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Technical Paper

Phosphate Uniformity of Aluminum Closure Sheet

Uniform phosphate deposition over the entire outer surface of automotive exterior body panels is necessary in order to optimize surface appearance after painting and to ensure long term corrosion performance. Non-uniformity of phosphate deposition can result from many factors, most of which are related to the processing parameters on the particular cleaning and phosphating line. In this study the phosphate uniformity on AA6111 body sheet material was examined. Phosphating was carried out on sheet metal in the as-annealed condition and after acid cleaning treatment. Phosphate deposition was rated both visually and by SEM analysis. Changes in oxide thickness and chemistry were evaluated by XPS methods. The effects of phosphate non-uniformity on paint adhesion and corrosion performance were also examined.
Technical Paper

Alternatives to Electrocoating Aluminum in Automotive Applications

The inherent corrosion resistance of aluminum is much greater than automotive steels. To demonstrate this principle in a fashion acceptable to the automotive industry, a test program was run which incorporated lab, test track and real life trials on both unpainted and painted aluminum and painted steel. The lab program consisted of neutral salt and cyclic corrosion tests. Having demonstrated that aluminum does not need electrocoating for good corrosion integrity, alternatives to electrocoating which would allow primers to be applied only where necessary for esthetic purposes were sought. Several primers were selected for study based upon current automotive usage. Factors such as the degree of pretreatment prior to primer application and the presence of residual lubricant on the metal were evaluated.
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.
Technical Paper

After Market Paint Repair of Aluminum Closure Panels

The rapid increase over the past few years in the use of aluminum closure panels on vehicles is anticipated to continue well into the next decade. Along with this increased usage comes the need for a greater amount of cosmetic repair of these closures in after market body shops. Although paint repair systems for aluminum have been available for some time, their effectiveness on newer alloys needed to be demonstrated. Some work in the recent literature has also questioned the corrosion and paint performance of aluminum closures after sanding. In this study, extensive surface analysis of AA6111 closure sheet material was carried out in the as-received condition, after phosphating and after sanding, cleaning and pretreating with selected paint repair systems from major automotive paint suppliers to determine the effects of surface residues or contaminants on subsequent corrosion performance.
Technical Paper

Cosmetic Corrosion of Aluminum Closure Panels: Lab Testing vs Field Performance

The correlation of lab test results with field performance for painted steel and galvanized steel automotive closure panels is now well established after many years. Although aluminum closure panels have been used on certain vehicles for many years, it has only been in more recent times that their usage has increased to a point where the issue of correlating lab and field corrosion data has become more essential. Many tests in the automotive industry were developed specifically for steel and their applicability to aluminum closures is uncertain. On the other hand, many of the standard corrosion tests used on aluminum were designed for aluminum applications other than automotive, e.g., architectural or packaging, so that the test environment and product requirements were very different. In this paper, a number of standard corrosion tests, including filiform, salt spray and cycling environment, were carried out on AA6111 and AA6016 closure sheet materials for comparison with field data.
Technical Paper

Corrosion Product Analysis of Aluminum Closure Panels

Painted aluminum panels subjected to several laboratory-based accelerated corrosion tests were examined using surface analytical techniques. This paper presents some of the results from these measurements, which indicated that the nature and the extent of the corrosion attack were greatly influenced by the salt spray conditions. In general, ASTM G 85 acetic acid salt spray produced the greatest amount of corrosion, while exposure to GM 9540P and ASTM B-117 resulted in the least amount of corrosion. Moreover, filiform corrosion was the most common corrosion attack observed from exposures to many of the salt spray tests. Finally, filiform corrosion appeared to propagate by the dissolution of the phosphate conversion coating, thus lifting off the paint layer from the aluminum substrate.