Storage tanks for sulfuric acid in strengths of 93% through 99% are usually fabricated from carbon steel. At normal temperatures there is some attack on the steel resulting in the evolution of hydrogen gas and the formation of an iron sulfate precipitate, most of which settles to the bottom of the tank. Wall thickness should be carefully calculated to provide for corrosion allowances and for the high specific gravity of the acid. Many storage tanks use anodic protection systems to minimize corrosion. Storage tank cleaning and inspection procedures are available on request. Other materials such as stainless steel and polyethylene are sometimes used for smaller tanks.