Optimization of plasma electrolyte oxidation process parameters for corrosion resistance of Mg alloy

Plasma electrolyte oxidation(PEO)is a surface treatment method with high dependency on the process parameters.This paper focuses on maximizing the corrosion resistance of PEO coatings applied on Mg-5Zn-0.4Ca(ZX504)alloy by optimizing the process parameiers.For this purpose,the Taguchi method based on LI8 orthogonal array with mixed level design was used for optimization and determining effective parameters.Main process factors including electrolyte concentration,current density,frequency and duty cycle were considered at different levels.The corrosion resistance,as the performance indicator,was obtained using electrochemical impedance spectroscopy technique.Surface characteristics were also evaluated using SEM(scanning electron microscopy),EDS(energy dispersive spectroscopy),profilometer and contact angle goniometer.The statistical analysis showed that the optimum condition could be obtained at a current density of 200 mA/cm^2,frequency of 500Hz and at a duty cycle of 30%,in an electrolyle containing 15 g/L NazPO·12H2O and 10g/L KF.

Microstructure characterization and effect of extrusion temperature on biodegradation behavior of Mg-5Zn-1Y-xCa alloy

Microstructure and biodegradation behavior of as-cast and hot extruded Mg-5Zn-1Y alloy containing different amountsof calcium （0.0%, 0.1%, 0.5%, and 1.0%, mass fraction） were explored. The extrusion process was conducted at three differenttemperatures of 300, 330, and 370 ℃. Chemical composition, phase constitution, microstructure, and biodegradation behavior of thealloys were investigated. The macro- and micro-scopic examination revealed that the addition of Ca refines the grain structure andforms an intermetallic phase, Ca2Mg6Zn3. The hot extrusion process resulted in breaking the intermetallic phases into fine particlesrouted to the extrusion direction. Moreover, dynamic recrystallization happened in almost all alloys, and more bimodalmicrostructure was formed in the alloys when the alloys were extruded at 370 ℃. Polarization curves showed no passive region,which indicated that active polarization dominated in the alloys; therefore, grain refining through Ca addition and dynamicrecrystallization over hot extrusion operation increased biodegradation rate. The results show that the as-cast Mg-5Zn-1Y-0.1Caalloy provides the highest corrosion resistance, and the extruded Mg-5Zn-1Y-0.5Ca alloy at 300 ℃ shows the lowestbiodegradation rate among the extruded alloys. Therefore, hot extrusion does not always improve the biodegradation behavior ofmagnesium alloys.

Effect of Phase Transformation During Long-Term Solution Treatment on Microstructure,Mechanical Properties,and Bio-corrosion Behavior of Mg–5Zn–1.5Y Cast Alloy

The influence of long-term solution treatment for various intervals on the microstructure,mechanical properties,and corrosion resistance of the as-cast Mg–5Zn–1.5Y alloy was investigated.Variation of secondary phases was studied during solution treatment through thermal analysis test and thermodynamic calculations.Tensile and hardness tests,as well as polarization and immersion tests,were performed to evaluate the mechanical properties and corrosion behavior of the ascast and heat-treated alloy,respectively.Results show that solution treatment transforms I-phaseinto W-phaseas well as dissolves it into the a-Mg matrix to some extent;therefore,the amount of W-phase increases.Moreover,prolonged solution treatment decreases the volume fraction of the phases.In the first stage of solution treatment for 14 h,the tensile properties significantly increase due to the incomplete phase transformation.Although long-term solution treatment sharply decreases the tensile and hardness properties of the alloy,it improves the corrosion resistance due to the transformation of I-phase into W-phase.In fact,it decreases corrosion potential and simultaneously dissolves intermetallic compounds into the a-Mg matrix,resulting in the reduction in galvanic microcells between the matrix and compounds.It is found that the optimum time for long-term solution treatment is 14 h,which improves both corrosion behavior and mechanical properties.