Fracture Mechanical Testing of Adhesion of Organic Coatings on Aluminium

Abstract

Applicability of the wedge test and the accompanying fracture mechanical data analysis, commonly used for testing of adhesively bonded joints, was inestigated in an attempt to develop a quantitative laboratory test for evaluating paint adhesion on aluminium. The test was further used, along with a relatively better accepted laboratory test for filiform corrosion of painted aluminium to study the mutual effects of corrosion and adhesion of various pretreatments and paint systems on extruded AA 6082 T6 aluminium alloy. Pretreatments used were commercial processes for deoxidising, chromating, hot AC anodising and Ti/Zr conversion coating. The paints were polyester TGIC and epoxy DICY, both pigmented commercial products. In particular, the compatibility of pretreatment-paint combinations were investigated The wedge test methodology and sample preparation developed proved to be a satisfactory approach for quantitative evaluation of pretreatment-paint combinations for aluminium. Ti/Zr treatment gave exceptionally good adhesion and corrosion results when coated with polyester. Epoxy coated Ti/Zr bars failed completely. However, hot AC anodising, although in general significantly more robust than Ti/Zr, was more compatible with the polyester than polyester coating. With the advent of chromate-free pre-treatments, the need to establish the compatibility of the modified aluminium surface and applied organic coating was demonstrated.