Are ASTM A123 and G90 equivalent galvanizing specifications?

G90

First of all, G90 is not a galvanizing specification; it is a coating thickness designation in the galvanizing specification ASTM A653. This specification is for hot-dip galvanizing steel sheet using the continuous (high-speed) galvanizing process. The designation G90 is simply a coating thickness of 0.9 oz/ft² as specified in A653 and is equivalent to 0.45 oz/ft² or about 0.76 mils per side (it is important to state per side because the oz/ft² method takes into consideration the coating thickness on both sides of the steel, such as in the case of sheet steel). Some other coating thicknesses in A653 you may have heard of include G60 (0.6 oz/ft²) or G185 (1.85 oz/ft²).

The coating produced by continuous galvanizing is uniform, has sufficient ductility to withstand deep drawing or bending without damage to the coating, and consists almost entirely of pure zinc. In the continuous process, the steel in sheet, strip or wire form is passed through the cleaning tanks and zinc kettles at a very rapid speed. The speed determines the coating thickness and can range upwards of 200 feet per minute. This speed, in combination with slight additions of aluminum to the zinc bath, helps suppress the formation of zinc-iron alloy layers. This results in a thin coating of almost pure zinc with very little intermetallic growth.

A variety of coating weights are available, ranging from 0.25 mils (6 µm) to just under 2 mils (50 µm) per side. The standard product is Class G90, which has 0.9 oz/ ft² of sheet (total both sides) or about 0.76 mils (18 µm) thickness per side. Batch galvanizing coating specifications are given in weight per surface area. The galvanized sheet coating parameters relate to the total for both sides of the sheet. The sheet coating thicknesses must be divided by two for an approximate comparison.

A123

ASTM A123, Standard Specification of Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, refers to products fabricated and then hot-dip galvanized using the batch process. The two methods of hot-dip galvanizing, batch and continuous, produce very different products with different characteristics. The hot-dip galvanized coating consists of a series of layers of zinc-iron alloy with successively higher zinc contents towards the coating surface. The coating is unique in that it is metallurgically bonded to the steel substrate, with the coating integral to the steel. The strength of the bond is measured in the range of several thousand psi.

If you are simply trying to specify a galvanized coating thickness that is 0.9 oz/ft² or greater, that should not be a problem using the batch galvanizing process (where the steel is fabricated and then dipped into a kettle of molten zinc), but if the steel is very thin, warpage/distortion is a possibility.

If you want a pre-galvanized sheet that can be formed into various shapes, then you will need to talk with a supplier of galvanized sheet. ¹ Pre-galvanized sheet tends to be very formable without cracking the galvanized coating because it is made almost entirely of eta layer (free zinc) and has very little alloying, unlike the hot-dip galvanized coatings produced using the batch process. This lack of alloying also means pre-galvanized sheet is less abrasion resistant than steel galvanized using the batch process.

Both coatings provide barrier and cathodic protection, a benefit unique to galvanizing. And both coatings protect steel from rust better than other corrosion protection systems. However, the coating thickness difference is significant. Each type of galvanized product, batch or continuous, has applications where it would be best utilized. If you are looking for more information that discusses the differences between continuous and batch hot-dip galvanizing, download the AGA publication Zinc Coatings, which gives in-depth explanations of both processes (as well as the other zinc coating processes), and the one-page comparison sheet, General Hot-Dip Galvanizing vs. Continuous Sheet Galvanizing

Footnotes:

1. Contact the Galv Info Center at www.galvinfo.com for a list of suppliers of galvanized sheet products.