Effect Of Crystallization On Corrosion Resistance Of Febased Bulk Metallic Glass Under Simulated Deep-sea Environment

Since the consumption of fossil energy on land has been exhausted,the exploitation of deep-sea fossil energy has gradually become one of the main ways to obtain new energy sources.Due to the complexity and harshness of the deep-sea environment,the corrosion resistance of common metal materials and protective coatings has deteriorated in deep sea.For example,the pitting potential of stainless steel in the deep sea will decrease.Polymer coating will accelerate aging in deep sea environment,and the chances of peeling coating increases significantly.This situation forces us to develop new high-strength and corrosion-resistant metal materials to improve the service performance and service life of materials and equipment in deep sea.Fe-based amorphous alloys have attracted widespread attention due to their high strength,excellent corrosion resistance and wear resistance.However,owing to the metastable nature of metallic glasses and bulk metallic glasses,crystallization is usually unavoidable more or less during their manufacturing procedures.Almost all of the previous studies on corrosion resistance were carried out under ambient conditions（1 atm）.However,how the crystallization affects the corrosion resistance of BMGs is rarely investigated in deep sea,and a deep understanding of the micromechanisms responsible for the different corrosion behaviors in partially crystallized BMGs is yet to be developed.Therefore,this thesis selects Fe₃₆Cr₂₃Mo₁₈C₁₅B₆Y₂（at.）,which is newly developed by our research group,as the research material.Cylindrical specimens were fabricated by copper mold casting method,and were then sealed in quartz tube in vacuum for heat treatments.The microstructure and phase compositions of as-cast and as-annealed specimens after various heat treatments were characterized by X-ray diffraction（XRD）,transmission electron microscopy（TEM）coupled with energy dispersive X-ray spectroscopy（EDX）,and differential thermal analysis（DTA）.The effects of crystallization on the corrosion resistance of Fe-based amorphous alloys under high hydrostatic pressure and corresponding corrosion mechanisms were investigated by electrochemical workstations,scanning electron microscopes（SEM）,and X-ray photoelectron spectroscopy（XPS）.It was found that when the crystallization rate was below 30%,only an ultrafine（Fe,Mo）₆C carbide precipitates in the amorphous matrix.In addition,the corrosion rate and passive film properties of partially crystallized sample remained almost unchanged and similar to that of the as-cast BMG with a fully amorphous structure.However,when the crystallization rate is above 30%,pitting-susceptive phase of（Fe,Cr）₂₃C₆ carbide precipitates and forms a percolated multiphase microstructure.Moreover,as the crystallization rate increases,the size of the nanocrystalline phase gradually increases.The results showed that the corrosion rate in deep sea of partially crystallized sample（crystallization rate>30%）increased sharply,and the passivation kinetics slowed down significantly.resulting in the thickness of the passivation film was below 3 nm（when the crystallization rate was below 30%,the thickness of passivation film reaches 10 nm）.A crystallinity-dependent transition on corrosion resistance and corrosion mechanism of Fe-based BMG in deep sea was observed.Below the critical value of the crystallization rate（30%in this study）,crystallinity did not appear to affect their corrosion resistance and passive film properties,while above the critical value,a dramatic drop in the corrosion resistance along with a significant change in the passive film dynamics and properties occurred.The approximate in-situ TEM corrosion immersion test and DFT simulation showed that the strength of adsorption on the（Fe,Cr）₂₃C₆ phase was always higher than that on the（Fe,Mo）₆C phase regardless of the hydrostatic pressure,indicating that the Cl^-adsorption on the（Fe,Cr）₂₃C₆ was more favorable.Also,the pitting is more likely to occur in（Fe,Cr）₂₃C₆.The SKP results showed that as the crystallization rate increased,the Volta potential variation increased gradually.We used the Volta potential variation to reflect the trend of galvanic corrosion.This means that as the crystallization rate increases,the microgalvanic effect is more significant.The underlying mechanism for such a distinct crystallization-dependent transition can be understood in terms of the competition among the passivation,pitting kinetics and microgalvanic effects associated with the annealing-induced crystallization.When the crystallization rate was below 30%,the corrosion mechanism was dominated by passivation and pitting initiation;when the crystallization rate was above30%,the corrosion mechanism was dominated by the microgalvanic effect and pitting growth.In addition,the effect of crystallization on the corrosion behavior of amorphous alloys was further studied under different hydrostatic pressures（1 atm,80 atm,120 atm）.The study found that the corrosion resistance of the alloy manifested in two stages as the crystallization rate increased under three different hydrostatic pressures.When the crystallization rate was below 30%,the corrosion properties of the samples did not change significantly.When the crystallization rate was above 30%,the uniform corrosion rate of the samples increased significantly.Such a transition was also confirmed at other pressures,demonstrating the universality of the crystallinity-dependent transition in the Fe-based BMG system studied.Notably,if the crystallization fraction is above 30%,the degradation in corrosion resistance increases as the hydrostatic pressure increases,indicates that for heterogeneous microstructures consisting of multiphases with various molar volumes,the microgalvanic effect could be essentially amplified at elevated HPs.On the other hand,the Cl-activity also increased with increasing HP,which further accelerated the breakdown of the passive films and triggered stable pitting growth.The finding provide a theoretical basis for guiding the structural control of amorphous materials（e.g.,amorphous coatings,3D printed amorphous alloys,etc.）and their applications in extreme marine environments.