Your search did not yield any results

Site Pages

Dr. Galv KnowledgeBase


Are ASTM A123 and G90 equivalent galvanizing specifications?


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.

Continuous Sheet Galvanizing

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.


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


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

Was this answer helpful? YES       | NO

Are you still looking for the right answer? Ask an Expert


Michael Hinton, Architect

Why is hot dipped galvanized approved and recommended for contact with pressure treated lumber and G90 is not?


Hi Michael, This recommendation comes from the pressure treated wood industry themselves based on performance testing. Furthermore, batch hot-dip galvanizing provides a significantly thicker zinc coating and therefore a longer service life. For more details on this topic, see our article on the pressure treated wood industry and relevant recommendations:


What is the proper process if newly install galvanized ductwork with a G90 coating needed to be painted? Is there a specific product or type of paint or primer needed?


Hi Joe, The Galv Info has several guides and procedures for preparing continuous galvanized sheet products for painting here: Please consult a paint manufacturer for a suitable paint to be applied over the galvanizing.

Nguyen Loi

Hi guys, What are the differences between the Zmax (G185) coating and Hot-dip Galvanized coating?


Hi Nguyen, Zmax is the same thing as galvanizing to ASTM A653 standard with a coating thickness designation of G185. A G185 coating requires 1.85 oz/ft2 (total both sides) and would be equivalent hot-dip galvanized coating thickness of 1.57 mils. For more information on the differences between ASTM A653 (continuous galvanizing) vs. ASTM A123 (batch hot-dip galvanizing), see the following:

Big D Builders, Inc.

Is 85 microns of galvanizing equal to G90


Hi Big Builders, No -- G90 is equivalent to 19.6 microns per side.

LB Foster

What would be the G designation for 4 mil of coating per side? Example: G235 has approximately 2.12 mils per side. Is there anything higher than G235? How many years of service life does 4 mils give you? 100 years?

Bill Mellor

If building specs for new construction require the "use of connectors, fasteners, and anchors in contact with pressure treated wood to be of greater corrosion resistance than standard galvanized connectors," then what would the term greater corrosion resistance mean? Is G90 a standard galvanizing?


Hi Bill, That is a relative term. What I can tell you is hot-dip galvanized (per ASTM A153 standard) and stainless steel fasteners are suitable for contact with pressure treated wood according to research conducted by the pressure treated wood industry. G90 is a coating thickness designation for continuous sheet products galvanized to ASTM A653 standard, which means it is not applicable to fasteners.

Randy Bannor

Can a coating be applied to G- 60 steel to get a G-90 rating?


Hi Randy, G90 is not a rating, is a coating thickness designation. Therefore, you cannot apply a paint to achieve a greater zinc coating thickness. Painting over HDG does result in a synergistic effect which can extend the life of both coating systems. More about that on our website here:

Add Your Comment