Salicylic and tartaric ions co-doped polypyrrole to enhance the corrosion protection of polypyrrole/zinc bilayer coating on ZK60 magnesium alloy in Hank's solution

To enhance the corrosion protection of the polypyrrole/zinc (PPy/Zn) bilayer coating on biodegradable ZK60 magnesium alloy in Hank's solution at 37 °C, sodium salicylate (NaSA) in different concentrations (CNaSA, 0.5–4.0 mM) is selected as a co-doping reagent of tartrate anions (TA−), and an electropolymerization method that combines potentiostatic and galvanostatic synthesis is developed to prepare PPy(TA + SA)/Zn bilayer coatings. The co-doping of SA− suppresses the growth of PPy and notably increases its synthesis potential (Epoly) with increasing CNaSA. The co-doped PPy(TA + SA) films have a surface morphology and thickness similar to the PPy(TA) film. However, their chemical composition, structure, and adhesion strength change notably due to their varying growth conditions. In the long-term immersion test of PPy(TA + SA)/Zn/ZK60 samples, the PPy(TA + SA)/Zn coatings exhibit high stability, and the coating prepared with CNaSA = 1.0 mM has the most prolonged protection time (54 ± 1.5 d). As CNaSA increases, the co-doped PPy(TA + SA) film becomes heavily over-oxidized to produce more defects on PPy chains due to the high Epoly, ultimately decreasing the protection time to 40 ± 1.0 d. The corresponding mechanism involved is thoroughly discussed.