Materials Science Forum Vols. 595-598

Abstract: The present study focuses on the high temperature oxidation of a AISI 304 chromiaforming nitrided alloy. Isothermal oxidations were performed in air, at 800°C. The effect of nitridation on the steel surface depends on the temperature of the treatment. It leads whether to a γN solid solution formation or to CrN formation. In situ X-ray diffraction has been used to follow the oxides formation. Results show the concomitant growth of CrN and Fe2O3 at the beginning of the test. Then, Cr2O3 quickly appears which leads to the formation of a protective oxide scale (a parabolic rate law is observed). Our conclusions suggest that nitridation increases the high temperature oxidation resistance of 304 steels at 800°C.

Abstract: This paper focuses on a systematic description of the alumina formers oxidation behaviour by referring to a variety of observables, features and/or processes and their evolution during the exposure. An attempt is also made to identify the key effects relevant to degradation process. An output of this approach consists of the ‘Oxidation Routes’ and the ‘Degradation Paths’.

Abstract: The oxidation of iron in dry O2 and in wet O2 (40% H2O) has been studied at 600°C. The oxide microstructure was investigated by SEM/EDX, FIB and XRD. Iron forms a layered scale in dry and wet oxygen at 600°C. The scale consists of a top hematite layer, a middle magnetite layer and a wüstite layer close to the scale metal interface. All three layers grow with time, but with different growth rates, the overall growth being approximately parabolic. The presence of water vapour increases the rate of oxidation and affects the evolution of the oxide microstructure. The higher rate of oxidation in the presence of water vapour is due to an increased growth rate of the magnetite layer and, especially, of the hematite layer, while the growth of the wüstite layer is not affected. It is suggested that water vapour influences grain boundary transport in the hematite layer.

Abstract: The oxidation behavior of Fe-20at.%Cr-10at.%Al alloys with a small amount of an additional element such as W, Cu, Mn, Nb, Mo, Re, Co or Ti was investigated at 900 °C for up to 625hr. The fourth element addition to the FeCrAl alloy could be classified into two groups; elements (Mn, Nb, Ti) that are contained in the Al2O3 scale, and elements (W, Mo, Re, Co) which are not present in the scale. In the latter case, the elements (W, Cu) caused scale spallation. The rumpling of alloys with Mn, Nb or Ti was smaller than that of the other alloys. The surface of the alloy with Ti was the smooth. Pt marker experiments suggested that the Al2O3 scale formed on the alloy with Ti grew by inward diffusion of O, whilst the Al2O3 scale formed on the FeCrAl alloy grew by both outward diffusion of Al and inward diffusion of O. This different growth behavior due to the elements incorporated in the Al2O3 scale could have an effect on the surface rumpling behavior.

Abstract: Thermal cycling has been observed to cause a transition from superficial alumina formation to internal oxidation and nitridation, an effect that was shown to depend on the specimen thickness and geometry, which can be described by a spalling-probability model. Once protection by a dense and adherent alumina scale got lost, the internal-corrosion rate is determined by the diffusivity and solubility of nitrogen and oxygen in the alloy. These parameters seem to depend not only on the temperature and the alloy composition but also on the applied mechanical stress. Internal nitridation under a superimposed creep loading was found to follow a higher rate constant than under just isothermal exposure. This effect can probably be attributed to dislocation-pipe diffusion, a mechanism which has been claimed also to be relevant for outward solvent diffusion during internal corrosion, a phenomenon, which was observed as a stress-relief mechanism during various internal-reaction processes

Abstract: A titanium alloy Ti-6 Al(wt%)-4V(wt%) was treated in air by Nd:YAG laser radiation (wavelength of 1.064 %m) in continuous mode. Targets were irradiated globally with different levels of energy (accumulated fluence) at constant power. Different focal lengths and beam displacement velocities were used. Cross section microstructural observations were carried out by scanning electron microscopy. Backscattered electron imaging reveals microstructural modifications in samples. A structural phase transformation of beta (bcc) to alpha prime (hcp) phase was observed. The depth of the transformed zone observed by phase transformation is dependent on the treatment parameters. Conformity between microstructural observations and the treatment parameters is discussed with reference to thermal simulations.

Abstract: A quantitative elucidation of the void formation in a growing scale with Schottky defects and p-type conduction during high temperature oxidation of metals. The evaluation of the divergence of ionic fluxes indicates that (1) Voids form in the scale preferentially in the vicinity of the metal/scale interface, (2) The volume of voids increases in a parabolic manner, (3) The volume fraction of voids and the scale is independent of time. The comparison between the calculation and the experimentally observed scale microstructure of NiO and CoO confirmed well the validity of the prediction.

Abstract: The oxidation and nitridation of pure chromium and of chromium alloys containing 0.5 to 5at% silicon was investigated at 1300°C in several atmospheres. Global methods like thermogravimetry were associated to thickness measurements and microstructural characterisation to evaluate oxidation mechanisms. The contribution of nitridation to weight gain during high temperature exposure is discussed, examining nitride volume fraction. Experiments demonstrate that the presence of nitrogen in the substrate is always consecutive to a breakdown of the oxide layer and does not result from diffusion through the Cr2O3 layer. Silicon, when present in chromium solid solution, slows down the oxidation kinetics and limits the progression of nitrogen in the chromium lattice and at grain boundaries. The absence of the Cr2N layer beneath the metal/oxide interface does not benefit to the adherence of the oxide scale. Oxidation of silicon in chromium leads to the formation of discontinuous particles of SiO2 at the metal/oxide interface, at the grain boundaries and dispersed in the bulk in the alloy sub-surface.

Abstract: In this study, the effects of alloying with Cu and external doping with CeO2 on the oxidation of nickel were evaluated. The materials studied were pure Ni, Ni-5wt% Cu, and Ni with the surface doped with CeO2 by pulsed laser deposition (PLD). The oxidation kinetics were measured using thermogravimetric analysis (TGA). The oxidation microstructures were observed by scanning electron microscopy (SEM) compositional analysis was performed with energy dispersive x-ray analysis (EDS) and sputtered neutrals mass spectrometry (SNMS). Phase identification was performed using X-ray diffraction (XRD). The Cu additions had a negligible effect on the oxidation kinetics but Cu was found to be present in the outer portions of the scale in significant concentrations. Doping with CeO2 resulted in a significant decrease in the NiO growth rate. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. This suggests that the poisoned interface model, proposed by Pieraggi and Rapp, does not describe the effect of the CeO2 dopant. Mechanisms are presented to attempt to describe the above observations.

Abstract: Mo-Si-B alloys are attractive due to their high temperature mechanical properties and high melting temperature. The oxidation of multiphase alloys develops in two distinct stages. First, there is a transient stage that corresponds to the evaporation of the volatile MoO3 and to an initial high recession rate. The steady state stage of the oxidation begins when the slower forming borosilicate layer becomes continuous and inhibits further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to inhibit the transient stage, a coating strategy has been developed to capitalize on the interdiffusion reactions and to employ a kinetic bias to modify interface reaction products in order to maximize the high temperature stability and performance. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to synthesize metal-rich silicide and borosilicide surface layers. The analysis of the enhanced oxidation performance indicates that a strategy based upon the operating principles of interface reactions in multicomponent systems is effective for developing stable and robust coating systems.