Filiform Corrosion Inhibition of Zn–Al–Mg-Coated Steel by Ti–Zr Conversion Coating: Crucial Role of Benzotriazole-5-Carboxylic Acid

Under-film filiform corrosion (FFC) poses a significant challenge to the durability and reliability of Zn–Al–Mg-coated steel (ZM-steel), necessitating the development of advanced protective strategies. Herein, benzotriazole 5-carboxylic acid (BTC)-modified titanium zirconium (TiZr) conversion coatings (CCs) were developed as a sustainable solution to mitigate FFC, enhance corrosion resistance, and improve the adhesion of epoxy (EP) coatings on ZM-steel. Through electrochemical impedance spectroscopy, potentiodynamic polarization, FFC, and neutral salt spray (NSS) tests, the optimized BTC-TiZr CC, prepared in a Ti–Zr solution containing 0.5 g/L BTC, a treatment temperature of 25 °C, and a short immersion duration of 120 s, exhibited superior corrosion resistance, robust adhesion strength, and exceptional FFC inhibition. Metal–organic complexes (MOCs) of Ti/Zr/Zn with BTC were identified alongside mixed Ti/Zr/Zn/Al/Mg oxides and fluorides within the BTC-TiZr CC. BTC not only exerted strong corrosion inhibition efficacy for ZM-steel but also facilitated the formation of MOCs with metal ions in TiZr conversion solutions (CSs). Density functional theory calculations revealed that BTC preferentially adsorbs onto the MgZn2 phase, effectively suppressing its dissolution within the ZM layer. These findings establish BTC-TiZr CCs as a promising, environmentally friendly solution for advanced corrosion protection of ZM-steel, especially FFC suppression.