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Corrosion tendency is one of the major inhibitors for increased use of magnesium alloys in automotive structural applications. Moreover, systematic or standardized methods for evaluation of both general and galvanic corrosion of magnesium alloys, either as individual components or eventually as entire subassemblies, remains elusive, and receives little attention from professional and standardization bodies. This work reports outcomes from an effort underway within the U.S. Automotive Materials Partnership - ‘USAMP’ (Chrysler, Ford and GM) directed toward enabling technologies and knowledge base for the design and fabrication of magnesium-intensive subassemblies intended for automotive “front end” applications. In particular, subassemblies consisting of three different grades of magnesium (die cast, sheet and extrusion) and receiving a typical corrosion protective coating were subjected to cyclic corrosion tests as employed by each OEM in the consortium.

One of the key coating layers that inhibits corrosion on modern automobiles is the pretreatment film. This layer, which is typically a tri-cationic zinc phosphate material, provides both corrosion protection and enhanced paint adhesion to the base metal. Recent tightening of environmental regulations has made the use of this coating more difficult. In response to these pressures, pretreatment suppliers have been developing a new generation of metal pretreatments based on zirconium oxide. Characterization of these new materials is challenging as the zirconium oxide-based coatings are over ten times thinner than the current zinc phosphate coatings. Methods that are currently employed for studying zinc phosphate films such as coating weight determination by weighing, and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS) are not sensitive enough to fully characterize these materials.

The pretreatment of aluminum sheet material in preparation for further paint application can be challenging due to the presence of a thick oxide layer. The composition of the oxide layer is primarily aluminum oxide, but it may also contain magnesium that is typically dispersed unevenly throughout the oxide layer. Zinc-phosphate systems remove much of the oxide layer on aluminum, but questions remain on the extent of removal of the oxide layer by zirconium oxide-based pretreatments and how these oxide layers may affect the zirconium oxide-based pretreatment deposition on aluminum. Several methods have been used to characterize the coating of zirconium oxide-based pretreatments on aluminum. Scanning electron microscopy at very high magnification reveals a coating on aluminum that is significantly different in morphology than the same coating chemistry on steel substrates.

An electrochemical testing protocol for assessing the intrinsic corrosion-resistance of creep-resistant magnesium alloys in aqueous environments, and effects of passivating surface films anticipated to develop in the presence of engine coolants is under development. This work reports progress in assessing the relative corrosion resistance of the base metals (AMC-SC1, MRI-202S, MRI-230D, AM50 and 99.98% Mg) in a common test environment, based on a near-neutral pH buffered saline solution, found to yield particularly stable values for the open-circuit or corrosion potential. This approach was found to provide a platform for the eventual assessment of the durability of certain passivating layers expected to develop during exposure of the magnesium alloys to aqueous coolants.