Interfacial bonding and surface chemistry of silane–zirconium hybrid conversion coatings for durable protection of ADC12 aluminum alloy

Zirconium-based conversion coatings (ZCCs) are a promising chromium-free alternative for aluminum alloys but are often suffer from weak interfacial bonding with organic topcoats and limited long-term durability. To address these challenges, a silane–zirconium hybrid conversion coating was developed by incorporating KH-560 silane into a ZCC formulation for ADC12 aluminum alloy. The influence of silane concentration on coating microstructure, interfacial chemistry, adhesion and corrosion performance was systematically investigated using SEM, AFM, XPS, and electrochemical techniques, complemented by density functional theory (DFT) calculations. An optimal KH-560 concentration of 20 g/L (ZCC-KH560-20) produced a dense, crack-free, and hydrophobic hybrid film through the formation of a cross-linked Si–O–Si network and covalent Si–O–Zr bonds, leading to enhanced coating compactness and interfacial stability. Consequently, the corrosion current density decreased by two orders of magnitude, while epoxy–substrate adhesion increased nearly fivefold (from 2.61 to 12.32 MPa). Extended electrochemical impedance monitoring revealed that the ZCC-KH560-20–EP system maintained impedance values above 10 8 Ω·cm 2 after 60 days of immersion in 3.5 wt% NaCl solution. The enhanced durability arises from a synergistic mechanism combining a robust physical barrier with active self-passivating. This study provides mechanistic insight and a quantitative strategy for designing sustainable hybrid conversion coatings for aluminum alloys.