Continuous Sheet Galvanizing (G60, G90) Vs. Batch Hot-dip ...

Login

Close

Login

Forgot password?

Home » KnowledgeBase » Continuous Sheet Galvanizing ASTM A653 (G60, G90) vs. Batch Hot-dip Galvanizing ASTM A123

Continuous Sheet Galvanizing ASTM A653 (G60, G90) vs. Batch Hot-dip Galvanizing ASTM A123

December 1, 2010

Authored by Dr. Tom Langill

Note: Have questions about continuous hot-dip galvanizing? Contact the GalvInfo Center

Continuous hot-dip galvanizing and after-fabrication batch hot-dip galvanizing are often confused because they are both hot-dip galvanized products. Though similar, these two methods of hot-dip galvanizing differ in terms of application process, available product that can be coated, and coating thickness.

The continuous hot-dip coating process according to ASTM A653, Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process, is a widely used method to coat sheet products prior to fabrication. In this process, steel is passed as a continuous ribbon through production and into a bath of molten zinc at speeds up to 600 feet per minute. Once the product is removed from the bath, high-pressure air is used to remove any excess molten zinc to create a closely controlled coating thickness. Afterward, the steel is cooled and rolled into large coils for eventual fabrication. Today, the continuous sheet process is used to make seven different types of hot-dip coated products, including galvanized (zinc), galvannealed (90-92% zinc / 8-10% iron alloy), two alloys of zinc and aluminum (55% aluminum / 45% zinc alloy and 95% zinc / 5% aluminum alloy), two aluminum based alloys (100% aluminum, 89-95% aluminum / 5-11% silicon alloy), and the terne coating (85-97% lead / 3-15% tin alloy).

Batch hot-dip galvanizing according to ASTM A123, Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, refers to the process where steel articles are suspended by chain, hook, or wire, and then coated by immersing the articles in a kettle containing molten zinc. This method is suitable for products to be galvanized after-fabrication such as plate, poles, beams, frames, trailers and other assemblies.

Both galvanized products provide barrier and cathodic protection, but have applications where they are best utilized. If you are simply trying to specify a hot-dip galvanized coating weight that is anywhere from 0.3 - 3.6 oz/ft2, this can be achieved using either process. However, if the steel is very thin (less than 7 gauge), warpage/distortion is a possibility with the batch process. Furthermore, if it is required to form the steel into various shapes after hot-dip galvanizing, then continuous galvanizing is preferred because it is very formable without cracking the galvanized coating. This is because the coating formed by the continuous process 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.

There are also significant differences between coating thickness and longevity. ASTM A123 requires a minimum average coating thickness based on material category and steel thickness and has no maximum value for coating thickness. Meanwhile, continuous sheet galvanizing per ASTM A563 contains a list of zinc coating thickness designations such as G30, G60, and G90. To clarify, G30/G60/G90 are not a galvanizing specifications; they are coating thickness designations found in the continuous galvanizing specification ASTM A653 that refer to a dual-sided coating weight in oz/ft2. These coating weight values must be divided by two to provide an approximate comparison to the single-sided coating thickness values provided in ASTM A123. Refer to Tables 1 & 2 below to compare some of the common coating thickness values achieved using each process (refer to each specification for a full list of coating grades).

TABLE 1: CONTINUOUS GALVANIZING (PRE-GALVANIZED SHEET PRODUCTS 7 GAUGE [3/16"] TO 32 GAUGE [0.102"] THICK), MINIMUM AVERAGE COATING THICKNESS [MILS]

Coating Grade ASTM A653	Coating Weight (Total Both Sides)	Equivalent Coating Thickness (one side)
G30	0.30 oz/ft2	0.26 mils
G60	0.60 oz/ft2	0.51 mils
G90	0.90 oz/ft2	0.77 mils
G115	1.15 oz/ft2	0.98 mils
G140	1.40 oz/ft2	1.20 mils
G165	1.65 oz/ft2	1.40 mils
G185	1.85 oz/ft2	1.57 mils
G210	2.10 oz/ft2	1.79 mils
G235	2.35 oz/ft2	2.00 mils
G300	3.00 oz/ft2	2.55 mils
G360	3.60 oz/ft2	3.06 mils

TABLE 2: BATCH HOT-DIP GALVANIZING (AFTER-FABRICATION), MINIMUM AVERAGE COATING THICKNESS [MILS]

Steel Thickness
Material Category ASTM A123	<1/16 [5<1.6]	>1/16 to <1/8 [>1.6 to <3.2]	>1/8 to <3/16 [>3.2 to < 4.8]	>3/16 to <1/4 [>4.8 to <6.4]	>1/4 to <5/8 [>6.4 to <16.0]	>5/8 [>16.0]
Structural Shapes	1.8 mils	2.6 mils	3.0 mils	3.0 mils	3.9 mils	3.9 mils
Plate	1.8 mils	2.6 mils	3.0 mils	3.0 mils	3.0 mils	3.9 mils
Pipe & Tubing	1.8 mils	1.8 mils	3.0 mils	3.0 mils	3.0 mils	3.0 mils

The most commonly encountered coating thickness designations for continuous hot-dip galvanized products (G30, G60, and G90) are significantly thinner than those achieved though batch hot-dip galvanizing. Since coating thickness is directly related to service life, greater longevity is achieved when specifying batch hot-dip galvanizing. Generally, a hot-dip galvanized coating thickness of at least 3.0 mils is recommended for long-term corrosion protection in exterior environments or contact with soil (for example, solar infrastructure commonly considers both coating options). For interior or mild environments (ductwork, appliances, etc.) continuous hot-dip galvanizing is likely to provide suitable longevity.

If you are looking for more information about the use and performance of continuous sheet galvanized products, please contact the GalvInfo Center.

See Also:

Zinc Coatings Publication

© 2024 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.

Was this answer helpful? YES | NO

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

Dr. Galv KnowledgeBase

• All (Sort by New)
• Appearance
• Coating Thickness
• Design & Fabrication
• Duplex Systems
• Embrittlement
• Fasteners
• HDG Process
• HDG Specifications
• Health, Safety & Environment
• Inspection
• Performance & Use
• Reinforcing Steel
• Steel Selection & Steel Chemistry
• Touch-Up & Repair
• Zinc Coatings

Necessary (Required) Cookies that the site cannot function properly without. This includes cookies for access to secure areas and CSRF security. Please note that Craft’s default cookies do not collect any personal or sensitive information. Craft's default cookies do not collect IP addresses. The information they store is not sent to Pixel & Tonic or any 3rd parties. Name: CraftSessionId Description: Craft relies on PHP sessions to maintain sessions across web requests. That is done via the PHP session cookie. Craft names that cookie “CraftSessionId” by default, but it can be renamed via the phpSessionId config setting. This cookie will expire as soon as the session expires. Provider: this site Expiry: Session Name: *_identity Description: When you log into the Control Panel, you will get an authentication cookie used to maintain your authenticated state. The cookie name is prefixed with a long, randomly generated string, followed by _identity. The cookie only stores information necessary to maintain a secure, authenticated session and will only exist for as long as the user is authenticated in Craft. Provider: this site Expiry: Persistent Name: *_username Description: If you check the "Keep me logged in" option during login, this cookie is used to remember the username for your next authentication. Provider: this site Expiry: Persistent Name: CRAFT_CSRF_TOKEN Description: Protects us and you as a user against Cross-Site Request Forgery attacks. Provider: this site Expiry: Session Statistics Statistic cookies help us understand how visitors interact with websites by collecting and reporting information anonymously. Marketing Marketing cookies are used to track visitors across websites. The intention is to display ads that are relevant and engaging for the individual user and thereby more valuable for publishers and third party advertisers. Details Hide Details