Effect of sodium citrate on the electrodeposition of biodegradable Fe-Zn alloys

Fe-Zn alloys for degradable vascular stents were prepared via electrodeposition technique. The effect of the concentration of the complexing agent sodium citrate (C₆H₅Na₃O₇·2H₂O) in the plating solution on the solution state, alloy composition, and microstructure was investigated. The complexation mechanism of Fe³⁺, Fe²⁺ and Zn²⁺ by C₆H₅Na₃O₇·2H₂O was analyzed using cyclic voltammetry. In vitro degradation properties of the alloy were evaluated through electrochemical and static immersion methods. Results showed that an increase of C₆H₅Na₃O₇·2H₂O concentration led to a decrease in pH change of the plating solution, deposition rate, and cathodic current efficiency. With increasing concentration of C₆H₅Na₃O₇·2H₂O, the reduction potential of Zn²⁺ gradually shifted positively while the reduction potential difference with Fe²⁺ decreased; this resulted in increased Zn content in the alloy and reduced grain size indicating promotion of co-deposition by C₆H₅Na₃O₇·2H₂O. However, excessive sodium citrate caused an increase in the reduction potential difference between Fe²⁺ and Zn²⁺ leading to decreased Zn content as well as coarsened grains suggesting significant influence on alloy composition from complexing agent concentration. Alloys with different compositions exhibited a single α(Fe) phase with a (211) preferred orientation crystal plane. Polarization curves, electrochemical impedance spectra, and long-term immersion experiments indicated that the corrosion sensitivity of the alloys increased and the corrosion rate elevated with an increase in Zn content in the alloys. These results indicate that reasonable control over the complexing agent, C₆H₅Na₃O₇·2H₂O, is expected to obtain Fe-Zn materials for biodegradable stents with good microstructure and performances.