Hybrid surface treatment with eco-friendly citrate pickling for enhanced bonding performance in CFRP-steel interfaces

• Proposes a hybrid surface treatment for corroded steel prior to CFRP bonding. • Incorporates cellulose nanofibers to enable practical on-site applications. • Provides mechanistic insights into citrate pickling via electrochemical analysis. • Elucidates chemical–mechanical coupling effect on surface morphology. • Validates enhanced interfacial durability of CFRP–steel bonded joints. CFRP bonding reinforcement of steel members is often limited by interfacial degradation caused by inadequate surface treatment. A hybrid acid-pickling (HA) method was developed, consisting of sequential mechanical–chemical–mechanical steps, with chemical pickling as the core process. In this study, cellulose nanofiber (CNF) was added to citrate solution to form a pickling slurry with strong surface adhesion, thereby enhancing on-site applicability. Rust removal efficiency and substrate improvement were further optimized by adjusting the formulation, and examined through electrochemical tests, surface characterization, and mechanical evaluation. The results revealed that a 10 wt% citrate medium with over 4 h of pickling time efficiently removed the embedded pitting rust and increased the surface roughness. Coupled with subsequent mechanical grinding, the treatment reshaped empty pits into semi-closed geometries with anchoring capability, resulting in an approximately 4.5-fold improvement in adhesion strength compared with mechanical treatment alone. Furthermore, mechanistic analysis revealed that the CNF-assisted citrate pickling exhibited rust removal behavior similar to that of the citrate solution in the initial phase, proceeding rapidly before gradually slowing. In the later phase, however, an additional reaction peak emerged, induced by accelerated moisture evaporation within the CNF matrix. This CNF-induced effect altered surface topography by eliminating smaller pits. Atmospheric exposure test further validated that the modified surface condition enhanced environmental durability and mitigated interfacial degradation in CFRP–steel bonded joints. These findings highlight an eco-friendly citrate-based pickling for steel surface treatment on-site that generates solid, non-hazardous waste, promoting the wider adoption of CFRP-based reinforcement strategies in steel infrastructure.