Electropolymerization of acrylic acid on steel for enhanced joining by plastic deformation

Resource-efficient electrochemical polymerization of acrylic acid (AA) on steel (DC04), from mildly acid Zn²⁺/ AA-containing aqueous solutions, is presented as an effective and sustainable thin film technology which allows for a precise control of the polymer thickness and morphology. The steel substrates were potentiostatically polarized to -1.4 V (vs. Ag/AgCl) or cycled between +0.1 to -1.4 V (vs. Ag/AgCl) in a degassed 2.0 M AA and 0.2 M ZnCl₂-containing aqueous electrolyte solution at pH 6. FE-SEM, AFM, FT-IRRAS, XPS, and electrochemical measurements shed light on the structural and chemical features exhibited by the as-prepared polyacrylic acid (PAA) thin films. When PAA-modified DC04 plates are joined to Al EN AW-1050A H24 specimens by plastic deformation using cold pressure welding (CPW), tensile strength testing unveiled the macroscopic interfacial adhesion-promoting capabilities exhibited by PAA layers to the opposing aluminum oxide surfaces. Carboxylate moieties present in the PAA are shown to form stable chemical bonds with metal oxide surfaces and with amine-epoxy-based resins. Interestingly, higher maximum shear force values are obtained for the PAA films when Zn metallic deposits are not present in the organic layer, but, when these welded specimens are heated up to 200 °C in a N₂ atmosphere, it occurred exactly the opposite: Zn particle-containing PAA films showed the highest maximum shear force values for the same deposition time.