| Summary: | 碩士 === 國立中興大學 === 材料工程學研究所 === 84 === The effect of corrosion resistance of GX-5 Cr Ni Mo Nb
stainless steel has been explored by pre-oxidizing oxide
film in this study. The problem of the difference of
corrosion resistance between different temperature grown
oxide films can be investigated by some methods such as
electrochemical dynamic polarization , electrochemical
impedance spectroscopy and immersion test. The surface
morphology of the corroded species can also be observed by
the OM (Optical Microscopy), SEM (Scanning Electron Micr-
oscopy). TGA (Thermal Gravitation Analysis), AES ( Auger
Electron Spectroscopy) and X-ray diffraction, which are
very helpful to propose the oidation mechanisim of the
species. It
is found that the pitting potentials of the species at
lower oxidized temperatures(573K-673K) are higher than the
others, because of in the matrix is rich in chronium
chromium under the oxide film is richer and the oxidized
film is uniform and adherent. At higher temperatures
(773K-973K) the corrosion resistances of the oxide films
are worse and intergranular corrosion can be observed,
because they are within sensitizing temperature range.
Above 1073K the oxide film is out of the sensitizing
temperature range and the corrosion resistance is increased
by the higher concentration of chromium within the oxide
film. But at 1173K oxidation over thirty minutes will
decrease the corrosion resistance again. By
the use of TGA and AES analyses, the weight gain
against time of different materials at different temperatures
can increase the understanding of the oxidation mechanism.
It is found that the critical temperature near 934K, above
which the oxidation rate of chromium will surpass that of
the iron. On the other hand the oxide film will be
dominated by the iron rich oxide below the critical
temperature.
Through the electrochemical impedance spectroscopy
(EIS) analysis, it is found that the corrosion resistance is
increased with the increasing charge transfer resistance Rct
and the simulation by the software of equivalent circuit
(EQU) is also helpful to increase the understanding of
corrosion mechanism.
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