The pre-oxidation and corrosion resistance of Gx-5 Cr Ni Mo Nb

• Main Authors: Yeh, C.J., 葉琦珍
• Other Authors: S.K.Yen
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/82199519331829094223

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.