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) whils t the others are chromate-free (BONDERITE 777 and EP2472) and not wide ly used for adhesive bonding. In the present study, the degree of surf ace modification introduced by these treatments has been determined us ing 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-onl y and grit-blasted adherends were used as controls. Overall, the conve rsion coatings provided better durability performance than the mechani cal treatments. The developmental treatment EP2472, a chromate-free co nversion coating, out-performed the established chromate-phosphate pro cess at low applied loads (less than or equal to 0.5 kN). All three co nversion coatings performed similarly at the higher loads(greater than or equal to 1 kN).