Filiform corrosion of electrocoated aluminium alloy: Role of surface pretreatment

Commercial Oxsilan, 3-aminopropyltriethoxysilane and sol–gel siloxane coatings were applied on AA6016 aluminium alloy before electrocoating. Standard filiform corrosion test demonstrated the pre-treatment role on the under-paint corrosion rate. All pre-treatments increase the coating adhesion in water electrolyte (EIS). Anodic undermining is the mechanism of the coatings de-adhesion (SKP). Electromotive force (difference in potentials of defect-surrounding interface) and interface ability to promote the chloride migration are rate-determining factors of the filiform corrosion. Contrary to other pre-treatments, the aminosiloxane decreases the interfacial potential, facilitates the chlorides migration and leads to anodic de-adhesion of large coated areas (Tof-SIMS and EDXS).

► Adhesion improvement of an electrocoating by four investigated silane pre-treatments.
► Anodic undermining is the mechanism of the coating de-adhesion.
► Galvanic cell and interface ability to promote chloride migration are determining factors of the filiform corrosion.
► The activation mechanism of filiform corrosion by aminosiloxane layer is proposed.