective NiCr2O4 spinel layer. With an increase in time and the amount of cerium, the internal oxidation of aluminum developes into the alloy substrate, resulting in the improvement of the adhesion of the oxide scale. The internal oxidation of cerium is also observed in the alloy containing 0.43% Ce when oxidized for 200 hours.

Mapping of the oxidation products in the ternary Co-Cr-Al system

The kinetics and products of oxidation of alloys in the Co-Cr-Al system have been studied and four mechanisms of oxidation identified. For the first mechanism, the rate-controlling process is cation diffusion of cobalt cations through CoO, and the main oxidation product is CoO. In the second mechanism, cation diffusion through CoO is still rate controlling, but the oxidation is strongly inhibited by an inner discontinuous spinel layer. The major oxidation products are CoO and CoCr2O4. The third mechanism of oxidation consists of preferential oxidation of chromium coupled with internal oxidation of aluminum while the fourth mechanism is the preferential oxidation of aluminum with the subsequent formation of Al2O3 scales, providing the best oxidation resistance. From the oxidation data obtained, an “oxide map” from which the oxidation behavior of various alloys may be deduced is drawn for the Co-Cr-Al system at 1100°C.