About Corrosion and Surface Science

This group specializes in the surface microstructure of light metal alloys, specifically aluminum and magnesium, with focus on corrosion and paint adhesion. The microstructure of the as-processed surface is therefore of prime interest along with actual corrosion and adhesion performance in service. Naturally, understanding the complex properties of an industrially-processed surface often requires work with idealized model surfaces. However, the present approach deviates from the usual practice of removing the as-processed surface in preparing specimens for research purposes in the laboratory. The as-processed surface microstructure is almost always very different from the bulk, and this structure determines certain crucial properties of the material in practical application, such as corrosion, optical and tribological.

Both thermomechanically and chemically processed surfaces are of interest. To give an example, figure 1 shows the existence of a highly-deformed nanocrystalline surface layer on the surface of hot-rolled AA8006 aluminum alloy. The microstructure of the near surface region in cross section, as shown in the figure, can be revealed only by use of advanced specimen preparation and electron-optical analysis techniques. As also sketched in the accompanying figure 2, this layer is characterized by grains which are about an order of magnitude smaller than the bulk grains, oxide particles entrained along the grain boundaries as a result of hot rolling and nanoscale intermetallic particles which precipitate along the grain boundaries as a result of high-temperature annealing following cold rolling.

The structure is pinned by the presence of grain boundary particles; recrystallization is not always possible by heat treatment. In addition, the surface is contaminated by the trace element Pb, which is present in most commercial alloys at the ppm level. These properties have a significant effect on the electrochemical and corrosion characteristics of such surfaces with wide consequences in practical application. Once one is aware of the presence and properties of such layers, many problems in application can be prevented simply by removing an adequate amount of material by chemical means or modifying the processing route.

Figure 1: Surface layer on hot-rolled AA8006 aluminum alloy

Figure 2: Schematic illustration of near-surface microstructure on hot-rolled product.

The microstructure and chemistry of a chemically-processed surface can also be quite different from the as-formed surface and the bulk microstructure because of selective dissolution of the more active component aluminum. This enriches the more noble elements both on the solid solution matrix and the intermetallic compounds with consequences to the electrochemical behavior and corrosion. The surface oxide film also becomes modified with added consequences to adhesion of paint, given most outdoor applications of aluminum alloys require the use of coatings.

The foregoing phenomena become progressively more important with increased use of recycled metal. At present, only about 25% of aluminum metal in use globally is recycled. This is expected to go up to about 70% (same level as steel today) in the near future, particularly in relation to increasing use in the automotive industry. The resulting modification of the type and content of trace elements are expected to introduce significant, so far unknown changes to the surface properties as well as mechanical properties. These changes can be detrimental needing mitigation or beneficial with possibilities of exploitation. An important objective for the group in the future is to characterize the modified surface properties of recycled alloys by use of advanced surface-analytical and electrochemical techniques and understand the underlying mechanisms.