Study of factors affecting corrosion of aluminum automotive wheels

Abstract

Aluminum automotive wheels, which have been used since the mid 1970’s, have proven to be very rugged, but early wheels discolored with time. Since 1976 the machined “bright” exterior surfaces, have been coated with a chromium based “conversion” pretreatment, and painted with a “clearcoat” of polyester or acrylic to preserve appearance. These coated wheels can exhibit “filiform” corrosion between the Al surface and the clearcoat (Fig. 1). The corrosion is filamentous and closely follows aluminum grain boundaries. Corrosion results from oxidation of the aluminum (anode) in the presence of water and oxygen at a break (cathode) which intrudes through the clearcoat and conversion coat into the aluminum (Fig. 2). Current specifications require ≤ 1.6 mm filiform corrosion from a scratch after 400 h of accelerated corrosion testing. This paper correlates accelerated corrosion with the composition of the clearcoat paint, the UV absorbance of the paint and the microstructure of the Al / Cr conversion coat / clearcoat interface to learn the necessary parameters to produce Al wheels with enhanced protection. Production Al wheels, from a variety of sources, wheels prepared to our specifications and a design of experiments (DOE) wheel series prepared in our laboratories were examined.The accelerated corrosion test exposes the test piece to UV weathering (400 h) with four water spray cycles every 23 h. After UV exposure the surface is scratched through the surface layers into the Al metal with a carbide scribe, followed by exposure to HCl vapor (1 h).