Materials Science Forum Vols. 522-523

Abstract: A better understanding of the transport properties of gases in oxides is certainly very important in many applications. In the case of metals, a general protection measure against corrosion implies formation of a dense metal oxide scale. The scale should act as a barrier against gas transport and consequently it needs to be gas-tight. This is often assumed but rarely, if ever, confirmed. Hence there is a need for characterization of micro- and/or meso- pores formed especially during the early oxidation stage of metallic materials. This paper presents a novel and relatively straightforward method for characterization of gas release from an oxide previously equilibrated in a controlled atmosphere. The geometry of the sample is approximated to be a plate. The plate can be self-supporting or constitute a scale on a substrate. A mathematical model for calculation of diffusivity and gas content is given for this geometry. A desorption experiment, involving a mass spectrometer placed in ultra high vacuum, can be used to determine diffusivity and amount of gas released with aid of the mathematical model. The method is validated in measurements of diffusivity and solubility of He in quartz and applied in characterization of two Zroxides and one Fe oxide. From the outgassed amounts of water and nitrogen the H2O/N2 molar ratio can be used to estimate an effective pore size in oxides.

Abstract: Phase equilibria in the Ni-Al-Cr system at 1150°C were investigated by using annealed ternary alloys and diffusion couples. The phase constitutions, microstructures, and tie-lines were determined by conventional techniques, including X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The solubility limits of Cr was found to be about 9at.% in the γ’ and 17at.% in the β phase. The size of second or third phase precipitates in the alloys was found to affect the determination of the tie-lines by the micro probe analysis.

Abstract: We propose a multi-element hopping model, which shows the flux equations of various cations under high electric field. The model is applied to the initial oxidation of binary alloys which produces a homogeneous single layer solid solution oxide film. The flux equations by this model for two kinds of cations were substituted to the oxidation growth rate definition equation. Finally, it is shown that the initial oxidation rate law of dilute binary alloys can be obtained as a type of inverse logarithmic rate law.

Abstract: This review discusses key papers presented at an EPRI sponsored Workshop on “Scale Growth and Exfoliation in Steam Plant” that was held at the National Physical Laboratory (NPL) in September 2003 [1]. Additionally, some more recent developments on modelling both scale growth and exfoliation are described. Scale exfoliation in the steam circuit of power plant boilers leads to tube blockages and, further downstream in the power plant, to erosion of the steam turbine blading; and this can have serious consequences for plant performance. Factors controlling this behaviour are reviewed. These include the thermochemistry of oxide formation as a function of operating conditions, scale microstructure and scale growth rates. It is well known that the oxidation rate of steels in steam is about an order of magnitude greater than that in air or oxygen, but the mechanism responsible for this increased rate is still unclear. Various hypotheses, which consider transport of volatile species through cracks and pores, diffusion of OH - or protons and direct access of steam to the metal oxide interface, are proposed to account for the increased rates of reaction in steam compared with air. Modelling exfoliation of thick oxide scales is considered in a number of ways. The basis of the original model by Armitt et al [2] has been extended and further developed. A popular approach is to assume that an oxide layer develops through-thickness cracks when a critical tensile stress (the oxide strength) or strain (the oxide strain to failure) is encountered. Another approach applies fracture mechanics principles to defects that are assumed to exist in the oxide layer, although there is great uncertainty regarding the relevant defect size distributions that control behaviour. A third lower bound (and conservative) approach is to consider the energetics of steady state through-thickness cracking that involves the fracture energy for through-thickness cracking and avoids the difficult issue of needing to know the defect size that initiates through-thickness cracking. Additionally, the need to incorporate kinetics of scale growth into the developing exfoliation models is briefly discussed.

Abstract: Three commercial martensitic steels have been oxidised in steam at 600 and 650 °C for times up to 10000 h. The partition of minor elements within the oxide scales has been determined. Silicon forms an additional oxide layer beneath the spinel. Chromium, molybdenum and tungsten concentrate in the spinel and manganese is present in both the spinel and magnetite. Several proposed mechanisms for steam oxidation have been examined to explain the observed effects of alloy composition. Modification of the oxide defect structure and oxidant gas penetration through microcracks were identified as the mechanisms most able to explain the influence of alloy composition.

Abstract: For long time it is known that protons in aqueous solutions have a detrimental effect on metallic materials. Relatively recently, it has also been observed in aqueous solution that the pitting corrosion resistance of Cr, stainless steel 304 and 310 decreases and the anodic dissolution rate increases due to the presence of hydrogen in the metal. In gas phase a high oxidation rate has been observed for hydrogen containing Cr and Fe. Hydrogen in the substrate can also enhance the oxidation of Fe in SS 316 and As in GaAs. All these results suggest enhanced dissolution in aqueous solution and enhanced oxide growth at the oxide/gas interface in gas phase oxidation due to hydrogen promoted outward-transport of substrate components. A possible mechanism for such out-transport is an increased metal ion diffusivity in the metal-oxide due to a high abundance of metal ion vacancies generated by hydrogen. In contrast to all the above examples, also positive effects of hydrogen have been identified under certain conditions. In an attempt to understand both the negative and the positive effects the concept of a beneficial, balanced oxide growth is used. In this concept a certain amount of hydrogen can be beneficial in the oxidation by improving the balance between oxygen-ion and metalion transport, leading to more dense and protective oxides. Depending on the temperature, H2 in air is considered as either a sink or a source for hydrogen in materials.

Abstract: The effect of S in steels on high-temperature steam oxidation resistance was investigated with respect to the content and the state in high Cr ferritic steels. The beneficial sulfur effect on high-temperature steam oxidation resistance was verified in high Cr ferritic steels. It was considered that Cr was enriched in the vicinity of the segregated S on the specimen surface because of a strong affinity between Cr and S atoms, resulting in the easy formation of the passive Cr2O3 oxide layer on the surface even after the steam oxidation test for a short time. It was found that the precipitated S operated more effectively to the improvement of the steam oxidation resistance compared to the solid-solution state of S in the steels. Furthermore, the sulfur effect on the high temperature steam oxidation resistance was related strongly to the amount of dissolution hydrogen in the high Cr ferritic steels.

Abstract: The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at 650°C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H2 produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H2 affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H2-H2O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H2 formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.

Abstract: Hydrogen dissolved in the Cr2O3 scale formed on the stainless steel in the H2O-containing atmospheres is observed by TDS (thermal desorption spectroscopy) measurements. The amount of dissolved hydrogen in the Cr2O3 scale reaches a maximum about 0.32 mol% when the H2O concentration in the gas reaches 20%. It was found from GDS (glow discharge spectroscopy) measurements that hydrogen may exist at the oxide scale / substrate interface or in Cr2O3 scale bounded that interface. However, results from the Vickers hardness and the observation of scale morphology by SEM (scanning electron microscopy), hydrogen dissolved in the Cr2O3 scale would have little effect on a decrease in the mechanical property of the Cr2O3 scale. Therefore, hydrogen dissolved in the Cr2O3 scale may not be main factor of the deterioration of the Cr2O3 scale.

Abstract: Development and use of high-speed steels for manufacturing the roll outer shell of hot strip mills represented a major recent technological advancement in the hot rolling field. However, it was observed that the oxidation behavior of these steels was different from that of conventional rolls. The high-speed steel oxidation rate is about four times higher than the alloys previously used. Furthermore, the rolling conditions are quite aggressive. Contact of the roll with the hot strip, air and water of the cooling system is expected to increase the oxidation of the roll surface in this wet atmosphere. Therefore, it is necessary to study their oxidation behavior in order to achieve the full potential of high-speed steel rolls for hot strip mills. In the present work, the oxidation behavior of three high-speed steels with differences in chromium content was studied. Corrosion tests were carried out in a thermobalance under dry and moist (12.5 % H2O) atmospheres at 765oC for 240 minutes. The corroded samples were examined by X-ray diffraction, scanning electron microscopy and energy dispersive micro-analysis. It was found that the variation of chromium content of the high-speed steels studied was sufficient to influence the oxidation behaviour. Samples with high chromium contents presented smallest final mass gain. The presence of humidity had a significant effect on the oxidation behavior.