Influence of steam-based pre-treatment using acidic chemistries on the adhesion performance of powder coated aluminium alloy AA6060

In this study, the adhesion of a commercially applied powder coating on a steam treated AA6060 surface with pure steam and steam with citric and phosphoric acid chemistries has been investigated. Contact angle, roughness, and nanoscale pull off forces were determined as a function of the steam treatment prior to application of the powder coating. A focussed ion beam technique was used to examine the cross section of the powder coating, interface adhesion, and fracture morphology after the boiling test and interface indentation method. Transmission electron microscopy was used to study the fracture after indentation. Regardless of the steam treatment method, the wettability of the AA6060 surface was increased after the steam treatment. Addition of citric and phosphoric acid resulted in a low degree of hydrophobicity for the oxide layer compared to the use of pure steam. Steam generated oxide with citric and phosphoric acid resulted in poor penetration of the adhesive over the intermetallic particles, and the interface showed mixed (cohesive/adhesive) fracture during interface indentation, while the pure steam treated surface showed dominant cohesive fracture.