Weathering grade steels, also called high strength low-alloy (HSLA) steels, have been available to the engineering profession for many years. The cost-effectiveness of the material has been demonstrated in both short and long-term savings since the additional cost of the material can be often offset by the elimination of the need for initial painting or galvanizing. Weathering steel has the unique characteristic that, under proper conditions, it corrodes by forming a dense and tightly adherent oxide barrier that seals out the atmosphere and retards further corrosion. This is in contrast to other steels that form a coarse, porous and flaky oxide that allows the atmosphere to continue penetrating the steel. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.
Although cleaning and handling of the material can affect the short term appearance of the product, the overall corrosion resistance and ultimate appearance of weathering steel is not affected by cleanliness. The appearance, texture, and maturity of this oxide coating depend on three primary interrelated, natural factors: time, degree of exposure, and atmospheric environment.