Method of evaluating corrosion resistance of metal material, method of designing alloy of high corrosion resistance, method of diagnosing corroded state of metal material, and method of operating plant

摘要

The composition of an alloy is inhomogeneous, so that the Fermi level of electrons in the surface of the alloy differs depending upon positions. It is accordingly considered that a part susceptible to corrosion and a part less susceptible thereto will coexist in the alloy. The corrosion rate of the alloy is indicated as the exponential function of a potential difference (ΔΦH) within an electric double layer. The potential difference remains unchanged as long as the Fermi level lies within the forbidden band of the electrons. However, in a range in which the Fermi level falls within the valence band of the electrons, the lowering thereof leads to the increase of the potential difference. Accordingly, a corrosion-resisting alloy is designed in accordance with the following guidelines: a) The electron energy level (Ev) of the valence band is low, b) an oxide film to be formed on the alloy is an n-type semiconductor, c) a band gap (Ec - Ev) is wide where Ec denotes the conduction band of the electrons, and d) a flatband potential (Ef1) is low. Further, the operation of a plant and the evaluation of a corrosional damage can be based on such a theory.