oxidation rate

1 متالوژی:: سرعت اکسیداسیون

Despite the rapid initial reactions, the longer-term oxidation rate of iron under isothermal-oxidation Instead, the parabolic oxidation rate constant decreased with time, which was attributed to the formation of voids by the coalescence of vacancies at the scale-iron interface. As will be discussed later, there exists a critical oxidizing gas flow rate at a given oxidation temperature, below which the iron oxidation rate increases with the gas flow rate, and above which the iron oxidation rate becomes flow-rate independent. The parabolic oxidation rate constants of cold-worked samples at 400 to 550C are typically high initially, drop steeply after the initial stages, and then approach constant values at longer oxidation times.65,68 As in the case of oxidation at 580C and 600C, the scale struc tures developed at 400 to 550C on cold-worked samples are more adherent to the iron substrate, and more uniform in thickness, comprising a thin hematite layer and a thick magnetite layer.65,68 More severe cold working applied to the sample surface has been reported to result in more uniform, thicker, and more adherent scale, together with a lower hematite-magnetite thickness ratio The thickness ratios between the hematite and magnetite layers developed at 500 to 550C on samples prepared by coarse mechanical polishing or grinding were in the range 1:10 to 1:20. The oxidation rates of annealed samples are typically much lower and the parabolic rate constant decreases progressively with time due to the gradual loss of contact between the scale and the iron substrate.69 The scales are also nonuniform in thickness; at locations where the scale remains in contact, it is relatively thick, whereas at locations where the scale-iron con tact was lost, it is very thin.

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