Gunnar Leijon, Dan Persson, Maria Öhman, Caroline Celsing, Mats Larsson, Arne Melander

Abstract

The report summarises the study on prediction of durability of adhesive bonded aluminium joints for automotive applications. Specimens of adhesively bonded alloys AA6060, AA5182, AA6016 and hot dip galvanised steel were subjected to on-vehicle exposure on bus for two years and accelerated corrosion tests. Aluminium samples with different surface states have been examined: re-oiled sheets with no pretreatment, extruded profiles after sulphuric acid anodisation (SAA) and phosphoric anodisation (PAA), profiles and sheets pretreated with Ti-Alodine 2040 and sheets pre-treated with Ti-Zr-SiO2 Gardobond X4591 (incl. a version with additional dry lube application). The samples have been bonded with epoxy Betamate1480, polyurethane adhesive Betamate 7185, rubber based adhesive Betaguard RB10 to lap shear specimens. Parts of the bonded specimens were subsequently E-coated. The lap shear specimens were mechanically tested with tensile testing, fatigue testing and high-speed testing in as received condition and after on-vehicle exposure and accelerated corrosion tests.

On-vehicle exposure resulted in varying results concerning the degradation of the mechanical properties and the corrosion for the different material combinations. The response to the environmental exposure was strongly related to both the surface pre-treatment and the type of adhesive. Similar corrosion behaviour was obtained after different accelerated corrosion tests and the on-vehicle exposure and the correlation between these exposures was generally good.

The influence of climatic parameters on the corrosion was studied using samples with an adhesive bead on top (epoxy Betamate 1480). These samples consisted of AA6016 pre-treated with Ti-Zr-SiO Gardobond X4591 and AA5182 pre-treated with Ti- Alodine 2040, as well as AA6016 and AA5182 in no pre-treated condition. The samples, which were given a proper surface treatment, showed good durability. AA5182 without pre-treatment showed unsatisfactory durability. Filiform corrosion was observed on AA6016 sheet with epoxy adhesive after on-vehicle exposure and after accelerated corrosion testing, both without prior surface treatment and with insufficient surface treatment leaving an enrichment of Mg at the surface. Limited durability degradation was also observed on some other systems, such as AA6060 with Ti- Alodine 2040 and AA5182 with Ti-Alodine 2040 and Ti-Zr-SiO2 Gardobond X4591 after on-vehicle exposure.The corroded area, observed under the adhesive of adhesive bead specimens, showed some correlation with the reduction in strength of lap shear specimens. The results of the accelerated corrosion test S1/D70/T24/NS2, correlated well with on-vehicle exposure results for shear specimens of aluminium bonded with epoxy adhesive, and hence this test is recommended for the evaluation of similar adhesive joint systems.