Pitting corrosion of soil-side bottoms of Aboveground Storage Tanks (ASTs) causes the formation of holes that allows product leakage into the environment. This specific type of corrosion forms in the gaps or "vapor spaces" between the metal tank bottom and the soil/concrete slab it rests on. These "vapor spaces" are formed when the metal bottom flexes up and down when product is filled and removed from the AST. In addition, when the AST is constructed using bituminous beds below the tank bottoms (a common practice), it leads to attack from the acids in the surrounding soil, which seep through these same beds. Because of the leaks caused by corrosion, tank operators have to periodically stop tank operations for repairs or to replacement of the entire bottoms. The stoppage time can come within the first few years of a tank's life (2 to 4 years), or at scheduled inspection intervals (6 to 10 years depending on relevant API 650 guidelines).