Linear oxidation behavior was detected for all samples investigated in this work. A start of parabolic shape was noticed at the very beginning of the experiments, when the temperature was still not stabilized. The non-isothermal condition prevented to calculate the parabolic constant. The transition between the parabolic behavior, which means a protective oxide layer, to the linear behavior, is marked by the transformation point. Some samples have shown two different slopes during the oxidation process, in particular at the lower temperatures, 600 and 700℃. Therefore two linear constants were calculated. The second linear constant was considered to compare the linear constant of the difference samples, since it has more effect on the overall mass gain.
In Fig. 1 the linear rate constants were plotted (a) for experiments in the temperature range from 600 to 900℃ and (b) up to 1100℃. The linear constant values of the plates at 600 and 700℃ are the same order of magnitude of the linear rate constant at 1000 and 1100℃, therefore they are presented in Fig. 1b. In Fig. 1a the values between 0.01 and 0.009 g/m2 /s are shown, whereas in Fig. 1b the values not visible in the previous graph are shown in the range from 0 to 0.7 g/ m2 /s. Hence, the values for the plates at 600 and 700 ℃ lay in the second graph, being comparable to the oxidation linear constant at 1000 and 1100℃.
Fig. 1 a Linear constants of the samples oxidized at 700, 800 and 900℃ b and at 600, 700, 1000 and 1100℃
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