The Benefits of Aluminum and Zinc.

Bethlehem developed Galvalume® sheet in the mid-60�s. Since that time, Galvalume sheet has enjoyed the fastest growth rate of any steel product on the world market. It has become the preferred material for roofing applications which formerly would have utilized aluminum-coated or galvanized sheet steel.

The composition of the Galvalume sheet alloy coating is typically 55% aluminum, 1.6% silicon and the balance zinc, nominal percentages by weight. The coating is applied to a substrate of steel by a patented, hot-dip process developed by Bethlehem research engineers. The result is a highly corrosion-resistant sheet steel that delivers the optimum combination of features of aluminum and zinc: the barrier protection and long-term durability of aluminum and the sacrificial or galvanic protection of sheared edges that is characteristic of zinc.

Excellent Long-Term Durability.

Galvalume sheet has been subjected to long-term testing for resistance to atmospheric corrosion. Test specimens of Galvalume sheet and competitive products are exposed at sites in four different environments: severe marine, marine, industrial and rural. After more than 23 years of exposure, Galvalume sheet continues to demonstrate an optimum combination of durability, edge protection and resistance to crevice corrosion. Typical durability for Galvalume sheet is two to four times that of galvanized G90 sheet.

The accompanying photographs show atmospheric corrosion test specimens of G90 galvanized. Galvalume and aluminum-coated, type 2 steel that have been exposed to moderate marine and rural environments for 23 years.

(After 18 years at the severe marine and industrial sites, the galvanized G90 test samples have corroded to 100% red rust and were removed from test. Thus, a full visual comparison among the three competing materials is not possible for these test sites. However, the outdoor testing program continues and, after 23 years, clearly shows the excellent corrosion resistance of Galvalume sheet as typified by the photographic comparison at the right for the two test sites where all three materials remain on exposure.)

The photographs show the appearance of the skyward surface of the test panels after 23 years of exposure at the 250-meter, moderate marine site at Kure Beach, N.C. and the rural site at Saylorsburg, Pa.

Galvalume sheet (left), galvanized G90 (center), aluminum-coated, type 2 (right) after 23 years' exposure at the moderate marine site.		Galvalume sheet (left), galvanized G90 (center), aluminum-coated, type 2 (right) after 23 years' exposure at the rural site.

Cut Edge Performance.

Galvalume sheet provides superior cut edge protection compared to G90 galvanized sheet. This is mainly because the 55% Al-Zn coating remains on the steel sheet providing sacrificial protection for a much longer time period (2 to 4 times) than the pure zinc coating. Compared to the aluminum-coated, type 2 panels, the sheared edges of the Galvalume sheet panels show less corrosive attack at the rural site. At the moderate marine site, both materials show evidence of red pinpoint and white corrosion products characteristic of aluminum-bearing coatings exposed for long periods in chloride-containing atmospheres.

This level of performance is particularly important in applications where sheared edges will be exposed or where localized damage to the coating may occur during fabrication or service.

Good Resistance to Crevice Corrosion.

Galvalume sheet also demonstrates that it is more resistant than aluminum-coated, type 2 sheet to crevice corrosion in chloride-containing atmospheres.

The difference in performance is dramatically shown in the photo below. After five years� exposure in a severe marine environment, crevices formed by 180-degree flat bends have perforated from interior corrosion on aluminum-coated sheet. On the other hand, the 55% Al-Zn coated sheet remains unaffected. This kind of performance can be especially important for lockseamed roofing panels used on structures in aggressive marine environments.