Understanding the corrosion behavior of poly(3-hydroxybutyrate) coated ZK60 Mg alloy under dynamic m-SBF electrolyte flow conditions

Electrochemical impedance spectroscopy (EIS) has been used to quantify the extent of degradation of a bare ZK60 magnesium alloy under static immersion and dynamic flow (circulation using a submersible pump) conditions in a modified simulated body fluid (m-SBF) electrolyte. Dynamic flow conditions showed aggressive corrosion due to the absence of protection offered by the corrosion product film, as seen by a nearly 2-fold lower polarisation resistance (R_P) compared to static immersion. Further, 1 wt. % (thin) and 5 wt. % (thick) biodegradable poly(3-hydroxybutyrate) (PHB) coated ZK60 were studied under such dynamic electrolyte flow conditions over 9 days. Fitted impedance spectra revealed that the thick PHB coating with lower pore density significantly delayed the corrosion of the ZK60 alloy by acting as a barrier to the ingress of the electrolyte, with a nearly 2-fold higher R_P than the thin PHB coating. Grazing-incidence X-ray diffraction (GI-XRD) confirmed the greater extent of PHB degradation for the thin compared to the thick coating after 9 days. Complementary X-ray photoelectron spectroscopy (XPS) analysis also quantified the greater degree of formation of corrosion products for the former compared to the latter.