A model for the oxidation of mild steel in CO2 is proposed, which is an extension of the ideas of Bruckman, Romanski, and Mrowec. A single layer of magnetite forms initially by short-circuit solid-state transport of cations. Lattice vacancies are injected into the underlying metal and eventually cause loss of scale-metal adhesion in some areas. Microchannels develop in the overlying oxide and the scale continues to grow at both oxide-metal and oxide-gas interfaces. In this duplex stage of growth, inner layer oxide nodules form in the vacancy condensation volume produced by departing metal. Their growth is restricted by a build-up of CO released by the oxidation reaction, so that a microporous structure is perpetuated. Breakaway oxidation is the result of local destruction of CO when a catalyst for the Boudouard reaction eventually forms. The inner layer crystals then grow in an atmosphere of higher oxygen potential, and deposited carbon produces a very porous structure.

A model for mild steel oxidation in CO2

Reflectance spectra of mafic silicate-opaque assemblages with applications to meteorite spectra

Fe-C alloys containing 0.1, 0.5, and 1.0% C were oxidized in 1 atm O2 at 500°C. Two specimen preparations were used: annealed followed by slow cooling to form coarse pearlite plus proeutectoid ferrite or cementite; and cold-worked by abrading after annealing. The cold-worked alloys oxidize more rapidly. Annealed pearlite oxidizes faster than annealed ferrite. The differences in oxidation rate are caused by differences in the Fe3O4 grain size, that is, by the number of oxide grain boundaries available to act as easy diffusion paths for the outward diffusion of Fe through the Fe3O4. The oxidation rate constant is 10 times larger for fine-grained poly crystalline oxide than for oxide in which the Fe3O4 is monocrystalline.

Oxidation of Fe-C alloys at 500°C

https://digital.library.unt.edu/ark:/67531/metadc927862/m2/1/high_res_d/960276.pdf

Oxidation kinetics of high strength low alloy steels at elevated temperatures

Archaeological evidence suggests that a wide range of metalworking techniques was employed during the Iron Age in Britain. This study examines the metalworking tools which have survived, principally those made of iron, and includes hearth implements, and tools for forming, decorating, and finishing metals. The ferrous metalworking tools are analysed in terms of typology and technology. Their occurrence in different types of archaeological contexts is examined, in particular relationships with metalworking residues. Functional and social use of the tools is discussed. Forty-one edge tools and six hearth implements are examined by metallography. The results are discussed according to tool type, and are further assessed by comparison with other categories of Iron Age ferrous artifacts (from published sources), and with metalworking tools of similar date from the Continent.

Pre-roman iron age metalworking tools from England and Wales: their use, technology, and archaeological context

The formation of thin oxide films (<1000 A) on polished iron-carbon alloys in the temperature range of 200–350°C and in 100 Torr of dry oxygen was investigated. A combination of vacuum microbalance and transmission electron microscopy techniques were employed to characterize the oxidation process kinetically and morphologically, respectively. The initial oxide formation on the iron-carbon alloys obeyed a two-stage logarithmic kinetic mechanism with activation energies of 0.26 eV and 0.32 eV for the first and second stage, respectively. These kinetics and activation energies are in agreement with the two-stage logarithmic kinetic theory developed by Uhlig, assuming electron transfer as the rate-controlling mechanism. Logarithmic kinetics were followed by parabolic kinetics with an activation energy of 1.08 eV, indicating cationic grain boundary diffusion as the rate controlling mechanism. The oxide formed over the iron-carbon alloys at 300°C was primarily due to the oxidation of the ferrite phase, but a thin protective oxide film (~ 150 A) was formed on the carbide phase. At later stages, a lateral growth of the oxide crystallites formed over the ferrite phase resulted in a gradual coverage of the carbide phase. This lateral growth was the major effect of the second phase on the oxidation of two-phase iron-carbon alloys. A systematic study of the effect of the phase boundary length on the oxidation of iron-carbon alloys was quantitatively evaluated.

H. J. Kim

Papers

The oxidation of iron-carbon alloys at low temperatures