The present study is composed of two parts. One is the study on "Effects of Alloying Elements (Mo, W) on the Localized Corrosion and Sigma Phase Formation of 25%Cr Duplex Stainless Steels", and the other is the study on "Investigation of the Electronic Properties of passive films on Fe, Cr and Fe-25Cr alloy by photo-elctrochemical methods and electrochemical impedance spectroscopy".
[STUDY Ⅰ]
Effects of W addition on the localized corrosion resistance and sigma (σ) phase formation rate of 25Cr% duplex stainless steels were investigated. 25 Cr-7Ni-xMo-yW-0.25N (x=1.5∼3, y=0∼3) duplex stainless steels were designed to have the same pitting resistance equivalent (PRE) value of 42 by varying the content of Mo and W. Effects of W on the pitting and stress corrosion for solution annealed or aged alloys in chloride solution were examined by anodic polarization test, critical pitting temperature (CPT) measurement, and slow strain rate test. Influences of W on the degradation in mechanical properties of alloys caused by the precipitation of secondary phases during aging at 850 ℃ were investigated by Charpy impact and tensile tests, XRD, and back scattered scanning electron microscopy. For the designed alloys, the resistance to pitting and stress corrosion increased with the alloying ratio of W to Mo content. During aging, the alloys were rapidly embrittled by the precipitation of σ phase with the rate of embrittlement being delayed significantly with an increase in W content. Thus, the alloy containing 3W-1.5Mo among the designed alloys exhibited the highest resistance to pitting and stress corrosion in the solution annealed condition, and also the highest resistance to the embrittlement induced by aging. The degree of degradation in the corrosion and mechanical properties of the alloys during aging was closely associated with the amount of precipitates. The addition of W to 25% Cr duplex alloys retarded the nucleation and growth of the σ phase during aging, thereby delaying the degradation in corrosion and mechanical properties of the alloys. The retardation of the precipitation of σ phase by W during aging is due to its inherently slower diffusion rate compared with that of Mo. Furthermore, W in the alloys caused a preferential precipitation of the chi (χ) phase along the grain boundaries, and hence inhibited the nucleation and growth of the σ phase by depleting the W and Mo around the c precipitates. This beneficial effect of W on the retardation of σ phase precipitation was found to be most dominant in the alloy containing 3W and 1.5Mo.
[STUDY Ⅱ]
Electronic properties of passive films formed on Fe, Cr and Fe-25Cr at various potentials in pH 8.5 buffer solution were examined by the photo-electrochemical methods. Influences of applied potential were also examined on the passive film on these metals formed at the anodic potential close to the transpassive potential of each metal through photo-electrochemical method and impedance spectroscopy. Photocurrent of passive films on Fe, Cr and Fe-25Cr were measured using a monochromatic illumination provided by a scanning digital monochromator automatically controlled by stepping motor.
Structure and composition of the passive film formed on Fe at potentials 0 to $800mV_{SCE}$ were investigated by photocurrent measurement. It was confirmed by the photocurrent spectra that the passive film on Fe primarily composed of $\gamma-Fe_{2}O_{3}$ whose structure have known to be spinel with some vacancies. In addition, the adsorbed hydroxide layer, $\gamma$-FeOOH, was formed at potentials above $600mV_{SCE}$. Electronic band structure of the passive films on Fe was described by the band structure model of the spinel which causes two types of electronic transition under illumination, i.e., the p-d ($O^{2-}+Fe^{3+}$ → $O^{-}+Fe^{2+}$) and the d-d ($2Fe^{3+}$ → $Fe^{2+}+Fe^{4+}$) transition. Photocurrent spectrum at low photon energy region was dominated by the d-d transition, on the other hand photocurrent spectrum at high photon energy region was formed by the p-d transition as well as the d-d transition. Each photocurrent spectrum of the passive film on Fe was analyzed through dividing themselves into two spectra depending on the transition type by which the photocurrent was raised. Contribution of the d-d transition on the photocurrent spectra increased and the indirect band gap energies for the d-d transition decreased from 3.0 to 2.8 eV as film formation potential increased from 0 to $800mV_{SCE}$. Effect of applied potential on the passive film formed on Fe at $800mV_{SCE}$ was investigated by photocurrent measurement and impedance spectroscopy, and the result was interpreted by the model above. Structure and composition of the passive film on Fe were changed with an increase of applied potential by the formation of adsorbed $\gamma$-FeOOH, which was confirmed from Mott-Schottky plot and the photocurrent intensity vs. applied potential plot.
Structure and composition of the passive film formed on Cr at potentials -500 to $300mV_{SCE}$ were investigated by photocurrent measurement. The passive film on Cr formed in pH 8.5 buffer solution revealed dual structure which involves inside hydrated $Cr_{2}O_{3}$ film and outside $Cr(OH)_{3}$ type hydroxide film. The passive film formed on Cr at low anodic potential ($-300mV_{SCE}$) was mainly composed of $Cr(OH)_{SCE}$ type hydroxide with 2.5 eV band gap energy. As the film formation potential was increased, the hydrated $Cr_{2}O_{3}$ film with 3 eV band gap energy appears to grow at the inside of $Cr(OH)_{3}$ type hydroxide. Effect of applied potential on the passive film formed on Cr at $300mV_{SCE}$ was investigated by photocurrent measurement and impedance spectroscopy. According to Mott-Schottky plot and photocurrent spectral change with applied potential for the passive film on Cr formed at 300 mV, the hydrated $Cr_{2}O_{3}$ shows n-type, but $Cr(OH)_{3}$ shows p-type semi-conducting behavior. These results were explained by the schematic band structure model with two spcace charge layers - one is between the hydrated $Cr_{2}O_{3}$ film and the $Cr(OH)_{3}$ and the other is between the $Cr(OH)_{3}$ ant electrolyte.
Structure and composition of the passive film formed on Fe-25Cr at potentials 0 to $900mV_{SCE}$ were investigated by photocurrent measurement. Electronic properties of the passive film on Fe-25Cr were found to be changed with film formation potential through the compositional change of the passive film induced by the dissolution of Cr. The passive film formed on Fe-25Cr at potentials 0 to $900mV_{SCE}$ showed n-type behavior in its photocurrent spectra. Band gap energies for the passive film on Fe-25Cr were 3.0 ∼ 3.2 eV when the film formation potential were 0 ∼ 600 $mV_{SCE}$, and abruptly decreased as the film formation potential was increased to be higher than 600 mV. Effect of applied potential on the passive film formed on Fe-25Cr at $800mV_{SCE}$ was investigated by photocurrent measurement and impedance spectroscopy. Donor density of the passive film formed on Fe-25Cr at $800mV_{SCE}$ evaluated from the Mott-Schottky plot was 3 times higher than that of Fe or Cr, and such a high donor density presumably induced the inversion in the passive film on Fe-25Cr at comparably low anodic applied potential thereby causing the passive film on Fe-25Cr to deviate from ideal n-type film behavior.
