| Nanocrystallization has been shown to improve the corrosion resistance of stainless steel thin films by promoting Cr enrichment in the passive film as well as inhibiting the adsorption of CT ions. Unfortunately, our understanding of the mechanism of this process remains inadequate. In this work, we analyzed the corrosion behavior of a magnetron sputtered nanocrystalline (NC) Fe-Cr alloy, by means of traditional electrochemical tests, surface analysis techniques as well as the first-principle calculations, with focus on the passivation behavior, Cl" surface adsorption behavior and Cl- transport behavior within the passive film.Our findings revealed the passive films on both the coarse conventional (CC) Fe-20Cr and the NC thin film in 0.15M B(OH)3+0.075M Na2B4O7·10H2O(pH8.6) borate buffer solution to be bilayer structured, composed of an outer hydroxide layer and an inner (barrier) layer of metal oxides. Compared with CC Fe-20Cr, the NC thin film manifested Cr enrichment in the passive film and the barrier layer/substrate interface, respectively. First principles computational simulations in the framework of the density functional theory were adapted to model the experimentally observed Cr enrichment in the interface and the passive film by constructing a FeO/Fe interface and Fe12O18 cell as the model systems. The results identified the passive film/metal interface as the most stable position for Fe substitution by Cr, with the passive film becoming more stable as Cr content increased. Combining our results with the point defect model (PDM), it becomes obvious that the nanocrystalline microstructure can accelerate Fe dissolution and Fe vacancy diffusion within the interface layer, thereby promoting Cr enrichment.The influence of alloy composition and structure on chloride ion adsorption was assessed using alloys with different Cr contents (Fe-10Cr, Fe-20Cr, Fe-30Cr, Fe-50Cr, Fe-75Cr). The results revealed that chloride ion adsorption increased steadily with Cr content up to 30% (Fe-lOCr, Fe-20Cr, Fe-30Cr) and then decreased thereafter, with Fe-30Cr having the highest amount of adsorbed chloride. It means that the alloys with Cr content> 30% (i.e. Fe-50Cr and Fe-75Cr) most likely formed Cr oxide as the main constituents of passive films, which prevents the adsorption of chloride ions. The Cl-/Fe interface was again modeled in order to explore the most stable adsorption site for chloride ions on the metal surface, as well as compute the Cl-/Fe adsorption energy and adsorption distance. The computations show that both the adsorption energy and the distance decreased with increasing Cr content, which can be related to the valence electron structure. Cr, with more 3d unoccupied orbitals. undergoes stronger hybridization with chloride ion thereby favoring chloride adsorption.The adsorption and diffusion behavior of chloride ions on the NC Fe-20Cr thin film as well as the corresponding CC alloy was also studied in HCl+NaCl solution (pH= 2, [Cl-]= 0.1M) by potentiostatic measurements. The inhibition of chloride ion absorption and diffusion within the passive film formed on Fe-20Cr NC thin film. Based on XPS analysis, we observed that nanocrystallization inhibited oxygen and chlorine adsorption and accelerated passive film formation as well as Cr enrichment in the passive film was also discussed. Furthermore, first principles calculation revealed that oxygen vacancies are more likely to form at the oxide/metal interface and that the interface with Cr enrichment would inhibit the diffusion of chloride ions. In addition, nanocrystalline promotes Fe-20Cr alloy passivation ability and enhance the corrosion resistance from the electronic perspective. |
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