Study On Smelting And Corrosion Mechanism Of Zn-Al-Mg Alloys

By simulating sea water corrosion experiment and electrochemical testing,this paper studied metallurgical behavior,corrosion mechanism and cathodic protection performance of Zn-Al-Mg alloys and will provide basic data for metallurgical application.Samples of Zn-Al-Mg alloys were prepared by induction melt furnace.By analyzing phase diagram,we determined melting process and parameters of Zn-Al-Mg alloys primarily and produced a series of Zn-Al-Mg alloy samples by the melting furnance.Phases and microstructure of different Zn alloy samples are studied by XRD,OM and SEM.By immersed corrosion testing in the 3.5% Na Cl solution,self-corrosion rates of different Zn-Al-Mg alloy samples were studied.Corrosion products were analyzed by XRD.Microtopography of Zn-Al-Mg alloys corroded in the 3.5% Na Cl solution and composition of Corrosion products were observed and analyzed by SEM.Polarization curves,self-corrosion potentials,self-corrosion currents and electrochemical impedance spectroscopies of Zn-Al-Mg alloys were tested by electrochemical testing in the 3.5% Na Cl solution.Corrosion resistance and corrosion mechanism were studied.By cathodic protecting experiments of sacrificial anode methods in the 3.5%Na Cl solution,cathodic protection potentials,cathodic protection currents,capacitances and current efficiency were tested and cathodic protection performance of different Zn-Al-Mg alloys was studied.Research results indicate that melting loss rates of Al and Mg are 3% and 12%when using Zn,5083 and AZ31 B to prepare Zn-Al-Mg alloy samples.Smelting temperature is in 700?.The best casting temperature is in 550?.Zn-6.02Al-3.1Mg and Zn-10.98Al-2.94 Mg alloys close to the eutectic component are achieved.Zn-6.02Al-3.1Mg and Zn-10.98Al-2.94 Mg alloys consist of dendritic Al solid solution containing Zn and Mg,massive Zn solid solution containing Al and Mg,netty ternary eutectic structures?Zn-hcp/Al-fcc/Mg Zn2?.Self-corrosion rate of Zn-6.02Al-3.1Mg alloy corroded in the 3.5% Na Cl solution is 0.0287mg/m2.h and it accounts for One-eighth of Zn;There is slight passivation in anode polarization curve.Self-corrosion potential is-1.25 V.Self-corrosion current density is 1.31 m A/cm2.AC impedance is 13500?.cm2.Compared to Zn,Zn-5.7Al and Zn-10.98Al-2.94 Mg alloys,Zn-6.02Al-3.1Mg alloy has many advantages,such as low self-corrosion rate,negative self-corrosionpotential,small self-corrosion current density and slight passivation etc.Working potential and working current of Zn,Zn-5.7Al and Zn-10.98Al-2.94 Mg alloys as anode materials are unstable when conducting the sacrificial anode experiment.Zn-6.02Al-3.1Mg alloy has stable working potential of-950 m V and working current from 0.62 m A to 0.66 m A after 24 h.It also has the maximum current efficiency of 80.43%,stable potential thermodynamic driving force,stable working current and most ideal cathodic protection performance.Corrosion mechanism of Zn-6.02Al-3.1Mg alloy relies mainly on oxidizing reaction of Zn,Al and Mg.Surface corrosion products consist mainly of Zn₅?OH?₈Cl₂.H₂O which is insoluble colloidal substance with compact structure,good adhesion and low electrical conductivity.Membranes similar to the honeycomb structure do not prevent anodic oxidation and anodic corrosion process.Corrosion products of Zn5?OH?8Cl2.H2 O form dense protective films and corrosion towards the inner of Zn-6.02Al-3.1Mg alloy is prohibited.It is different from porous corrosion products of Zn O and Zn?OH?₂ on surfaces of Zn.With the addition of Mg,the reaction from Zn?OH?₂ to Zn₅?OH?₈Cl₂.H₂O is enhanced and the reaction from Zn?OH?₂ to Zn O is prevented.