Adhesion mechanisms of waterborne epoxy coatings to steel substrates: A molecular-level investigation

For optimization of adhesion strength and long-term performance, an understanding of the interfacial chemistry between waterborne epoxy coatings and steel substrates is crucial. While it is generally accepted that physical interactions, such as Van der Waals forces, contribute to adhesion, direct evidence for covalent Fe–O–C bonds in epoxy–steel systems have not been reported. In this study, a multi-technique approach was used to investigate molecular interactions at the coating–steel interface. Depth-resolved X-ray photoelectron spectroscopy (XPS) with line-scan analysis revealed a transition from organic oxygen groups (–C–O, –C = O) at the air–coating surface to inorganic signals of the steel substrate (Fe–O–C bond) at the buried interphase. Complementary attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) showed accumulation of unreacted epoxide groups, reduced β-hydroxyl formation, and suppressed epoxy–amine curing, together revealing an interphase with a significant degree of covalent Fe–O–C bonds. The results highlight the importance of interfacial chemistry in waterborne epoxy coatings and provide mechanistic insight for optimization of coating formulations and surface preparation strategies to enhance adhesion performance.