The Magnesium Hatchback of the 3-Liter Car: Processing and Corrosion Protection

The hatchback of Volkswagen's 3 liter car (3 l fuel consumption per 100 km) consists of an inner component of die casting magnesium (AM50) covered with an aluminum panel from the outside. This hybrid design requires a new manufacturing process: The pre-coated magnesium part will be bonded and folded with the bare aluminum part. Corrosion protection is provided by an organic coating system which both protects against general corrosion and galvanic corrosion.

The corrosion of the Al / Mg sandwich has been examined with hybrid samples which are similar to the hatchback. Several powder coatings (epoxy resin, polyester resin, hybrid resin), wet paints and cathodic electro-coating paints of different thicknesses and compositions have been applied to the magnesium part. They show that only powder coating provides adequate protection. Galvanic corrosion at the points of attachment of the hatchback might be possible (for example the bolted joint of the hinge). Resistance against galvanic corrosion has been tested with samples that had a damaged paint coat in order to simulate a defect. Furthermore different contact elements (Fe, Al, Zn, etc.) have been attached with electrically conductive fixings. Under laboratory conditions, all test samples where subjected to a corrosion load of 30 cycles in the environmental corrosion test.