Results from a number of atmospheric corrosion testing programs show that a linear bilogarithmic law between corrosion penetration and time is valid. When this law is valid, the mean corrosion rate and the instantaneous corrosion rate are also related to the time of exposure by a linear bilogarithmic law. Such laws are valid for a number of materials (carbon steels, low-alloy steels, galvanized steels, aluminized steels) in a number of environments (rural, marine, industrial). The linear bilogarithmic laws are helpful in extrapolating results of corrosion tests to long term (extrapolation up to 20-30 years from four years of tests is reasonable). -- AATA