scale steel interface

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The stress can be generated by the outward diffusion of iron and the associated inward diffusion of vacancies during steel oxidation,97-99 different thermal-expansion coefficients of the scales and the substrate, or by gas build-up at the scale-steel interface as a result of carbon oxidizing from the steel substrate.95 Second, the scale possesses sufficient plasticity. It has been shown that at 800C and above, the wu ¨stite phase is deformable.100-102 At lower temperatures (750C), interface separation may occur, but because of the lack of plasticity of the scales, blisters are usually not observed.93,103 Third, the scale-steel interface adhesion is weakened at cer tain locations, which could be the result of vacancy condensation.104 Finally, a gas phase accumulated inside the blisters to prevent them from collapsing or healing after they have formed.90 The gases inside the blisters can come from carbon oxidation, or from the inert gases traveling through macro- or microcracks through the scale from the surrounding atmosphere, as sug gested by Griffiths.90 Without these gases occupying the space under the blistered scales, the localized separation as a result of vacancy deposition may be healed through various vacancy-annihilation mechanisms, e.g. A thin layer of graphite was detected by electron diffraction at the scale-steel interface, which was believed to have inhibited the oxidation process. Once it exceeded a certain level, it would cause blistering or rupture of the scale.115 The carbon-bearing gases generated during steel oxidation can also be transported via the outward movement of voids originated at the scale-steel interface.49,118,120 As discussed earlier, a large number of vacancies are left at regions at or near the scale-steel interface with the continuous outward diffusion of iron through the scale. rapid iron oxidation must then either remain at the scale-steel interface or

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