Covering the bare steel surface with a zinc coating makes galvanized sheet metal. One of the most important aspects of galvanized products is the zinc coating thickness. The effectiveness of the zinc coating to protect the steel substrate from corrosion in any given environment is directly proportional to the coating thickness.

This galvanized (zinc) coating is measured in ounces per square foot. The weight of the coating may be specified as a coating designation or as a minimum coating test, i.e. the Triple-Spot Test or Single-Spot Test.

Explanation of the coating weights:

Triple-Spot Test: The average of the edge-center-edge sampling. A 3" circle of material is removed from each spot. The zinc is separated from the steel and the zinc is weighed.

Single-Spot Test: Only one (1) 3" circle is removed. The zinc is separated from the steel and the zinc is weighed.

The higher number translates to more zinc per square foot of surface.

ASTM accepts a Single-Spot Test for G-60. ASTM requires a Triple-Spot Test for G-90.

G-90 is hot dip galvanized with a triple spot test per ASTM.

It is a well-established fact that the corrosion resistance of a galvanized sheet of metal is directionally proportional to the coating weight (thickness).

G-90 has almost twice (1.80) the corrosion resistance of G-60.

With G-90 Spiral Duct the end user (owner) will receive a product with almost twice the life span expectancy and corrosion resistance as the G-60 product.