Influence of pre-treatments on adhesion, barrier and mechanical properties of epoxy coatings: A comparison between steel, AA7075 and AA2024

In this work five independent pre-treatments – mechanical polishing, alkaline and acid etching, sol-gel primer, and anodizing – were applied on mild steel and aluminum (AA7075 and AA2024) substrates before the deposition of epoxy coatings. The results demonstrate that efficient adhesion can be achieved by simple methodologies and widely available chemicals as an alternative to toxic conversion layers or highly energetic processes. All the treatments provided higher adhesion (up to 75%, 61% and 14% on AA7075, AA2024 and steel, respectively) and better anti-corrosion performance (increase of impedance values at low frequency up to four orders of magnitude) than the polished reference. The anodizing of aluminum alloys yielded good anti-corrosion performance, but slightly lower mechanical properties compared to sol-gel primer and alkaline etching. The hydroxyls density was found to play a major role in strong adhesion prevailing over surface roughness, contact area, oxide thickness and hydrophobicity. Despite the efficient performance of hydrogen bonds from acid and alkaline etching on aluminum, these connections do not yield sufficiently stable bonds that make an impermeable barrier of epoxy coatings on steel. Covalent bonds (Me-O-Si) represent the key to achieve a significant improvement in mechanical and chemical resistance against the ingress of water and polymer chain relaxation. This makes the sol-gel primer a simple, economical, and environmentally alternative towards efficient bonding at a molecular level.