Fabrication of Superhydrophobic Calcium Phosphate Coating on Mg-Zn-Ca alloy and Its Corrosion Resistance
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JMEPEG DOI: 10.1007/s11665-017-2994-y
Fabrication of Superhydrophobic Calcium Phosphate Coating on Mg-Zn-Ca alloy and Its Corrosion Resistance Lashuang Zhang, Yue Jiang, Wei Zai, Guangyu Li, Shaocheng Liu, Jianshe Lian, and Zhonghao Jiang (Submitted May 6, 2017; in revised form July 26, 2017) A novel superhydrophobic calcium phosphate coating was prepared on a magnesium alloy substrate by a highly effective chemical conversion process and subsequent chemical modification. Different methods were employed to characterize the surface morphology and chemical composition as well as measure the wettability of the coating. It was demonstrated that the as-prepared superhydrophobic calcium phosphate coating has a typical three-level hierarchical structure consisted of micro-protrusions, submicro-lumps and nano-grains, conferring excellent superhydrophobicity with a water contact angle of 159°. The electrochemical measurements and appropriate equivalent circuit revealed that the corrosion-resistant performance of the superhydrophobic calcium phosphate coating was significantly improved as compared with that of the substrate, the corrosion potential of the superhydrophobic coating increases from 21.56 to 21.36 V, and its corrosion current density decreases from 1.29 3 1024 to 1.3 3 1026 A/cm2. The anticorrosion mechanism of the superhydrophobic coating was also discussed. It can be indicated that the corrosion inhibitive properties of the coating are in accordance with its hydrophobicity, which is owing to the presence of a protective layer of air trapped in the grooves of the coating surface to isolate the underlying materials from the external environment. Keywords
calcium phosphate coating, corrosion resistance, hierarchical structure, magnesium, superhydrophobicity
1. Introduction Magnesium and its alloys as the lightest prospective engineering metal materials have been available to aerospace, automobile, military and electronic industries owing to their remarkable properties such as cutting performance, low density, electromagnetic compatibility and high strength/weigh ratio (Ref 1-3). Nevertheless, magnesium alloys are very active and easily oxidized and corroded in aggressive environments containing corrosive ions such as Cl, which intensely impedes their application range (Ref 4). The details and parts of these alloys lose their functionality rapidly during the operation in corrosion-active medium. The development of corrosion-resistant magnesium alloys will enable a much wider range of applications based on these materials. So far, many methods have been used for conferring corrosion resistance on magnesium alloys; for example, the mixture with rare earth elements (Ref 5), the control of metallurgical factors by increasing the purity of the alloy (Ref 6) and the use of a rapid solidification processing have been investigated (Ref 7). Alternative approaches for elevating corrosion resistance include surface modification by laser processing, deposition of a protective Lashuang Zhang, Yue Jiang, Wei Zai, Guangyu Li, Shao
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