Galvalume (55% Aluminium-Zinc Coated Steel)

Galvalume is the best-known trade name for steel sheet carrying a 55% aluminium-zinc alloy coating. The coating is roughly 55% aluminium, 43 to 44% zinc and about 1.5% silicon. The silicon controls the growth of the alloy layer between coating and steel and helps the coating stay attached when the sheet is roll-formed. GALVALUME is a registered trademark of BIEC International Inc., used under licence by steel producers; the same type of coating is sold under many other trade names around the world, such as ZINCALUME.

How it protects

The aluminium-rich part of the coating forms a durable barrier, while the zinc-rich part supplies some sacrificial (galvanic) protection at scratches and edges. Producers and coating specialists describe its atmospheric corrosion resistance as considerably better than that of galvanized steel; the GalvInfo Center reports that it outlasts galvanized steel by two to four times, depending on the environment. Like galvanized coatings it is consumed over time, and it is thinnest at cut edges.

Specification

In North America the product is covered by ASTM A792. Coating mass is given as total for both sides: AZ50 is 0.50 oz/ft2 (AZM150, about 150 g/m2) and AZ55 is 0.55 oz/ft2 (AZM165). The standard spans roughly 0.30 to 0.70 oz/ft2 (100 to 210 g/m2). Because aluminium is less dense than zinc, an AZ coating is thicker than a zinc coating of the same weight. AZ50 is often chosen where heavy forming is needed; AZ55 where corrosion resistance matters most. Comparable products are made to AS 1397 in Australia.

Limitations

• It is a poor partner for copper; water running from copper onto it, or direct contact, accelerates corrosion.
• Many roofing warranties restrict or exclude Galvalume installed close to salt water.
• Some producers neither recommend nor warrant it for animal confinement buildings (swine, cattle, poultry), where waste by-products attack the coating.

Galvalume takes paint well and is the usual base for factory-painted steel roofing.