An Investigation Of The Electrochemical Corrosion Behavior And Corrosion Resistance Of Plasma Electrolytic Oxidation Coated AZ31 Magnesium Alloy

Magnesium alloys with excellent mechanical properties have been used in aerospace, automotive and other industries, while the poor corrosion resistance restricts the further development. In this paper, the AZ31 magnesium alloy was treated by a home-made WH-1A plasma electrolytic oxidation(PEO) device, and the specimens were studied by the electrochemical methods after the salt spray corrosion test(SSCT). Moreover, the effect Na2 Si O3 and other additives in the base electrolyte was also investigated.The study shows that many discharge pores and cracks distributed on the surface of the PEO specimens. The reduced discharge pores and cracks of the PEO specimens were found after the SSCT for 24 h, and a number of corrosion products(Mg(OH)2) which were tested by XRD existed in the discharges pores and cracks. The electrochemical results indicated that the corrosion resistance of the outer layer increased while that of the inner layer decreased, and the type of semiconducting behavior transformed from n-type to p-type after the SSCT for 24 h. The transport of ions was blocked by the aggregation of corrosion products in the discharge pores and cracks of the outer layer after the SSCT for 72 h, and some micro cracks were produced by the extrusion stress at the interface between inner layer and outer layer after the SSCT for 120 h. The micro cracks promoted the subsequent corrosion processThe investigation of the effect of Na2 Si O3 showed that the surface roughness of the specimens increased with the increase of PEO time, and the mole percent of Mg O also increased, while the mole percent of Mg2 Si O4 and Mg Si O3 decreased. The electrochemical results indicated that the corrosion resistance of AZ31 magnesium alloy was the worst, and the corrosion resistance of specimens after PEO for 30 s was the best. Also, the corrosion resistance of the PEO specimens decreased with the increase of PEO time. The corrosion resistance of the PEO specimens treated for 1 min, 5 min and 15 min degraded to similar values after immersed in the 0.1mol/L Na2 Si O3 for 40 min, indicating that the inner layer play a dominant role in the corrosion resistance of the PEO specimens treated for different time.The investigation of the effect of electrolyte additives in the base electrolyte showed that the PEO specimens exhibit different surface and cross section morphologies, also the phase compositions are different. The results showed that Si O32- was conducive to the growth of ceramic layer, and the ceramic layer developing in the electrolyte which contained Al O2- showed a typical double-layer structure. The corrosion resistance of P-coating was higher than that of the Si-coating and Al-coating, and the corrosion resistance of F-coating was the highest.