Study On The Corrosion Behavior Of ZK60 Magnesium Alloys And Methods To Improve Its Anti-corrosion Performance

Magnesium(Mg)alloys have been widely used in the automotive industry,aerospace,3C(computers,communications,consumer electronics),and biomedical fields due to their excellent comprehensive properties.However,the low corrosion resistance of Mg allo ys restricted their further application.ZK60 magnesium alloys are the current commercial Mg alloys with the best comprehensive mechanical properties,but few reports focus their corrosion behavior.The effects of microstructure,medium environment,alloying,and heat treatments on the corrosion of ZK60 alloys are still not clearly understood.Therefore,studying the corrosion behavior of ZK60 alloys in these areas is of great significance for a comprehensive understanding of the corrosion mechanism and finding proper ways to improve its corrosion resistance.In this study,a commercial as-cast ZK60 alloy was selected as the test material to study the influence of microstructure on its corrosion behavior.The corrosion development process in Na Cl solution and the comprehensive influence of Cl^-concentration and p H value were studied intensively by weight-loss,hydrogen evolution,and electrochemical tests companying with 3D optical observation and microscopic analysis.Based on these results,the feasibility of improving the corrosion resistance of as-cast ZK60 alloy by heat treatment(solution treatment and solution aging treatment),rare earth alloying(Ce and Nd),and conversion coating(fluoride coating)were discussed.The main conclusions obtained are as follows:1.Effects of microstructure and medium factors on the corrosion behavior of as-castZK60 alloyThe as-cast ZK60 magnesium alloy was composed of?-Mg phase,and large second-phase particles(Mg Zn₂)mainly deposited along grain boundaries.The second-phase particles and the Zr-rich regions in the grains were more stable than the grain boundary regions,resulting in a strong corrosion micro-galvanic effect.The corrosion of the as-cast ZK60 alloy in 0.1mol/L Na Cl solution originated from the areas around the second-phase particles in grain boundaries and developed in the grain boundary regions displaying filiform-like corrosion characteristics.With the development of corrosion,corrosion pits were formed under the corrosion filaments,and the localized corrosion gradually dominated the corrosion behavior of the cast ZK60 alloy.The corrosion product layer suppressed the general corrosion rate P_wand the pit growth rate Pdepth.With the increase in[Cl^-]and decrease in p H,the corrosion rate was increased.In p H<5 or[Cl^-]?0.5 mol/L Na Cl solutions,the p H or[Cl^-]played the leading role in the corrosion rate,respectively.[Cl^-]and p H had relatively little influence on the corrosion rate in Na Cl solutions with p H?5 and[Cl^-]<0.1 mol/L.Localized corrosion played a dominant role under all investigated conditions,and there was P_depth/P_w=7?15.2.Heat treatment and rare earth alloying on the corrosion behavior of ZK60magnesium alloySolution treatment(T4)dissolved most of the second-phase particles(Mg Zn₂)and madethe Zr-rich area disappear from the as-cast ZK60 alloy.New tiny second phase particles(Mg Zn₂)were reprecipitated during solution aging treatment(T6).Heat treatments effectively weakened the corrosion micro-galvanic effect between the second-phase and the Mg matrix.T4-and T6-treated alloys still displayed filiform corrosion characteristics in the early corrosion period,which originated in grains or from grain boundaries and propagated to any direction.With increasing corrosion time,localized corrosion played a dominant role,and the order of corrosion rate was:as-cast ZK60 alloy?T6-treated alloy>T4-treated alloy.The re-precipitated second phases reduced the long-term corrosion resistance of the T6-treated alloy.Ce(0.3-1.5 wt.%)and Nd(0.3-1.0 wt.%)significantly refined the grain size,uniformedthe microstructure of the ZK60 alloy,and formed the ternary rare-earth phases on the grain boundaries,improving the stability of the grain boundary region and weakening the corrosion micro-galvanic effect between the second phase and the matrix.The best addition contents were 1.0%Ce and 0.5%Nd.The corrosion of ZK60-1Ce and ZK60-0.5Nd alloys all originated from the second phase particles.However,the former developed in the grain boundary regions and displayed filiform-like corrosion features,and the latter propagated to the grain center and preferentially corroded the grains.The corrosion product film containing rare earth oxides inhibited the general and local corrosion rates.However,the localized corrosion still played a dominant role,and the density of corrosion pits was much larger than those of the as-cast ZK60 alloy.In general,adjusting of the microstructure and composition of the as-cast ZK60 alloythrough heat treatment and rare earth alloying could significantly improve the corrosion resistance in the early stage.However,the improvement in the long-term corrosion resistance was not obvious.3.Preparation of a fluoride coating on the as-cast ZK60 alloy surface and itscorrosion resistance studyFluoride coating with good adhesion was prepared on the surface of ZK60 alloy by the Na[BF₄]molten salt method.The preparation temperature and time had a significant impact on the fluoride coating's structure and performance.The fluoride coating consisted of the outer Na Mg F₃layer composed of Na Mg F₃ particles with a diameter of about 100-200 nm,and the inner Mg F₂ layer consisted of columnar grains.With increasing temperature and time,the thickness of the coatings increased,and the surface defects were gradually reduced.Specimens with the coating prepared at 440°C for 6 h had the best corrosion resistance.The protective effect of the fluoride layer mainly resulted from the inner Mg F₂ layer.The corrosive medium migrated to the ZK60 alloy substrate,resulting in the substrate corrosion expanded horizontally and vertically under the coating.The H₂ evolution and accumulation of corrosion products under the coating made it gradually disbond and crack and weakened its defensive performance.