| Amorphous alloys have gained much attention due to the attractive combination of excellent properties which makes them one of the most potential metallic materials.Due to the combined characteristics of glasses and metals and the lack of crystalline defects such as grain boundaries and dislocations,amorphous alloys possess ultra-high strength,high elastic limit,excellent corrosion and wear resistance.Besides,compared with their crystalline counterparts,amorphous alloys could retain amounts of beneficial elements to enhance their various properties.However,amorphous alloys forming ability is limited and the amorphous alloys present little ductility at room temperature.Thus,amorphous alloys are not suitable as structural components.Fortunately,these limitations could be overcome or suppressed when amorphous alloys are used as surface protective coatings.In this thesis,the HVAF sprayed SAM2X5 Fe-based amorphous alloy coatings were studied with traditional low alloy steel coatings and traditional electrodeposited chromium coating as the reference materials,using the electrochemical workstation,salt frog test and supercritical CO2 corrosion experiments to investigate its corrosion behavior.SEM,XRD,DSC and XPS were utilized to analyze the passive behavior of amorphous coatings under various conditions.The aim was to elucidate the synergistic effects of corrosion resistant components in the amorphous alloys.The main conclusions are summarized as follows:The SAM2X5 Fe-based amorphous alloy coatings were prepared by HVAF technology.Confirmed by the structure characterization,the coatings possess totally amorphous structure.The hardness of the coating was up to 900 HV indicating the excellent abrasive resistance.In neutral 3.5 wt.%NaCl solution,compared with 316L stainless steel coatings and electrodeposited chromium coatings,the amorphous coatings presented strong passivation,and its passive range was relatively wide and stable.In neutral salt frog test,the amorphous coatings could resist salt frog corrosion in 1008 h.After undergoing 168 h supercritical CO2 corrosion experiment,its corrosion rate was only 1/6 ratio on the contrast low alloy steel coatings.The thickness of passive film was in connection with the corrosion conditions.In the neutral NaCl solution,the thickness of films increased and then decreased slightly with the increase of applied potentials.This might be related with the preferential dissolution of the high valence oxides.The passive film formed at 0.6 VSCE was about 16 times thicker than 0.0 VSCE while the film formed at 0.8 VSCE was about 10 times thicker.The passive films formed in supercritical CO2 environment were 36 times thicker than 0.0 VSCE.The Cr element in amorphous alloys plays an important role in corrosion resistance.if the alloys only contain Cr element,such as electrodeposited chromium coating,the alloys would not passivate well at lower potential range and presents high passive current density and unstable passive region.If the alloys lack of Mn or W elements,such as 316L stainless steel,its corrosion resistance would be worse at higher potential range and it presents narrow passive range and lower transpassivation potential.In supercritical CO2 experiment,the passive film formed on the amorphous alloy coating presents ultra-high protection.By analyzing the results of XPS and XRD,it can be clearly seen that Mn and Cr oxidations enriched in the passive film keeping the coating from transpassivation. |
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