The Microstructure And Corrosion Resistance Of Hot-dip Al-Zn-Si-Mg/La Coatings

Hot-dipping is an effective technique providing physical blocking and sacrificial protection for steels.The Al-rich alloy coated steel was widely used in household appliances,lightweight steel structure,traffic signs and so on.Efforts has been made to improve the corrosion reistance of the Al–Zn–Si coating by adding Mg or La.However,the effects of Mg or La had many disputes.The corrosion resistance mechanism was also unclear.Al–Zn–Si–Mg and Al–Zn–Si–La coatings were manufactured with different contents of Mg or La.Scanning electron microscope（SEM）and transmission electron microscopy（TEM）were used to analyze the microstructure of the bath alloys and the coatings.The phase compositions were identified by X-ray diffraction（XRD）.The solidification process was confirmed by analyzing the results of the thermodynamic calculation.Moreover,the electrochemical measurements were used to study the corrosion resistance and the corrosion mechanism for acquiring the corrosion behaviors of the different coatings.Mg₂Si and MgZn₂ phases were formed in the interdendritic regions of Al–Zn–Si–xMg（x=0,1.25,3,4 wt.%）alloy coatings.The Mg₂Si phases were dendritic in the bath,but they formed equilateral triangles at the coating surface.When the content of Mg was higher than 3 wt.%,Mg₂Si phase was agglomerate.The addition of Mg decreased the weight loss 63%⁷7%.In thoses coatings,the Al–Zn–Si–3Mg coating had the best corrosion resistance.According to the electrochemical measurements,the corrosion behaviors of Al–Zn–Si and Al–Zn–Si–3Mg coatings were revealed.The corrosion resistance mechanisms included three main reasons:（i）the addition of Mg formed the Mg₂Si and MgZn₂ with cathodic protection capability;（ii）Mg²⁺promoted the formation of compact corrosion products Zn₆Al₂（OH）₁₆CO₃·4H₂O;（iii）the exposure of Fe–Al intermetallic layers was delayed,resulting the reduction of the galvanic corrosion.The corrosion models indicated the failure process of the Al-rich coatings and contributed to guiding the developments of new coatings.The low-content La inhibited the nucleation ofα-Al phase and enlarged the size of the Al-rich dendrite in the Al–Zn–Si–xLa（x=0,0.01,0.02,0.05 wt.%）alloy coatings.The intermetallic layers consisted of Fe₂Al₅,Fe₄Al₁₃ and a new Si-rich quaternary phase.The addition of trace La inhibited the dissolution of Fe and the formation of intermetallic compounds.The nucleation position,the driving force of nucleation and the stability of heterogeneous interface affected the“nucleation-growth”process between the solid-liquid reaction.Moreover,the electrochemical measurements demonstrated that the appropriate content of La could improve the corrosion resistance of coatings by suppressing the cathodic reaction.The Fe–Al solid-liquid reaction was revisited by theoretical calculation.The nucleation position,thermodynamic driving force and heterophase interface energy affected the solid-liuqid reaction.A“nucleation-growth”model,which was more consistent with the experimental results,was built.The doping effects of Zn,Si,Mg,Ti,La on the growth of Fe₂Al₅ phase were compared.The results indicated that Si,Ti and La suppressed the Al diffusion,but Zn and Mg had opposite effects.The doping effects on the stability of Al crystal were also studied.The high content of Ti,Fe and Ce promoted the nucleation of Al,but the doping of Zn,Si,Mg and La inhibited the nucleation.