Assessment of rivet coating corrosion effect on strength degradation of CFRP/aluminum self-piercing riveted joints

This study investigates the effects of different rivet coatings on the corrosion behavior and strength degradation of self-pierce riveting (SPR) joints between dissimilar materials. Carbon fiber-reinforced polymer (CFRP) and an aluminum alloy were joined using Almac (R)- and Zn-Ni-coated rivets. The corrosion phenomena and the sub-sequent strength degradation of the joints were examined through thirteen weeks of accelerated corrosion tests. The experimental results indicated that the Almac (R)-coated SPR joints suffered strength losses more than three times higher than those of the Zn-Ni-coated SPR joints owing to severe rivet head corrosion. Characterization of the corrosion products and potentiodynamic polarization tests revealed the corrosion mechanism. A strong galvanic coupling formed between the Almac (R)-coated rivet head and the CFRP sheet, thereby accelerating the dissolution of the rivet head coating and initiating rivet base metal (steel) corrosion early in the test. Consequently, the mechanical locking between the rivet head and the top sheet significantly decreased, and the Almac (R)-coated SPR joints exhibited a change in failure type (from rivet head-tail pullout to rivet head pullout), along with significant strength loss. In contrast, the weak galvanic coupling between the top sheet and the Zn-Nicoated rivets improved the corrosion resistance of Zn-Ni-coated SPR joints.