In this report, the structure of oxide film formed on the surface of 17 Cr steel, heated at 830°C in air for 100 s, is determined by means of X-ray diffraction, electron diffraction and X-ray photoelectron spectroscopy. The descaling mechanism in electrolytes of sodium chloride and sodium nitrate is experimentally investigate. Results obtained are given in following. (1) The layer of the scale is composed of two types of oxide with a corundum structure, which is identified to be Cr
2O
3 and α-Fe
2O
3. (2) In NaCl electrolyte, layer of the scale is not dissolved at all by the anodic dissolution but the base metal is dissolved in those area exposed to electrolyte through some scale defects. (3) In NaNO
3 electrolyte, layer of the scale is removed by the dissolution of Cr
2O
3 of the scale. The formation of passive film on descaled surface is confirmed by means of X-ray photoelectron spectroscopy. Two type of oxygen with different binding energies of 531. 3 eV and 529. 6 eV, which correspond to the binding energies of oxygen in hydroxides and oxides respectively, are observed in the photoelectron spectra of O 1s of passive film on the descaled surface.
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