| As the lightest structural metallic material,magnesium alloys exhibit high strength/density ratio,excellent specific stiffness and damping performances,and thus are becoming one of the most important materials for weight reduction of automobiles.However,in real service conditions,structural components made of magnesium alloys often suffer early failure due to the severe corrosion in certain chemical environments or stress corrosion cracking under altemating mechanical loads,which severely hinders their widespread applications in industry.Therefore,it is crucial to enhance the anti-corrosion properties,especially corrosion fatigue performances,of magnesium alloys for fabrication of structural components,while still satisfying their requirements of mechanical properties.To this end,the high-strength as-cast and wrought magnesium alloys of AZ31B,AZ80 and ZK60 are selected and utilized as substrate materials in this research.A variety of protective coatings with different microstructures are prepared on these alloys using micro-arc oxidation(MAO)method and micro-arc composite coating technique,which combines MAO and electrostatic spraying.The influences of microstructure,alloys type,and processing history of the magnesium alloys on the anti-corrosion mechanisms of different protective coatings,were studied systematically by means of electrochemical and standard salt spray corrosion tests,as well as the corrosion fatigue tests that evaluate the combined effects of load and chemical environment.An in-depth understanding of the 'base alloy elements_processing history_surface protective coating_corrosion/corrosion fatigue performances' interaction and relationship was established.The main conclusions are summarized as follows:(1)The influence rules of the MAO coating growing at different current densities on the corrosion performances were investigated via advanced electrochemical corrosion testing techniques.Physical and chemical models describing the charge transfer through the electrical double layer and ion adsorption processes on the MAO coated AZ80 alloy were established,which revealed the mechanism governing the charge transfer and balance processes of the corrosive Cl-during migration at the interface of MAO coating and magnesium alloy substrate.A comparison of MAO coatings deposited under different current densities showed that the MAO coating grown at 34 mA/cm2 had uniform surface morphology with fine micro-pores,few micro structure defects,and high fluoride content,which could significantly enhance the corrosion resistance of the magnesium alloy substrate.Using the Mott-Schottky test and analysis method,it is concluded that both the bare and MAO coated AZ80 alloy specimens exhibit characteristics p-type semiconductor.The MAO coating produced at 34 mA/cm2 possesses a low acceptor concentration and flat band potential.In a corrosive environment containing Cl-,the MAO coating can hinder the adsorption and migration processes of Cl-ions in the electrical double layer,maintaining a relatively stable equilibrium of surface charge on the specimens,and thence prevents the occurrence of corrosion and improves the corrosion resistance of the magnesium alloy substrate.(2)The electrochemical corrosion and dynamic/static corrosion behavior of MAO coatings processed under a current density of 34 mA/cm2 for different time durations were investigated.A suitable robust protective coating was identified for magnesium alloy structural components,which shows excellent corrosion resistance and outstanding corrosion fatigue strength.And the mechanism of dynamic corrosion of MAO coated magnesium alloys was revealed.The corrosion behavior in accelerated salt spray test and the corrosion fatigue life of AZ31B alloy,with and without MAO coating and micro-arc composite coating(i.e.zinc phosphate E-paint coated by electrostatic spraying process+MAO coating)were characterized.It is found that in the accelerated corrosion test environment,the time for the MAO coated AZ3 1B to start showing pit corrosion was 96 h.The magnesium alloys with micro-arc composite coating(E-paint+MAO)showed higher corrosion fatigue strength.The fatigue life and fracture morphology of AZ31B substrate with and without MAO coating and E-paint+MAO composite coating were analyzed.It was observed that there were micro-pores and micro-cracks on the surface of the MAO coating on AZ3 1B.These micro-pores and micro-cracks were deemed as crack initiation points of fatigue cracks and can accelerate the initiation and propagation of cracks under stress conditions,which led to a reduction of 55%in the corrosion fatigue life of the AZ31B alloy.The corrosion fatigue limit of AZ3 1B with micro-arc composite coating was 64.0±5.4 MPa,which is 59%higher than that of the bare AZ31B substrate.Under low stress condition(<80 MPa),the corrosion fatigue strength of the AZ31B alloy treated with E-paint+MAO composite coating was significantly improved.This is due to the fact that the E-paint coating can seal the micro-pores and micro-cracks in the MAO coating,forming a composite coating with microstructure/composition gradient and strong mechanical bonding/interlock between layers,and thereby improves the corrosion fatigue strength of the magnesium alloy.(3)A comparison study of the corrosion resistance of different magnesium alloys(i.e.AZ31B with 3%aluminum content,AZ80 with 8%aluminum content,and ZK60 with 6%zinc content)was conducted to reveal the static corrosion mechanisms of the magnesium alloys with and without MAO coatings under long-term service conditions containing Cl-ions.It was found that ZK60 alloy exhibits better corrosion resistance than the magnesium-aluminum alloys due to the formation of a microstructural continuous network of an electrochemically stable phase(?-MgZn2).The pitting initiation time for different magnesium alloy substrates(AZ31B,AZ80 and ZK60)treated with MAO coating was 96,144 and 336 h,respectively.The aluminum and zinc elements from the alloy substrate entered the MAO coating during coating growth process by forming complex amorphous compounds with the MAO processing bath,and the aluminum element was found to mainly segregated in the outer layer of the MAO coating,while zinc mainly existed in the inner layer,which in turn affects the corrosion resistance of the MAO coating.(4)The evolution rules of the forging process parameters such as forging temperatures(e.g.250,300,450?)on the micro structure of the AZ80 and ZK60 wrought magnesium alloys were analyzed.The correlation between the micro structure and corrosion performances of the substrate alloys,and the properties of the MAO coating,and the corrosion resistance of the MAO coated magnesium alloys was explored.With an increase in the forging temperature,the grain size of the magnesium alloy in dynamic recrystallization increases,while the volume fraction of the ?-phases(?-Mg17Al12 ? ?-MgZn)in the alloy decreases.At a forging temperature of 450?,the ?-phases dissolved and disappeared in the micro structure of the alloys.The corrosion resistance of wrought magnesium alloys mainly depends on their microstructure,including grain size,volume fraction and distribution of the second ?-phase.At the forging temperature of 250?(AZ80EF-250)and 300?(ZK60EF-300),AZ80 and ZK60 extruded alloys showed the microstructure with a smaller grain size and more desirable distribution of second ?-phase that's beneficial for corrosion performances,and thus better corrosion resistance(weight-loss corrosion rate:1.76 and 1.49 mg/cm2·d for AZ80EF-250 and ZK60EF-300,respectively).The higher forging temperature(450?)will lead to the formation of coarse grains and dissolution of secondary phases in the magnesium alloys,and thereby reduce their corrosion resistance.At a low forging temperature(250?),twin crystals were formed in ZK60 alloy,and the twinning accelerated the corrosion of the alloy.The corrosion resistance of the magnesium alloy treated with MAO coating is closely related to the alloying elements of the alloy substrate and its micro structure.The pitting initiation time of the magnesium alloy with MAO coating is determined by its alloying elements.After corrosion pits have developed in the MAO coating,the corrosion performances of the MAO coated magnesium alloy are predominantly controlled by the micro structure and properties of the alloy itself. |
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