Development and screening of (Ca-P-Si-F)-PEO coatings for biodegradability control of Mg-Zn-Ca alloys
Lara Moreno, M. Mohedano, R. Arrabal, E. Matykina
Abstract
PEO coatings were developed on MgZnxCay alloys for biomedical applications using different treatment times and a novel transparent electrolyte containing Ca, P, Si and F. Surface characteristics and chemical composition of studied materials were evaluated using scanning electron microscopy, X-ray diffraction and optical metrology. Corrosion investigations were performed for uncoated and coated materials using potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution measurements in three media (0.9wt.% NaCl, α-MEM and inorganic part of α-MEM). Findings revealed that Zn-rich intermetallics favour the formation of large voids in the coatings. Ca, P, F and Si are successfully incorporated into the coatings. Coatings consist of crystalline phases (MgO, MgF2 and Ca,Si-rich compounds) and amorphous material. Aminoacids and organic components of α-MEM solution accelerate corrosion of the bare substrates but have no significant influence on the corrosion resistance of coated specimens. Therefore, inorganic α-MEM can be considered as an appropriate medium for fast screening of PEO coatings. The relatively small amount of F content in the barrier layer facilitates early failure of the coatings, which show undercoating corrosion. Zn-rich intermetallic particles in the undercoating crevice induce a more aggressive corrosive environment and enhanced micro-galvanic couple effect with the Mg matrix. This leads to faster degradation of the coated alloys compared to the bare substrates.