Hybrid-acid pickling method for enhancing adhesion and durability at CFRP–steel bonded interface

A hybrid-acid (HA) pickling method has been developed to remove rust from corroded steel components prior to localized strengthening with carbon-fiber-reinforced polymer. This study conducted a time-dependent assessment of the pickling reaction and steel-surface characteristics. The rust removal mechanism was hypothesized at the mesoscale and electrochemical levels. Furthermore, a comparative study among conventional blasting, power-tool grinding, and the HA method was performed via the surface characterization of steel substrates and deterioration tests on composite joints. The results demonstrated that the HA method has excellent applicability to corroded steel surface with pitting behavior, and exhibits good comprehensive performance regarding adhesion, surface morphology, and oxide residue. In addition, the pickling process of 1 h reduces over 70 % intrinsic salt residue, while pickling over 4 h facilitates the formation of decentralized pit craters that provide an anchoring effect for adhesive bonding. The bond strength of the corroded steel surface prepared by the HA method is comparable to that of the blasted surface with a difference of ±10 %. Moreover, it provides excellent water resistance at the bonded interface, and the bond strength of the CFRP-steel joint increased by 10–30 % due to its special mechanical interlocking properties. This study offers new insights into advancing the applicability of both the HA method and CFRP technique for corroded steel structures.