Effect of Cyclic Deformation on the Corrosion of Coated AA6xxx Hem Flanges

Abstract

The cyclic corrosion and cyclic deformation behaviour of AA6022 and AA6111 coated au-tomotive hem flanges were investigated. Neutron radiography, in conjunction with a gado-linium (III) nitrate (Gd(NO3)3) solution, was used to reveal the existence of an interconnect-ed network of pores within the adhesive layer of the hem flange coupons. Cyclic corrosion tests revealed that the inner compressive surface of the outer sheet (area B) was the weak link in the corrosion resistance of a hem flange due to the high degree of plastic deformation combined with micro-cracking present in this area. Cyclic deformation test revealed that cyclic deformation caused cracking in the adhesive layer and subsequent loss of load bear-ing ability. The cyclic deformation of pre-exposed hem flange coupons showed that cyclic corrosion exposure resulted in adhesive embrittlement and an earlier initiation of cracking in the adhesive layer. The current evidence was not enough to demonstrate that the corrosion resistance of a hem flange coupon is altered by the damage accumulated during cyclic de-formation. An imposed low-level static stress on pre-cracked hem flange coupons was found to have a synergistic effect with the cyclic corrosion environment and resulted in ad-hesive failure at the adhesive-metal interface, thereby exposing new sites for corrosion.