Studies of Adhesive Delamination Using DC-Resistance Measurement

Abstract

Abstract Delamination of three different automotive epoxy adhesives (A, B, and D) from two different steel substrates (cold-rolled steel and electrodeposited zinc steel) upon exposure to distilled water and a 1.0 M NaCl solution was studied using DC-resistance measurements. The DC-resistance measurements were taken on sandwich adhesive bonded specimens having different combinations of adhesive and steel metal substrates upon exposure to a test solution. The measurements were taken on: (i) electrodeposited zinc steel substrates bonded by adhesives A and D in distilled water; (ii) electrodeposited zinc steel substrates bonded by adhesives B and D in a 1.0 M NaCl solution: and (iii) cold-rolled steel substrates bonded by adhesives A and D in a 1.0 M NaCl solution. Four stages of the resistance change across the metal/adhesive/metal specimens were proposed, corresponding to the different stages of bonding degradation. It was found that the initial resistance decrease across an adhesive-bonded specimen was governed by the concentration of the electrolyte produced in the adhesive matrix. The more the water-soluble fillers dissolved in water inside the adhesive, the higher the concentration of the electrolyte and the lower the resistance across the specimen. Hence, the amount of water required to dissolve the water-soluble fillers was governed by the water absorption rate and the rate of adhesive degradation. Adhesive delamination occurred when sharp resistance fluctuations were detected across an adhesive-bonded specimen. The sharp resistance fluctuations were a result of the consecutive breakdown and formation phenomenon of the interfacial secondary oxide or hydroxide film. The magnitude of the resistance fluctuations increased with the degree of adhesive delamination and the area of oxide or hydroxide film breakdown. Anodic delamination was found to occur in the metal/adhesive/ metal specimens according to the crevice corrosion mechanism. The degree of anodic delamination depended on the nature of the secondary oxide or hydroxide film at the adhesive/substrate interface.