Aluminum alloys possess a unique combination of mechanical properties and corrosion resistance. These alloys are roughly one-third as dense as steel with higher strength to weight ratios. Their elastic modulus is also about one-third that of steel, but with careful design, structures that are lighter and more rigid can be cast in aluminum than in steel. In oxidizing environments, aluminum alloys form a tight, protective layer of aluminum oxide on their surfaces which essentially prevents further oxidation. Aluminum alloys are almost always provided in the solution annealed and aged condition. Unlike steels, aluminum alloys do not have a fatigue limit and are therefore not usually selected for applications involving high cycle fatigue.