Effect Of Elements Nd And Electrolyte PH On Corrosion Resistance Of PEO Coatings On Magnesium Alloy

Magnesium alloy has the advantages of low density,good processability,etc.It has become the most lightweight material with the most development and application prospects in the 21 st century.However,the poor corrosion resistance of magnesium alloys restricts the application of magnesium alloys in practice.The method of plasma electrolytic oxidation modulates the process parameters and utilizes the arc discharge phenomenon on the surface of magnesium alloys to produce Oxide film layer with high corrosion resistance on the surface of magnesium alloys.This article from the perspective of basic research,the first part of the impact of rare earth elements Nd on the microstructure of the plasma electrolytic oxidation coating and film layer performance;the second part of effect of pH value of the AZ91 D magnesium alloy in potassium fluoride electrolyte on the microstructure and corrosion resistance of plasma electrolytic oxidation coatings was investigated.After the plasma electrolytic oxidation of NZ30 K magnesium alloy containing rare earth element Nd and AZ91 D magnesium alloy containing rare earth element yttrium,the microstructure and corrosion resistance of the plasma electrolytic oxide film are very different.The microstructure of the film was characterized by XRD,SEM and EDS.The results showed that the rare earth element Nd was enriched in the plasma electrolytic oxidation coating at the divorced eutectic phase of the rare earth magnesium alloy,and micropore penetrated loose layers in the film;AZ91D magnesium alloy plasma electrolytic oxidation coating is distributed with larger holes.The potentiodynamic polarization curves,salt spray experiments and hydrogen evolution experiments showed that the NZ30 K containing rare earth elements Nd had better corrosion resistance than AZ91 D,and the corrosion resistance of the film after plasma electrolytic oxidation was positively correlated with the corrosion resistance of the matrix.Through the observation of microscopic morphology,electrochemical impedance spectroscopy analysis and porosity calculation of the soaked substrate and film,the porosity of the AZ30 K and AZ91 D magnesium alloy plasma electrolytic oxide film is not much different,and the protection of the film layer to the substrate is limited.Corrosion ions enter the magnesium alloy matrix through micropores and cracks,and then the corrosion process of the magnesium alloy matrix plays an important role in the continuous corrosion process of theplasma electrolytic oxidation coating.The corrosion product of the NZ30 K rare earth magnesium alloy is subjected to matrix/plasma electrolytic oxidation.The accumulation of film interface slows down the corrosion rate.In order to reduce the content of magnesia in PEO ceramic film and improve the compactness and corrosion resistance of oxide film,potassium fluoride was used as the basic electrolyte to study the influence of different pH values on the microstructure and corrosion resistance of plasma electrolytic oxide film.XRD,SEM,EDS,potentiodynamic polarization curves and salt spray test results show that the electrolyte pH has a significant impact on the content and distribution of magnesium oxide and magnesium fluoride in the plasma electrolytic oxidation coating.The loose layer obtained under weak alkaline conditions is porous and has poor corrosion resistance.Under weak acidic conditions,the plasma electrolytic oxide layer is denser which has good barrier to corrosive ions,and has good corrosion resistance.