| Pure magnesium is seldom used as structural material due to its poor mechanical properties,and that used as industrial structural material are magnesium alloys which were alloyed with certain elements.The addition of alloy elements not only improves the mechanical properties of magnesium alloys,but also induces the formation of second phase.The second phaseβ-Mg17Al12 plays an important role in corrosion performance of magnesium-aluminum alloy.It is known that magnesium alloys are frequently used in various atmospheric environments.The main subject concerned in the present paper is how the atmospheric corrosion behavior of magnesium alloys when the atmospheres were contaminated with certain salt particles,which frequently were as deposits on the alloy surface.What the article particularly concerns how the variation of the volume fraction ofβphase affects the micro galvanic effect,and the influence ofβphase on the surface films.Three magnesium-aluminum alloys with different aluminum contents were prepared by ordinary casting methods,all of which contain the secondβphase (Mg17Al12) because the content of additive aluminum exceeds the solubility of aluminum in magnesium at ambient temperature.The volume fraction ofβphase increases with the increase of the aluminum content.The atmospheric corrosion tests with the presence of contaminant particles in laboratory simulation was conducted to the three magnesium alloys with different volume fraction ofβphase, the surface morphologies and cross sectional morphologies of corrosion products, and the matrix after the remove of corrosion products were observed by SEM.It was found that the atmospheric corrosion of magnesium-aluminum alloys initiated directly where beneath NaCl particles deposits,that is to say,the corrosion could take place at any phase as long as it is the position where NaCl particles was deposited.The corrosion behavior varied because of the different volume fraction ofβphase in the magnesium alloys.It was indicated from the results of corrosion mass gain that the corrosion rate of magnesium alloys increased as increasing the volume fraction ofβphase,resulting in the serious corrosion of the magnesium alloy substrate.Because the increase ofβphase enlarged the cathodal area of micro galvanic cells,consequently increasing the corrosion rate of magnesium alloys,which indicated that in dual phase magnesium-aluminum alloys,the effect of micro couple betweenβphase and a phase enhanced with the increase ofβphase.The formed corrosion product fills showed protectiveness to some extent, depending on the structure and state of the corrosion product films.The influence of the increased volume fraction ofβphase in magnesium-aluminum alloys on the composition,structure and performance of atmosphere corrosion products were investigated by XRD,EPMA and SEM.There were Mg(OH)2,MgCl2, MgCl2·2H2O,Mg2(OH)3Cl·4H2O in the corrosion products of the three magnesium alloys,and as theβphase in magnesium alloys increased,the Al-enriched phase(MgAl2O4) emerged.However,the formation of Al-enriched phase didn't improve the stability of corrosion product fill,in contrast,the higher the volume fraction ofβphase,the more defects and pores existing in the corrosion products.The reason is that the micro galvanic effect betweenβphase andαphase in the substrate aggravates corrosion,leading to the inhomogeneous distribution,loose structure and pores of the corrosion products,which decreased the protective function of corrosion products of magnesium in atmosphere.The protective ability of corrosion product films of the three magnesium alloys could be qualitatively determined by Electrochemical Impedance Spectral,and the results showed that with the increase ofβphase,the impedance values of corrosion product films decreased when exposed for the same periods,indicating the decline of protective effect.The in-situ Electrochemical Impedance Spectra measurements were conducted to investigate the earlier stage of atmospheric corrosion behavior of the three magnesium alloys with NaCl particles using Solartron 1260 impedance/gain phase analyzer and 1296 dielectric interface.The in-situ impedance spectra combined with corrosion morphology could preferably reflect the process of atmospheric corrosion of the three magnesium alloys.The high frequency capacitive impedance arc of the three magnesium alloys stabilize gradually with the increase of exposure time.The gradually increased low frequency capacitive impedance arc reflected the slowdown of the substrate corrosion and the enhancement of the protectiveness of corrosion product films.The diffusion tale on the impedance spectra corresponds to the migrating information of corrosion products,rather than the diffusion controlled by non-oxygen factors.The in-situ impedance spectra revealed the better corrosion resistance of magnesium alloy with high volume fractionβphase during the earlier atmospheric corrosion induced by NaCl particles centralized in the earlier ten hours,and faster corrosion occurred for Mg-21Al alloy after ten hours becauseβphase is corroded.As the exposure time prolonged,the protective function of corrosion product films in Mg-3A1 alloy is significantly superior to the other two alloys.The results of in-situ impedance spectra were well consistent with those of the above results,showing the feasibility of the impedance spectra analysis technology to investigate the earlier stage of atmospheric corrosion induced by NaCl particles for magnesium alloys.The surface performance ofβphase is the key determining the galvanic couple behavior betweenβphase and a matrix phase,as well as the whole surface corrosion.A model alloy of consisted of nearlyβphase was smelt according to the composition features ofβphase,and the composition,structure and electron characteristics of the formed passive films on which were studied in solutions conditions containing different Cl- concentration.The increase of Cl- concentration may decrease the corrosion potential and the pitting breakdown potential ofβphase,accelerate the formation and dissolution of films on the surface ofβphase, and affect semiconductor characteristics of surface films obviously.This is because there is excess Cl- doping the formation ofβphase surface film,resulting in the change of the properties of surface film as the Cl- concentration increases.It is indicated from the electrochemical behavior ofβphase in the solution that theβ phase in the micro-region of magnesium-aluminum alloy plays a role of corrosion in atmosphere condition.The high concentration of Cl- decreases the stability ofβphase surface film,attributing from the fact that the increase of volume fraction ofβphase in magnesium-aluminum alloy doesn't increase the stability of surface, conversely,emphasizes the function of micro galvanic couple.
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