Pretreatment has become a necessary process to enhance the surface chemistry and morphology of aluminium alloys before implementing subsequent corrosion protection measures, especially organic coating application. Traditionally, hexavalent-chromium-based pretreatment processes were frequently used in many industries, but hexavalent-chromium-based chemistries are now recognized as a potentially carcinogenic hazard and environmentally harmful. The development of alternative, non-toxic and eco-friendly corrosion inhibitor and pretreatment technologies has become of pivotal importance to industries as a basic license to operate and to reach a sustainable society as a whole.
In this work, sodium galactarate (NaGal) was investigated as a potential green corrosion inhibiting chemical for hexavalent-chromium-free pretreatment and to serve as a coating adhesion promoter on aluminium alloy AW3003. The pretreatment layer formation was examined at five different pHs; 3, 4, 7, 10, and 11. The surface analysis was performed by Fourier transform infrared – reflection absorption spectroscopy (FTIR-RAS), contact angle measurement and white light interferometry (WLI). Samples with pretreatment layers formed in acidic and alkaline environment showed an increase in polar energy and surface roughness, which are strongly related to the coating adhesion properties, in line with the failure stresses obtained from the pull-off adhesion tests. The corrosion resistance was evaluated through the electrochemical behavior that was measured by linear polarization resistance (LPR), potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements. Immersion testing was performed to evaluate the pitting corrosion behavior. The pretreated sample exhibited less pitting than the reference sample, especially in an alkaline environment where aluminium alloys severely corrode due to limited protectiveness of the oxide layer. The pretreatment prevented the corrosion products from accumulating on the sample surface, limiting stain and smut layer redeposition, subsequently reducing the pitting corrosion.