Aluminum and aluminum alloys can be coated with porcelain enamel slip which is fired at 1000°F for approximately 10 minutes. This produces a material with a surface layer (about 0.1 mm thick) which has the properties of porcelain and the basic mechanical properties of a light ductile metal. A problem termed spalling may arise after weathering or exposure to a corrosive salt solution, if the base metal is not chosen carefully or pretreatment of the metal is not correct. A typical failure of this type on a test panel is illustrated in Fig. 1. The spalling failure initiates at pin holes, scratches and edges, then spreads laterally by attack at the interface between the enamel and base metal. An electron fractograph at the edge of a circular spalled area on a test panel of an improperly pretreated 6061 alloy is shown in Fig. 2. The corroded metal is on the bottom and the porcelain on the top. The narrow band through the center is the fracture edge of the porcelain enamel adjacent to the interface.