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

• Published in: Journal of alloys and compounds 2015-06, Vol.635, p.278-288
• Main Authors: Chen, Lingling, Gu, Yanhong, Liu, Lu, Liu, Shujing, Hou, Binbin, Liu, Qi, Ding, Haiyang
• Format: Article
• Language: English
• Subjects: Ca/P coating; Calcium; Calcium phosphate; Coating; Hardness; Magnesium alloy; Magnesium base alloys; MAO; Microstructure; Nano layer; Pretreatment; Scanning electron microscopy; Surface roughness; UCFT
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2015.02.086

•Ultrasonic cold forging technology was used as the pretreatment for MAO coating.•Nano layer with the grain size of 30–80nm was formed on the UCFT treated surface.•Calcium phosphate contained coating was obtained by MAO process.•The remained nano layer underlying MAO coating could impact the corrosion resistance greatly. 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 48.67nm and the surface roughness is 17.03nm. 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 10g, 25g and 50g. 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.