Study on the influence of water ingress on the interface of high strength zinc coated steels and epoxy adhesives

This study aimed to assess the impact of humidity on adhesive joints comprising four different Zn-coated high strength steel substrates (SA, SB, SC, and SD) and two epoxy adhesives (EA and EB) at 70 degrees C. Several tests were conducted to analyse the effects of humidity on adhesive properties, under both dry and saturated conditions. Gravimetric tests were used to measure water absorption by the adhesives, while mechanical properties were evaluated through bulk tensile tests. Chemical structure changes were examined using Fourier-transform infrared spectroscopy (FTIR), and the glass transition temperature was determined using a DMA-like test. The water immersion behaviour of the substrates was studied through gravimetric tests and surface energy was analysed using contact angle measurements. The overall joint performance was assessed through gravimetric tests, and the mechanical impact of water was evaluated using single lap joint tests on both dry and aged joints, for one week and one month. The findings indicated that EB adhesive absorbed more water compared to EA, which did not reach saturation. FTIR analysis revealed significant chemical degradation in the EB adhesive after ageing, leading to greater deterioration in mechanical properties compared to EA. The bulk substrate tests showed similar surface energy among the substrates, with SA being the only substrate that did not corrode during water im-mersion. In terms of joint behaviour, cohesive failure was predominant in dry conditions, but as the joints aged, those with EB adhesive experienced a decrease in failure load due to adhesive degradation, while the interface remained strong. Joints with EA adhesive exhibited increasing interfacial failure, indicating weaker interfaces that allowed water ingress between the adhesive and substrate. Consequently, water uptake in the joints was higher for EA adhesive than for EB adhesive, contrary to the behaviour observed at the bulk level. SA and SD substrates demonstrated stronger interfaces with EA, likely due to their higher developed interfacial area ratio, which facilitated mechanical interlocking.