ASTM A792

ASTM A792/A792M, Standard Specification for Steel Sheet, 55 % Aluminum-Zinc Alloy-Coated by the Hot-Dip Process, covers steel sheet in coils and cut lengths coated with a 55% aluminium-zinc alloy. This coating is sold under trade names such as Galvalume and Zincalume and is one of the most common substrates for metal roofing and wall panels worldwide. The 2023 edition is A792/A792M-23.

What it covers

The standard defines:

• the coating alloy and how it is applied (continuous hot-dip process)
• coating mass designations, which state how much coating is on the sheet
• steel designations, including commercial, forming, drawing, high-temperature and structural steel grades
• tolerances, testing and ordering information

The product is intended where corrosion resistance, heat resistance or both are needed.

Why aluminum-zinc

The aluminium in the coating forms a stable oxide barrier that slows general corrosion, while the zinc provides some sacrificial protection at cut edges and scratches. In many atmospheres this gives longer life than plain zinc (galvanized) coatings of similar thickness. Aluminum-zinc coatings are less suited to direct contact with wet concrete, mortar or some treated timbers, and their cut-edge protection is lower than zinc's.

Use in roofing

Most painted steel roofing in North America is either A792 aluminum-zinc or ASTM A653 galvanized steel with a paint system on top. Bare (unpainted) A792 sheet is also used on agricultural and industrial buildings. Paint and finish warranties from coil coaters and panel makers usually specify the coating designation required.

International equivalents

• In Europe, aluminum-zinc coated steel is designated AZ under EN 10346, with coating masses such as AZ150 or AZ185 (grams per square metre, both sides).