A novel hydrophilic-to-hydrophobic convertible coating on biodegradable Mg-4Zn-1Mn alloy: Preparation, microstructure and corrosion behaviour

A series of hydrophilic-to-hydrophobic convertible coatings, consisting of copolymers made from poly (N-isopropyl acrylamide) (PNIPAAm) and chitosan (CHI) with various CHI/PNIPAAm ratios, were prepared on Mg-4Zn-1Mn alloy surface pretreated by micro-arc oxidation (MAO) to improve its corrosion behaviour. Microstructural analysis demonstrated successful deposition of the PNIPAAm-g-CHI coatings and their thickness increased with increasing CHI/PNIPAAm ratio. The bonding strength of the coatings became worse as the CHI/PNIPAAm ratio increased and the strongest bonding was obtained at CHI/PNIPAAm ratios of 0:10 and 1:9. The electrochemical corrosion tests indicated that the corrosion behaviour of the coatings deteriorated as the CHI/PNIPAAm ratio increased. The coating with a CHI/PNIPAAm ratio of 0:10 exhibited the optimal corrosion resistance, demonstrating an impedance of 9455.2 ​Ω ​cm² and a corrosion current density of 0.179 ​μA/cm², which was about respectively 2 times higher and 10 times lower than those of the MAO-treated sample. The surface wettability measurements revealed that the lower critical solution temperature (LCST) progressively became higher when the CHI/PNIPAAm ratio increased, resulting in a continuous decrease in the water contact angle. The coatings presented hydrophobic properties above the LCST, whereas they displayed hydrophilic below the LCST.