Effect of ultrasonic cold forging technology as the pretreatment on the corrosion resistance of MAO Ca/P coating on AZ31B Mg alloy

A calcium phosphate contained (Ca/P) coating was obtained on AZ31B Mg alloy by micro-arc oxidation (MAO) process under the pretreatment of ultrasonic cold forging technology (UCFT). The surface nanograins were introduced after UCFT pretreatment on AZ31B Mg alloy. Optical microscope (OM) was employed to observe the microstructures of the untreated and UCFT treated samples. Transmission electron microscopy (TEM) and atomic force microscope (AFM) were employed to observe the microstructures of nanograins and the surface roughness of the UCFT treated Mg alloys. The grain size of the UCFT treated Mg alloy is 48.67 nm and the surface roughness is 17.03 nm. The microstructures and the phase compositions of MAO samples were observed and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The XRD results show that the coating include Ca/P phase, including hydroxyapatite (Ca10(PO4)6(OH)2), HA), tertiary calcium phosphate (Ca3(PO4)2, TCP) and calcium phosphate dehydrate (CaHPO4⋅2H2O, DCPD). The hardness of the samples was measured by the micro-hardness tester under the loads of 10 g, 25 g and 50 g. 3D topographies of hardness indenter were characterized by 3D profiler. The immersion tests and potentiodynamic polarization tests were used to evaluate the weight loss rate and corrosion current density in simulated body fluid (SBF). The results show that the corrosion resistance of Ca/P MAO coating on Mg alloy was improved greatly by the pretreatment of UCFT.