Synergy of consecutive PEO and LDH surface treatments on the corrosion protection and adhesion strength of organic coatings on AA2024T3

Abstract

The properties of the interface AA2024-T3/organic coating is improved with a tailored combination of different surface treatments: i) a dual anodizing process comprising an energy-efficient (41 % reduction in energy consumption) plasma electrolytic oxidation (PEO) process just after conventional anodizing (to build a precursor layer), followed by ii) a conversion coating (CC) based on the in-situ growth of a MgAl-layered double hydroxide (LDH). This combination allows the formation of a thick PEO coating (40 μm) with an outermost LDH-CC layer. Prior to applying the organic coating, a characterization of the roughness, crystalline structure, morphology and composition of the protective systems (PEO and PEO-LDH) is thoroughly investigated. Corrosion performance is evaluated using electrochemical impedance spectroscopy (EIS) for all the systems, uncoated and epoxy-coated ones. In addition, the wet and dry adhesion properties of the epoxy-coated systems are evaluated. Results reveal that the PEO-LDH + Epoxy system shows superior wet adhesion and better corrosion resistance after 28 days of immersion in 3.5 wt% NaCl. Furthermore, the EIS diagrams for the PEO-LDH system demonstrate a synergistic effect of the PEO and LDH treatments, as evidenced by the capacitance and resistance values associated with the low-frequency time constant, outperforming the stand-alone PEO system.