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.