A Comparison of Chromate-Phosphate and Chromate-Free Conversion Coatings for Adhesive Bonding

Three different conversion coatings have been evaluated. One of these is an established chromate-phosphate treatment (BONDERITE 705) whilst the others are chromate-free (BONDERITE 777 and EP2472) and not widely used for adhesive bonding. In the present study, the degree of surface modification introduced by these treatments has been determined using Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Both initial single lap shear and stressed durability results have been obtained using a single part epoxide adhesive. Degreased-only and grit-blasted adherends were used as controls. Overall, the conversion coatings provided better durability performance than the mechanical treatments. The developmental treatment EP2472, a chromate-free conversion coating, out-performed the established chromate-phosphate process at low applied loads (≤0.5 kN). All three conversion coatings performed similarly at the higher loads (≥ 1 kN).