where x is weight gain (mg mm−2) and t is time (ks). The weight gain rate constant kp versus testing temperature curves were plotted in the Arrhenius diagram, as shown in Fig. 11. It can be found from the figure that the activation energy [8] in the 10 vol.% HCl is lower than that in the wet air and the 0.5 vol.% HCl. This result confirms that the growth of the oxide films is active as non-protective films and that the oxidation is accelerated by hydrogen chloride.