Yedong He

Oxidation behavior of Ni3Al and FeAl intermetallics under low oxygen partial pressures

Abstract Selective oxidation and formation of protective oxide layers are critically important for engineering alloys used at high temperatures. In this project, a solid-state electrochemical oxygen pump system was used to establish oxidizing atmospheres with controlled oxygen partial pressures, in the range of Po2 ≅ 10−15–10−22 atm, and selective oxidation behavior of nickel- and iron-aluminides under these low oxygen partial pressures was studied, special attention was paid to early oxidation stages. The results indicated that under Po2 > 10−18 atm, pure nickel particles formed on the Ni3Al specimen surface, while Al2O3 could not completely cover the surface after short time oxidation. It was believed that the outward diffusion of nickel was related to the formation of the internal Al2O3, which generated extra volume in the alloy substrate, resulting in compressive stresses. After a long time oxidation, external oxidation dominated the process; and the specimen was completely covered by an Al2O3 scale. Under the lowest tested oxygen partial pressure (Po2 ≅ 10−22 atm), formation of nickel particles was not observed, while an external Al2O3 scale formed on the surface. For FeAl samples, no outward diffusion and formation of iron particles were observed, and an Al2O3 scale formed under the present conditions.

Corrosion resistance of Zn-Al co-cementation coatings on carbon steels

Abstract Zn–Al co-cementation coatings on carbon steels have been obtained by pack cementation method. The coatings possess a two-layered structure. The outer layer is composed mainly of Fe 2 Al 5 and FeAl intermetallics with a small amount of Zn and the inner layer consists of Zn, Fe and a small amount of Al. The corrosion resistance of the Zn–Al co-cementation coatings was studied with rotary corrosion test in water and NaCl and H 2 S containing solutions. High-temperature oxidation and sulphidation were carried out in air and H 2 S containing atmosphere at 500 °C. The experimental results were compared with the coatings produced by Zn sherardizing and aluminizing, showing that the Zn–Al co-cementation coatings have much better corrosion resistance in both aqueous solutions and high-temperature environments.

Oxidation behaviour of micro-crystalline Ni–20Cr–Y2O3 ODS alloy coatings

Abstract Ni–20Cr–Y2O3 oxide dispersion strengthened (ODS) alloy coatings were deposited on the surface of Ni–20Cr alloy by a special electro-spark technique. Two methods were used to prepare these ODS alloy coatings: (a) using M754 ODS alloy as the electrode; and (b) using M754 electrode with nano-particles of Y2O3 on the surface of substrates. The ODS coatings possess micro-crystalline structure and metallurgical bonding with the substrates. The experimental results of high-temperature oxidation in air at 1000°C show that the formation of Cr2O3 scale has been promoted on these coatings with excellent scale spallation resistance. The mechanisms of the beneficial effects of these coatings were also discussed.

Modeling of oxidation kinetics of Y-doped Fe-Cr-Al alloys

Studies using advanced analytical techniques indicated that the reactiveelements (RE) segregate along the oxide grain boundaries and at theoxide–alloy interface during oxidation of α-Al2O3forming alloys. The segregation results in inward oxygen diffusion along theoxide grain boundaries as the predominant transport process in the oxidegrowth. The present work establishes a mathematical model based on themechanisms of inward oxygen diffusion along the grain boundaries and oxidegrain coarsening. This model has been used to describe the oxidationkinetics of Y-doped Fe–Cr–Al alloys. The results showed a muchbetter agreement with the experimental data than the parabolic rate law. Byusing this model, the exponential number for the grain coarsening of aluminascales during oxidation was calculated to be ∼3. The activation energyfor oxygen diffusing along the grain boundaries was 450 kJ/mol. They arealso in good agreement with values reported in the literatures.

Synthesis of Nanostructured Ni―TiO2 Composite Coatings by Sol-Enhanced Electroplating

A Ni―TiO 2 composite film was prepared by electrodeposition containing Ti precursor sol (sol-enhanced deposition). The electrochemical process, microstructures, and properties of the sol-enhanced and traditional composite coatings were studied and compared. The sol-enhanced Ni―TiO 2 composite coating possessed a smooth surface and a compact microstructure and showed higher mechanical properties (430 HV 100 ) compared with the traditional coatings (360 HV 100 ). It is believed that the strengthening effects resulted from the high dispersion of TiO 2 nanoparticles. The sol added coatings also showed slower growth of Ni grains along the [220] direction but did not change the orientation. It is suggested that the sol addition reduced the thickness of the diffusion layer and increased the limited current density. Therefore, the polarization control in the sol-enhanced process changed from the traditional concentration polarization to electrochemical polarization, avoiding the formation of loose and dendritic structures at high current density.

Electrodeposited thin oxide films

Many oxide films such as those of most alkali earth metals, most transition metals and rare earth elements can be obtained through a new process called electrochemical deposition-pyrolysis. In the present paper the formation kinetics and mechanisms and the structure and properties of AlOOH films are discussed as an example.

Oxidation Behavior of a Ti3Al–Nb Alloy with Surface Thin Oxide Films

Thin films of aluminum, cerium, and yttrium oxides were applied onto the surfaces of Ti3Al–11Nb samples using an electrodeposition technique. The oxidation behaviors of the Ti3Al–Nb alloy, with and without these surface-applied films, were studied in air at 800–1000°C. The results showed that the oxidation rate of the alloy can be reduced by Ce oxide and Y oxide films, and the greatest improvement comes from oxidation of the Y oxide-coated specimens at 800°C. With increasing oxidation temperature, the difference between the Co-oxide and Y-oxide films becomes smaller. The results also indicated that the Ce-oxide and Y-oxide films can significantly improve the oxide scale-spallation resistance. On the other hand, Al-oxide films result in detrimental effects on the oxidation and scale-spallation resistance of the Ti3Al–Nb alloy. Based on the experimental results, the effects of the different surface films on the oxidation mechanism are discussed.

Use of a Solid-State Oxygen Pump to Study Oxidation Kinetics of Cr and Mo

A new method for measuring the amounts of oxygen consumed during theoxidation of metals was recently developed for oxidation kineticstudies. This method uses a Pt|ZrO2 (Y2O3)|Pt solid-stateoxygen pump, based on electrochemical principles, and is especially suitablefor alloy systems where oxidation rates are difficult to measure withmass-gain methods. The oxidation kinetics of pure chromium and molybdenumhave been studied using this new method. The results show that this methodhas clear advantages over the traditional thermogravimetric methods inassessing the actual oxidation rates of Cr and Mo, which form volatile oxidespecies depending on temperature and oxygen pressure.

Reactive-element effect of electrodeposited Y2O3 oxide films on the oxidation of Fe−25Cr and Fe−25Cr−10Al alloys

Thin Y2O3 films were deposited by the electrochemical deposition-pyrolysis process on Fe−25Cr and Fe−25Cr−10Al alloys. The influence of the films on the oxidation behavior of the alloys was studied at 850°C and 1000°C. The results showed that Y2O3 films remarkably decreased the oxidation rate of Cr2O3-forming alloys and spallation of the scales, but they did not decrease the oxidation rate of the Al2O3-forming alloys, although they do reduce the spallation of Al2O3 scales. Y2O3 films remarkably change the morphology of the scales on both alloys, depending on the oxidation temperatures. These results show that the reactive-element effects of Y2O3 films on the Cr2O3 formers and Al2O3 formers are different.

Electrophoretic deposition of MCrAlY overlay-type coatings

A new process, which is a combination of electrophoretic deposition and pack cementation, was used to obtain MCrAlY overlay-type coatings. Using the electrophoretic-deposition process, rare-earth element is added to the deposits by using a special additive which contains yittrium, and excellent high-temperature corrosion resistance was observed after pack cementation.