D. Caplan

Effect of cold work on the oxidation of iron from 400–650 °C

Abstract The oxidation of Fe cold-worked to various degrees has been investigated at 400–650 °C in oxygen. From 400 to 600 °C cold-worked Fe oxidizes faster than annealed Fe and the rate is higher the greater the degree of cold work; the oxide formed on annealed Fe is porous and separates from the metal. At 650 °C cold work has no effect and solid scale only is formed. On annealed Fe below 600 °C the cation vacancies diffusing inwards through the thickening Fe 3 O 4 layer develop pores at the oxide-metal interface. This slows the oxidation rate by impeding metal transfer. On cold-worked Fe, the extra dislocations in the surface metal inhibit the nucleation of pores by acting as vacancy sinks.

Effect of carbon on cavity formation during the high-temperature oxidation of Ni

Ni containing 0.00002 to 0.003 wt.% carbon was oxidized in 1 atm O2 at 700 to 1270°C for 1 min to 20 hr. Cavity formation in the metal and oxide and at the oxide-metal interface was affected by the carbon content. Extensive cavitation developed at the grain boundaries of Ni containing as little as 0.0001 wt.% carbon but no cavitation occurred in decarburized Ni. Metal cavitation is dependent on the local concentration of carbon segregated at the Ni grain boundaries rather than on the overall carbon content. The cavities arise from hot deformation or creep of the metal substrate induced by the oxidation process. The cavities in the oxide formed on decarburized Ni remain near the oxide-metal interface; on Ni containing appreciable carbon the oxide cavities migrate outward by a dissociative mechanism assisted by the gaseous transfer of oxygen across the cavities with CO-CO2 acting as the carrier.

Oxidation of chromium at 890°–1200°C

Abstract Cr was oxidized in 1 atm of oxygen at 980, 1090 and 1200°C for periods up to 100 hr. Surface preparation has a large effect on scaling; electropolished Cr oxidizes rapidly. Non-uniform oxide layers form exhibiting nodule growth, blistering, wrinkling and multilayered ballooning. These and other observations indicate that compressive stresses develop during film thickening. This suggests that anion as well as cation diffusion takes part in the growth process and that new oxide forms within the oxide layer. The resulting continuous plastic deformation is considered in interpreting the oxidation kinetics. Best values of the rate constants are derived from measurement of layer thickness at selected areas on the metallographic cross-section.. Moisture did not affect the rate. Cr is at least as oxidation resistant as Fe-25 Cr alloy.

Comparison of the kinetics ofhigh-temperature oxidation of Fe as influenced by metal purity and cold work

Abstract The oxidation of annealed and cold-worked Fe of varying purity was investigated at 500and 550°C in Oa at latm. The apparent parabolic rate constant for cold-worked Fe is high initially, drops steeply, and approaches a constant value at long times. Annealed Fe oxidizes slowly and develops voids between oxide and metal. The faster oxidation of cold-worked Fe is caused not only by extra vacancy sinks in the metal suppressing void formation at the Fe304-Fe interface but because diffusion through the oxide is rapid initially. Three values of the oxidation “rate constant” can be distinguished —for cold-worked Fe, for annealed Fe with oxide in good contact, and for annealed Fe with separated oxide. Impurities affect the oxidation of cold-worked Fe by increasing the amount of cold work, and of annealed Fe by modifying the separation between oxide and metal.

The growth and structure of oxide films formed on Fe in O2 and CO2 at 550C

A study has been made of the structure of oxide layers formed at different times on abraded Fe oxidized in 1 atm O2 and CO2 at 550°C. A duplex Fe3O4 layer was formed and the inner layer was considered to grow by an oxide dissociation mechanism. The growth of both layers has been explained by a model, which correlates the overall kinetics with oxide grain growth. Derived values of the parabolic rate constant for lattice diffusion have been used to calculate self-diffusion coefficients, which were in good agreement with literature values for Fe diffusion in Fe3O4, but were very much larger than the values for either Fe or O in α-Fe2O3.

Effect of oxygen pressure and experimental method on the high temperature oxidation of pure Fe

Thermogravimetric measurements were carried out on annealed and cold-worked Fe at 500°C in Oa at 10 and 760torr. Runs were started by bringing the specimens rapidly to temperature in O2 or by admitting 02 to hot H2-reduced specimens maintained free of oxide in ultrahigh vacuum. Less oxidation occurred at 10torr than 760 owing to greater separation between oxide and metal. Still greater separation developed in the hot-bare type of experiment and oxidation was correspondingly slower. Cold-worked Fe showed separation and slower oxidation at 10torr but only after the oxide grew thick enough. The explanation proposed is that plastic deformation of the oxide is greater at 760torr causing collapse of the voids that form by condensation of cation vacancies at the Fe3O4-Fe interface. Oxidation is faster because there is less separation to hinder transfer of metal into the oxide. Oxidation of cold-worked Fe is the same initially at both pressures because extra vacancy sinks are present to suppress nucleation of voids; oxidation at 10torr becomes relatively slow subsequently when the thickening oxide has developed sufficient hot strength to resist the squashing effect of 10torr but not of 760torr.

Effect of cold work on the oxidation of FeCr alloys in water vapour at 600°C

Abstract Annealed and abraded specimens of Fe containing 0–26% Cr were oxidized in water vapour-argon at 600°C and the oxidation measured by the gain in weight. Cold work improved the oxidation resistance of high-Cr alloys by suppressing internal oxidation. With low-Cr alloys, annealed and abraded specimens both show internal oxidation and there is no beneficial effect of cold work; a short-term deleterious effect is observed. The oxidation of unalloyed Fe in water vapour was not affected by cold work.

Oxidation of Fe-C alloys at 500C

The oxidation of Fe-C alloys containing 0.5 and 1.0% C was studied in 1 atm O2 at 700° C. The oxidation rate is considerably slower than for pure Fe. The oxide scale formed is detached, multilayered, and overoxidized, containing little or no FeO. A thin film of graphite was identified at the metal-oxide interface by electron diffraction. It is proposed that the slow oxidation and abnormal scale are caused by a residue of graphite left at the metal surface from the oxidation of Fe3C. This inhibition of the oxidation of Fe by carbon at 700°C is in contrast to the stimulation observed at 500°C.

The growth and structure of oxide films on Fe. II. Oxidation of polycrystalline Fe at 240–320°C

The influence of surface pretreatment and metal orientation on the oxidation of coarse-grained polycrystalline Fe has been studied at 240 to 320°C in 5×10−3 Torr O2 using electron diffraction, electron microscopy, and Mössbauer spectroscopy to complement kinetic data. Consistent with previous studies on Fe single crystals, differences in oxidation kinetics for surfaces covered with an electropolish film from those with a similar thickness prior oxide formed by dry oxidation at room temperature are interpreted in terms of differing densities of leakage paths in the oxide layers. The more complex kinetics for electropolished polycrystalline Fe are a result of the leakage path density, the degree of oxide separation, and the extent of α-Fe2O3 formation varying with substrate orientation. Where adherent Fe3O4 layers are formed on polycrystalline and single-crystal Fe surfaces, the parabolic rate constants give an activation energy which is consistent with a previous value of 32 kcal · mole−1, suggesting that at these low temperatures the transport mechanism for magnetite growth is cation diffusion via easy diffusion paths in the oxide.

The effect of FeO grain size and cavities on the oxidation of Fe

Abstract The oxidation of coarse-grained and fine-grained Fe was studied at 640–805°C in 1 atm O 2 for 0.1–18 h. The FeOFe 3 O 4 Fe 2 O 3 oxide scale that formed on annealed or cold-worked, fine-grained Ferrovac Fe was uniform, adherent, and thickened with normal parabolic kinetics. Coarsegrained Battelle Fe behaved similarly when cold-worked, but when annealed it oxidized more slowly and formed non-uniform, less adherent scales. On annealed Ferrovac, cold-worked Ferrovac and cold-worked Battelle Fe, dislocations and grain boundaries suppress cavity nucleation by providing sinks for the cation vacancies arriving at the metal surface. For annealed Battelle Fe, however, some orientations develop large areas of monocrystalline FeO, separated from the metal by a narrow gap. This gap forms either because vacancy sinks are too few on these orientations to prevent cavity formation or because, in the absence of FeO grain boundaries to facilitate hot deformation, the scale cannot maintain contact with the retreating metal surface. The separation between the monocrystalline FeO and the metal is the principal reason for the slower oxidation of annealed coarse-grained Fe.