Research On Structure And Corrosion Resistance Of Oxide Scale Of Hot-strip Steel

Oxide scales with different structure were prepared on SS400 hot-strip steel substrate by different technics of heat treatment. Corrosion resistance property of oxide scales on the strip was evaluated by continuous neutral salt-fog accelerated corrosion test and cyclic wet-dry immersion accelerated corrosion test. The electrochemical characteristics of different structure scales were studied by polarization curves. The morphology and structure of oxide scales with different corrosive times were investigated by optical microscope (OM), X-ray diffraction instrument (XRD), energy dispersive spectrdmeter (EDS) and scanning electronic microscope (SEM). Moreover, the effect of temperature of heat treatment, holding time, cooling-down method and oxygen content on structure and atmospheric corrosion resistance of oxide scale of hot-strip steel was studied systematically, too. The experimental results indicated that:1. Both in the middle and the margin of SS400 hot-strip steel, average thickness of original oxide scale is between 7μm and 9μm, the oxide is not continuous and there are many cracks inside of it. There is only Fe₃O₄ in the middle and are Fe₃O₄ and Fe₂O₃ in the margin.2. On the same condition of furnace cooling after holding 30min, oxidation markedly accelerated with the rise of temperature, and FeO is detected only in the oxide scale prepared at 800℃which is not denser than 700℃and 600℃. The oxide kinetics of SS400 hot-strip steel obeys parabolic law and it has some antioxidant property. With extend of holding time, growth stress is accumulated and crack appears gradually between oxide scale and matrix. With increment of oxygen content, trend of oxide increment is accelerated at the beginning and slow subsequently. There are Fe₃O₄ and Fe₂O₃ in oxide scale when oxygen is enough, and is only Fe₃O₄ when it is lacking.3. Air cooling retains the residual FeO, but it is not conducive to the combination of oxide scale and matrix. The oxide scale composed by three phrases is layered, the surface layer is Fe₂O₃, the middle is Fe₃O₄, and the inner is FeO which can prevent the further expansion of cracks inside.4. On the same condition of furnace cooling after holding 90min, corrosion resistance of oxide scale prepared at 700℃is superior to 600℃and 800℃. Thin oxide scale and surface defects lead corrosion to be accelerated, and corrosion products ofα-FeOOH can restrain the further expansion of corrosion. Corrosion resistance of oxide scale is not always consistent with the thickness. Salt-fog corrosion resistance of oxide scale through air cooling after holding 60min at 600℃is better than 30min and 120min.5. The results of continuous neutral salt-fog accelarated corrosion test and cyclic wet-dry immersion accelarated corrosion test indicated that, on the condition of equal thickness, corrosion resistance of oxide scale composed by three phases(Fe₃O₄+Fe₂O₃+FeO) is the best of the three structure, and the two phases(Fe₃O₄+Fe₂O₃) is better than the one phase(Fe₃O₄). The result of potentiodynamic polarization curves indicated that corrosion potential of oxide scale composed by three phases(Fe₃O₄+Fe₂O₃+FeO) is more positive, and corrosion current is lower, so this kind of oxide scale has the best corrosion resistance, and it is consistent with the results of continuous neutral salt-fog test and cyclic wet-dry immersion test.