（1. College of Materials Science and Engineering, Shandong University of Science and Technology,
Qingdao 266590, China;
2. State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China;
3. School of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China;
4. MagIC-Magnesium Innovation Center, Helmholtz-Zentrum Geesthacht,
Zentrum für Material-und Küstenforschung GmbH, Geesthacht D-21502, Germany;
5. National Engineering Center for Corrosion Control, Institute of Metals Research,
Chinese Academy of Science, Shenyang 110016, China）

Abstract:Zinc calcium phosphate (Zn-Ca-P) coating and cerium-doped zinc calcium phosphate (Zn-Ca-Ce-P) coating were prepared on AZ31 magnesium alloy. The chemical compositions, morphologies and corrosion resistance of coatings were investigated through energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), electron probe micro-analysis (EPMA) and scanning electron microscopy (SEM) together with hydrogen volumetric and electrochemical tests. The results indicate that both coatings predominately contain crystalline hopeite (Zn3(PO4)2·4H2O), Mg3(PO4)2 and Ca3(PO4)2, and traces of non-crystalline MgF2 and CaF2. The Zn-Ca-Ce-P coating is more compact than the Zn-Ca-P coating due to the formation of CePO4, and displays better corrosion resistance than the Zn-Ca-P coating. Both coatings protect the AZ31 Mg substrate only during an initial immersion period. The micro-galvanic corrosion between the coatings and their substrates leads to an increase of hydrogen evolution rate (HER) with extending the immersion time. The addition of Ce promotes the homogenous distribution of Ca and formation of hopeite. The Zn-Ca-Ce-P coating has the potential for the primer coating on magnesium alloys.

Key words: AZ31 magnesium alloy; cerium; zinc calcium phosphate; chemical conversion coating; corrosion resistance