Effect of benzoxazine chemical structure on the corrosion protection of AZ31 Mg alloy in saline and acidic solutions

The interactions between the chemical groups of polymers and metallic substrates play a crucial role in corrosion protection, particularly for magnesium (Mg) alloys, which have been less studied in this context. To address this, benzoxazine resins with varying chemical structures were synthesized, and the impact of each structure on the corrosion protection of the resulting coatings was evaluated using electrochemical impedance spectroscopy (EIS). Four types of coatings exhibiting different levels of flexibility and hydrophobicity were synthesized and applied on Mg alloys via drop casting. The protective effects of these coatings were examined in both saline and acidic environments. Our results demonstrated that the length of the alkyl chains and the presence of additional aromatic groups within the benzoxazine resin backbone significantly influence the corrosion resistance of the resulting coatings. Adhesion of the coatings to the substrates was assessed before and after 35 days of immersion in 0.1 M NaCl solution. Coatings derived from monomers with longer alkyl chains exhibited superior corrosion protection in saline solutions. Water contact angle measurements were conducted to evaluate the hydrophobicity of the coatings, revealing that the most hydrophobic sample, with a contact angle of 105 degrees, featured both a longer alkyl chain and additional aromatic rings. Furthermore, the presence of these aromatic groups directly attached to a tertiary amine was also found to enhance the coatings' resistance to 0.1 M H2SO4 solution.