Coating Weight and Thickness Designations for Coated Sheet Steels Questions are frequently asked about the coating weight and thickness designations of the coated sheet steel products used in the metal building industry. This Technical Bulletin is designed to answer these questions, provide you with the appropriate specification references and allow you to quickly convert between coating weights and thicknesses. The most commonly used coated sheet steel products in the metal building industry today are: Galvalume�, galvanized, and aluminum-coated, type 2. Another, newer product is Galfan�. All of these coatings are applied in a continuous coil line operation using the hot-dip coating method: the hot- or cold-rolled steel strip is passed through a molten metal bath followed by gas wiping to control the amount of coating applied. Because of the different densities of the coatings, the coating thickness will vary for a given coating weight when comparing these various products. Coating Weights:The table below gives the ASTM Specification for the corresponding coating, as well as examples of coating weight designations used in the metal building industry.
The numbers within the designations refer to the total coating weight on both sides of the sheet in hundredths of an ounce per square foot (oz/ft2) of sheet. Thus, an AZ55 Galvalume coating would have a minimum total of 0.55 oz/ft2 of coating, or, analogously, a T2 100 aluminum, type 2 coating would have a total coating weight of at least 1.00 oz/ft2. Galvalume is a registered trademark of BIEC International, Inc. Galfan� is a registered trademark of International Lead Zinc Research Organization, Inc. The concept of "per square foot of sheet" is significant. When coating weights are specified in this manner, as most hot-dipped coatings are, the total amount of coating on both sides of the sheet is referenced, not the amount on each individual side. In most cases, the coatings are applied relatively equally to both sheet surfaces. However, hot-dipped coatings are difficult to control precisely from side to side. Generally, the ASTM specifications covering hot-dipped coatings include a stipulation that "...it can normally be expected that not less than 40% of the single-spot check limit will be found on either surface." This means, for example, that a G90 galvanized coating will have at least 0.32 oz/ft2 of coating on each side of the sheet. This value is derived from the single-spot requirement for G90 of 0.80 oz/ft2 total on both sides of the sheet. 40% of 0.80 oz/ft2 is 0.32 oz/ft2. Coating Thickness:When coating thicknesses are compared for the products above at a given coating weight, there are large differences in thickness due to the different densities of the coating metals. The nominal densities for the coatings are given in the table below.
The densities listed above were used to prepare the conversion table on the following page, which will provide a quick reference for comparing the coating thicknesses at a given coating weight. Included in the table for your convenience are the ASTM Specification designations as they correspond to the coating weight values. Listed below are standard conversion factors which you may find useful, some of which were used in the preparation of the conversion table.
Coatings are sometimes designated in metric units, for example, grams per square meter (g/m2). These values are provided in the table as well. To convert oz/ft2 of sheet to g/m2 of sheet, multiply by 305.
* Coating thicknesses, weights and masses include the total coating on both sides of the sheet. |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
