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Technical Paper

Aluminum Wheel Alloy Variants: Effects on Corrosion Performance

Previous work has shown that variations in wheel alloy chemistry, particularly with respect to copper and iron levels, can have a pronounced effect on filiform corrosion performance. In this study, an examination of A356.2 alloy chemistry variants and their effects on corrosion was carried out in greater detail. The emphasis was on copper and iron variants, both alone and in combination. Copper levels ranged from 0.005 to 0.22% and iron from 0.04 to 0.23%. The effect of manganese additions was also examined, with levels ranging from 0.002 to 0.07%. In addition to the alloying variants, the level of dispersed oxides in the castings was varied to determine any effects on corrosion performance. Although filiform corrosion performance of painted samples was the primary focus of this study, the corrosion behaviour of unpainted samples was also evaluated for comparison purposes.
Technical Paper

Effects of Alloy Composition and Condition on Filiform Corrosion Performance of Cast Aluminum Wheels

The movement towards extended warranties in the automobile industry has focussed attention on corrosion performance of many components, particularly cast aluminum wheels. Filiform corrosion is of particular concern since it can severely affect the appearance of the wheel. The appearance and the choice of wheel design are the most attractive features to customers. In order to enhance the filiform corrosion resistance of cast aluminum wheels, cleaning, pretreatment, coating and alloy parameters are critical and need to be optimized. In this paper, the effects of alloy composition and condition on filiform corrosion are reviewed. A series of cast discs were prepared with variations in iron, zinc and copper levels around the standard A356.2 alloy composition. Apart from composition, certain specimens were subjected to different heat treatment and ageing conditions. The effects of porosity and different machining procedures were also evaluated.
Technical Paper

Reduced Finishing Costs for Aluminum Intensive Vehicles

Due to the inherently superior corrosion resistance of aluminum compared to automotive steels, phosphating and electrocoating are not necessarily required to provide good corrosion protection to aluminum intensive vehicles. This allows the potential for significant cost savings in the overall finishing process by eliminating these steps. Advantage can also be taken of the movement towards the use of powder primer surfacers to reduce solvent emissions in that the powder coating can be applied directly to a suitably pretreated aluminum surface. Pretreatments which are optimized for aluminum and much simpler to control than phosphating were chosen for trials based upon discussions with chemical suppliers. In this paper, the adhesion and corrosion characteristics of these selected pretreatment/powder primer systems were compared to standard phosphated and electrocoated AA6111 automotive closure sheet.
Technical Paper

Galvanic Corrosion Prevention of Steel-Aluminum Couples

Efforts towards weight reduction are leading towards increasing use of aluminum components on automobiles. Although aluminum on its own has inherently superior corrosion resistance to steel, galvanic action between the aluminum and steel or galvanized parts can lead to severe corrosion. Straightforward and effective methods of preventing galvanic corrosion from the subject of this paper. Since many aluminum components are connected to steel structures by mechanical fasteners, protective coatings on fasteners were evaluated as well. Galvanic test couples were prepared in a manner simulating typical automotive assembly conditions while incorporating features which would lead to enhanced corrosion. A variety of chemical treatments and coatings on the fasteners as well as barriers between the dissimilar metals were evaluated for corrosion prevention between the aluminum and cold rolled or galvanized steel. Comparison between neutral salt spray and cyclic corrosion tests is provided.