Base carbon/iron steel coated with an alloy of aluminum and zinc is known as Galvalume steel. When aluminum is added with zinc, both the positive and negative attributes of aluminum are magnified. Because aluminum itself is a very corrosion-resistant metal, Galvalume steel is also very corrosion resistant: The aluminum/zinc alloy provides barrier protection, as opposed to galvanic. The negative aspect of aluminum in the alloy, however, is that Galvalume does not selfprotect scratches or cut edges nearly as well as galvanized steel does. Additionally, Galvalume steel is also more susceptible to a process known as “Tension Bend Staining.”
When the steels are formed into the various metal roofing profiles, the Galvalume zinc/aluminum and the galvanized zinc coatings are spread very thin over areas in the metal where there are deep folds or tight bends – so thin, in fact, that the coating has a tendency to form microscopic cracks. Because of galvanic action of zinc, galvanized steel is able to protect these scratches with little harm. With Galvalume steel, however, the aluminum in the alloy somewhat neutralizes zinc’s galvanic properties and therefore the Galvalume steel is not able to self-protect the cracks, or other scratches in general. Tension Bend Staining occurs when moisture or other corrosive elements permeate these cracks and facilitate rusting. The result is “stains” of rust in areas with folds and bends in the metal.