| 其他摘要 | In this work, 309 stainless steel nano-crystalline coatings fabricated by magnetron sputtering were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and electron transmission microscope (TEM). The electrochemical corrosion behavior of the nano-crystalline coating and the bulk 309 stainless steel was investigated in acidic solutions by using potentiodynamic polarization, potentiostatic polarization, Mott-Schottky plots measurements, electron probe microanalysis (EPMA) and X-ray photoelectron spectrograph (XPS). The effects of nano-crystallization on the passivation, pitting resistance and transpassivation of 309 stainless steel were studied. Besides, the influence of phase constituent and hydrogen on the corrosion resistance of the nano-crystalline coating was also discussed, respectively. The comprehensive investigations on the electrochemical behavior of the nano-crystalline coating would facilitate its practical application.
The effects of nano-crystallization on the electrochemical corrosion behavior of 309 stainless steel were studied in 0.25M Na2SO4 + 0.05M H2SO4 solution. The coating showed self-passivation. Once formed, the passive films of the bulk steel and the coating dissolved at the same speed. The bulk steel and the nano-crystalline coating showed similar corrosion resistance because the compact passive films with the same electronic characteristics, i.e. p-n type semiconductor, were formed on the both materials.
The pitting resistance of the nano-crystalline coating and the bulk steel was also investigated in 0.5M NaCl + 0.05M H2SO4 solution. The homogeneity the stainless steel surface was improved by nano-crystallization. Besides, the Cr content of the passive film was greatly enhanced by nano-crystallization; the electronic properties of the passive film were changed, and the carrier density of passive film was also decreased. Thus, the pitting corrosion resistance of 309 stainless steel was greatly improved by the nano-crystallization; furthermore, the pitting susceptibility to Cl- ions was also largely decreased by nano-crystallization.
The corrosion behavior of the nano-crystalline coating and the bulk steel in the transpassive region was studied in 0.5M Na2SO4 (pH=2) solution. In the whole transpassive region, the corrosion current density of the nano-crystalline coating was lower than that of the bulk steel. The nano-crystallization improved the homogeneity of Cr element in the passive film, decreased the carrier density of oxide film in the transpassive region and decelerated the dissolution of the oxide film. Thus, the corrosion resistance of the stainless steel in the transpassive region was greatly enhanced by nano-crystallization.
A single-phase (α) and a duplex-phase (α+γ) nano-crystalline stainless steel coating were prepared by adjusting the sputtering parameters. The effects of phase constituent on the electrochemical behavior of the nano-crystalline coating were discussed. The duplex-phase coating exhibited a lower localized corrosion resistance than the single-phase coating in 0.5M NaCl + 0.05M H2SO4 solution. The Ni-rich austenite in the duplex-phase coating resulted in transpassivation at higher potentials. Therefore, compared with the single-phase coating, the duplex-phase coating possessed worse stable passive film with the increase of film formation potential.
With a large amount of grain boundaries, the nano-crystalline coating provides many diffusion paths and facilitates hydrogen entering the coating. During the formation of the passive film in 0.5M NaCl + 0.05M H2SO4 solution, nano-crystallization promoted hydrogen delaying the growth of the passive film. The semiconductive structure of the passive film on the bulk steel after hydrogen charging was hardly changed. However, the passive film on the nano-crystalline coating was converted from p-n type semiconductor to n-type semiconductor after hydrogen charging, that is, nano-crystallization facilitated the change of the electronic structure of passive film. The increase in OH-/O2- ratio of the passive film on the nano-crystalline coating was much larger than that on the bulk steel; thus, the pitting resistance of the nano-crystalline coating was greatly decreased. In conclusion, hydrogen deteriorated the corrosion resistance of the nano-crystalline coating more obviously than that of the bulk steel. |
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