A stearic acid/polypyrrole-based superhydrophobic coating on squeeze-cast Mg-Sr-Y-Ca-Zn alloys for improved salt-water corrosion

Abstract

A polypyrrole (PPy) and stearic acid (SA)-based superhydrophobic coating was developed to improve the corrosion behavior of squeeze cast Mg-1.0 wt% Sr-1.0 wt% Y-0.6 wt% Ca-0.5 wt% Zn alloy in NaCl solution. The structure and morphology of the coating were analyzed using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), three-dimensional optical surface profilometer, glancing angle X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS) reveals variable coating thickness ranging from ~25–120 μm (depending on Mg alloy pretreatment with/without NaOH prior to coating) comprised of primarily Mg, C, O, Cl, and Zr species ascribed to a mixed composition of PPy, Mg(OH)2, and low amount of ZrO2 with mud-crack features and petal-like clusters. Although SA/PPy-based superhydrophobic coating (contact angle of static water droplets ~1640) can improve the corrosion resistance of bare Mg alloy drastically owing to the presence of thin air cushion between NaCl solution and Mg alloy surface preventing saltwater infiltration and advancement of Cl−-ions towards reactive Mg surface, however, the surface pretreatment in NaOH salt bath before superhydrophobic surface treatment has a profound effect on the interphase, bonding, and stability of the coating with Mg and associated corrosion phenomena. The Mg alloy without NaOH pretreatment (NPT) demonstrated excellent instantaneous corrosion resistance in 0.1 M NaCl confirmed by potentiodynamic polarization, polarization resistance (RP), and coating resistance (Rcoat) measured by electrochemical impedance spectroscopy (EIS). On the contrary, NaOH pretreated Mg alloys (1MPT, 5MPT) exhibited excellent corrosion protection ability (up to ~4-5 h) during long-term EIS study owing to the presence of Mg(OH)2 thick film deposited during pretreatment acting as a protective barrier preventing PPy degradation and Mg2+-ion diffusion to the outward surface. Overall, SA/PPy-based superhydrophobic coating juxtaposed with/or without surface pretreatment can shed new insight into the corrosion phenomena of squeeze cast Mg alloy in NaCl solution.