Influence Of Rare Earth Element And Corrosion Medium On Corrosion Behavior Of Al-Based Amorphous Alloys

Amorphous alloys permit increases in both mechanical properties and corrosion resistance over their crystalline counterparts owing to their chemical and structural homogeneity.They are appropriate reservoirs containing active species beyond solid solubility,avoiding the formation of intermetallics.They have been regarded as ideal model materials to elucidate the intrinsic features against corrosion of addition elements.Al-based amorphous alloys possess high strength to weight ratio relative to other systems.Therefore,Al-based amorphous alloys are the best choice as substitutes of traditional aluminum alloys in aircraft industries.For the traditional aluminum alloys,a number of intermetallics precipitate phases could enhance their mechanical properities,but they also increase the susceptibility to localized corrosion.Corrosion inhibitor is an feasible method to improve the corrosion resistance of aluminum alloys in aggressive aqueous environments.To date there exist some controversies about the efficiency of corrosion inhibitors resulted from the structrual heterogeneities.Al-based amorphous alloys are suitable materials to clearfy the relevant mechanism.Considering that currently the Al-based bulk metallic glasses are difficult to manufacture due to its limited glass forming ability,thus they are more suitable for serving as coating materials.The researches on corrosion behavior of Al-based amorphous alloys and their coatings are of great importance.The Al-Y binary amorphous system was adopted to study the effect of yttrium on corrosion behavior,excluding the disturbances resulted from transition elements.The stability of passive film formed on Al-Y amorphous alloys was evaluated by Kelvin probe force microscope.The Al86Ni6Y4.5Co2La1.5 alloys,holding the best glass-forming ability in the Al-TM-RE alloy systems,were chosen to uncover the inhibition mechanism of sodium nitrate.The efficiency of inhibitons to metastable pitting on Al-based amorphous alloys were analyzed by electrochemical noise technology,together with the observation of passive film by TEM.Finally,the high performance amorphous coatings with low porosity and high volume fraction of amorhous phase were prepared by HVAF.The present study would promote the understanding of intrinsic role of alloying elements and corrosion inhibitors in Al-based amorphous,and advance the application of Al-based amorphous coatings.The main results are summarized as follows:The influence of yttrium on surface chemistry and stability of passive film in Al-based binary amorphous has been investigated in Al100-xYx(at.%)system with x=9,10,11.From the polarization behavior of these three Al-Y amorphous alloys,it was found that the corrosion potential increases with increasing yttrium content.The passive current density is reduced,but the pitting potential is insensitive to yttrium content.The mean value of the metastable pit peak current is only 11 pA for the high yttrium content amorphous alloys,and the surface remains stable passivation state.The radius of mestable pit on low yttrium content sample can reach 120 nm,and the morphology of passive film is loose and porous.The growth tendency of metastable pits to stable pis is depressed by the increase of yttrium content.The passive film formed on Al-Y amorphous alloy exhibits a p-type semiconducting behavior and the carrier density is decreasd from 4.95×1022 cm-3 to 0.53×1022 cm-3 with the increase of yttrium content.From the analyses of depth profiles,it indicated that there occurs an apparent depletion of yttrium in the inner layers of passive film for the lower Y-containing samples.The higher Y-containing samples possess a homogeneous single layer passive film rich in yttrium.The formationn of highly resistant YAlO3 increases the impedance of passive film.The surface potential of Al-Y amorphous alloys are enhanced by increasing yttrium content,and the stability of passive is promoted.The Al89Y11 alloys exhibits the best corrosion resistance.The Al86Ni6Y4.5Co2La1.5 systems,with the best glass-forming ability,were adopted to investigate the efficiency of sodium nitrate as a corrosion inhibitor.The pitting potential of Al-based amorphous alloys was found to be increased by 100 mV with the minor addition of NaNO3 into 0.01 M NaCl solution.When the concentration of nitrate is up to 0.03 M,the pitting corrosion was completely avoided,and the polarization behavior transfrormed from pitting to transpassivation.The passive current density is independent of the concentration of NaNO3,but the cathodic reaction rates are accelerated.The electrochemical noise of Al-based amorphous alloys in 0.01 M NaCl solution with the addition of different concentrations of NaNO3 has been recorded continuously.The transient current noise is inhibited during the immersion tests,and the noise resistance in the condition with 0.05 M NaNO3 is the highest.The pitting index remains below 0.3,indicating the characteristics of uniform corrosion.In the corrosive medium with 0.05 M NaNO3,a compact passive film,with a thickness of 8.6 nm,formed on Al-based amorphous alloys.The incorporation of NO3-into passive film is verified by EDS line scanning,and the invasion of Cl-is inhibited efficiently.The amorphous powders,with the composition of Al86Ni6Y4.5Co2La1.5(at.%),are deposited on AA 2024 via high velociety air fuel spray to get the Al-based amorphous coating.The feedstock powders were prepared by high-pure nitrogen gas atomization.They are highly spherical with smooth surface,and the primary size is 20-45 mm,meeting the powder dimension requirements for thermal spraying.The as-sprayed coating is uniform and dense with a thickness of 125?m,and its porosity is only 0.12%verified by X-ray diffraction topography.The coating exhibits a broad hump with some diffraction peaks of ?-Al precipitation phase,indicating that a small amount of nanometer-sized A1 crystals coexist in the amorphous matrix.The volume of amorphous phase is up to 83.7%.The microhardness of Al-based amorphous coating is 400 HVo.os,which is almost three times of AA 2024(137 HV0.05).The amorphous coating exhibits a form of passivation with a corrosion potential lower than AA 2024,which can provide an effective protection for the substrate.The resistance of passive film on Al-based amorphous coating is higher than AA 2024,and the driving force for the adsorption of aggressive anions is smaller on the coating than on AA 2024 substrate.The wear rate for coating is 5.6 × 10-4 mm3N-lm-1,which is one order of magnitude lower than the substrate.The worn surface of amorphous coating is uniform and smooth,and a protective oxide film formed on Al-based amorphous coating.This work prepares a high performance Al-based amorphous coating,which can provide an effective protection for the substrate under the condition of corrosion and abrasion.