Three documents answer most hot-dip galvanizing questions. ASTM A123/A123M covers fabricated structural steel and other products. ASTM A153/A153M covers hardware and fasteners. ISO 1461 is the main international equivalent of A123 for fabricated articles. In the US, you will usually cite A123 for the steel and A153 for the bolts, then use ISO 1461 only when a project, supplier or export market requires it. This guide explains what each one covers, how coating thickness is set and checked, and how to write a galvanizing clause that a galvanizer can actually follow.
TL;DR
- ASTM A123 is for fabricated steel products, A153 is for hardware and fasteners, and ISO 1461 is the international standard for fabricated articles.
- Coating thickness is a minimum, not a target. A123 sets minimum average grades by material category and steel thickness, and ASTM generally asks for more zinc than ISO.
- ISO 1461 measures thickness magnetically or by weighing (gravimetric), and uses the gravimetric method to settle disputes.
- Designers and fabricators are responsible for a galvanizable design (vents, drains, suitable chemistry). Galvanizers answer for compliance once the work is properly built.
- ASTM A36 is a structural steel grade, not a galvanizing specification. Whether it galvanizes well depends on its chemistry, mainly silicon.
Table of Contents
- The standards at a glance
- What ASTM A123/A123M covers
- Hot-dip galvanizing thickness under ASTM A123
- ASTM A153 for bolts, nuts and hardware
- ISO 1461 and how it differs
- Which standard should you cite?
- Is ASTM A36 galvanized?
- Preparing steel so it passes
- Our perspective: write the clause before you buy the steel
- Galvanized guardrails and steel supply from Gq-international
- FAQ
- Sources
The standards at a glance
A coating standard answers four questions: what products it covers, how much zinc must be on them, what the surface must look like, and how the zinc is verified. Different standards split the product world differently.
| Standard | Covers | Typical use |
|---|---|---|
| ASTM A123/A123M | Rolled, pressed and forged shapes, castings, plates, bars and strips, including fabricated assemblies | Structural steel, guardrail beams and posts, poles, frames |
| ASTM A153/A153M | Iron and steel hardware | Bolts, nuts, washers, small castings |
| ISO 1461 | Fabricated iron and steel articles | International projects, EN-based specifications |
| ISO 10684 | Fasteners | International equivalent for hardware |
| ISO 14713 (Parts 1 and 2) | Design guidance, not a coating spec | Corrosion resistance and design for galvanizing |
The split between A123, A153, ISO 1461 and ISO 10684 follows Advisory Note 28 on hot-dip galvanizing standards, which also lists ISO 14713 as the design guidance companion. It describes A123 as the most-used American specification and ISO 1461 as the most-used international one for the same general application.
What ASTM A123/A123M covers
A123 applies to hot-dip zinc coatings on iron and steel products made from rolled, pressed and forged shapes, castings, plates, bars and strips. It covers both unfabricated products and fabricated ones, including structural steel fabrications, large tubes and wire work. The 2024 edition of the text excludes products galvanized on specialized or continuous lines, as shown in the A123/A123M-24 document. Sheet coil, wire and similar products galvanized in continuous lines have their own standards.
The specification addresses coating thickness, finish, appearance and adherence. According to the American Galvanizers Association overview of A123, all thickness values in the specification are minimums, and there is no maximum thickness requirement.
If you order in SI units, say so when you place the order. The 2013 edition requires testing and inspection by the metric equivalent methods in that case, as the A123/A123M-13 text explains.
Who is responsible for what
A123 draws a line that matters in disputes. The designer and fabricator must make sure the product is properly designed and built before galvanizing. The galvanizer is then responsible for meeting the specification. The AGA summary of the ASTM specifications states this split plainly.
In practice that means vent and drain holes are a design decision. If a galvanizer has to add work such as drilling vent holes, the 2024 edition says this must be communicated and resolved with the customer. Settle it in the drawings, not at the kettle.
Hot-dip galvanizing thickness under ASTM A123
Thickness is the question most buyers actually have. A123 handles it with a table, not a single number. Table 1 sets minimum average coating-thickness grades by material category and by the measured thickness of the steel. In the 2024 edition the grades range from 35 to 100 µm, with heavier steel getting a heavier minimum, per the A123/A123M-24 text.
Three points follow from that design:
- Thickness depends on the steel, not on the order. A thin bracket and a heavy beam in the same assembly have different minimums. The standard assigns a grade to each material category and thickness range within a product.
- Minimum average, not absolute. The grade is an average across the specimens tested. Read the table in the current edition for the exact local minimums before you write a number into a spec.
- More zinc is not automatically better. Steel chemistry and thickness drive how much zinc forms. Asking for an arbitrary heavier coating without discussing it with the galvanizer can cause trouble. Specify the standard and the environment, and let the grade follow.
The previous editions work the same way: the 2013 text also gives a grade for each material category and thickness range in Table 1. If an older project specification cites A123-13 or earlier, check which edition governs before comparing figures.
ASTM A153 for bolts, nuts and hardware
A123 is for fabricated products. Hardware goes under A153. The AGA’s guide to A153 for hardware says coating thickness or weight depends on the material category and steel thickness, with values in its Table 3. Threaded areas are not subject to the coating thickness requirement, and the finish must be continuous, smooth and uniform.
Two practical notes from that guide:
- Internal threads on nuts are over-tapped after galvanizing, so the zinc on the bolt threads protects the uncoated nut threads.
- High-strength fasteners above 150 ksi, and castings, can be subject to embrittlement. ASTM A143/A143M gives precautions. If your bolted connections use high-strength fasteners, raise this with the galvanizer early.
A common spec mistake is to write “all steel to ASTM A123” and forget the fasteners. Name both.
ISO 1461 and how it differs
ISO 1461:2022 specifies general properties and test methods for hot-dip galvanized coatings on fabricated iron and steel articles. It excludes continuously galvanized sheet, wire and mesh, tube and pipe galvanized in automatic plants, and products covered by specific standards, per the ISO 1461:2022 text. The scope is very close to A123. The differences lie in thickness values and inspection.
Thickness
The two standards do not have identical coating-thickness limits. The AGA’s comparison of ISO 1461 and ASTM A123 notes that ASTM specifications typically require more coating thickness than ISO for most steel types and thicknesses. If a drawing set mixes the two, do not assume a coating that passes one will pass the other.
Measuring thickness
Under ISO 1461, local coating thickness comes from measurements inside a reference area. You can measure with magnetic methods under ISO 2178 or gravimetrically under ISO 1460. At least five magnetic readings are taken in each reference area, and the gravimetric method resolves disputes. All of this is in the ISO 1461:2022 text.
Appearance
ISO 1461:2022 also states that darker or lighter areas, cellular patterns and surface unevenness such as orange peel are not grounds for rejection. This helps on large fabrications where color varies with steel chemistry. Inspectors should be briefed on this before they walk the yard.
Which standard should you cite?
Use this as a decision sequence:
- US public or private project, fabricated steel: cite ASTM A123/A123M.
- Hardware and fasteners on the same project: cite ASTM A153/A153M.
- Export project or EN-based specification: cite ISO 1461 for fabricated articles and ISO 10684 for fasteners.
- Design and detailing questions: consult ISO 14713, whose Part 1 covers design principles and corrosion resistance and Part 2 covers design for hot-dip galvanizing.
- Mixed-origin supply chain: state which standard governs each component, and which edition. Never leave two standards to “whichever is stricter” without defining how you compare them.
For design details, the corrosion-resistance questions in ISO 14713 overlap with material selection. Our guide to choosing a structural steel coating system puts galvanizing next to the alternatives. If the structure will sit near the sea, coastal steel protection coatings are worth a look when you select a system.
Is ASTM A36 galvanized?
No. ASTM A36 is a structural carbon steel grade. A grade name tells you about the steel, not about its coating. A36 plate and shapes are routinely sent to galvanizers, and the finished product is then galvanized under A123. The standards cover different things: one describes the base metal, the other the zinc coating.
What matters for galvanizing is chemistry. A typical owner specification calls for steel with carbon below 0.25, phosphorus below 0.04 and manganese below 1.3, with silicon at 0.04 or less, and says that steel in that range will normally develop a typical coating under conventional galvanizing, per a published galvanizing specification. Steel outside those ranges can still be galvanized, but the coating may look and behave differently, so tell the galvanizer which heat or mill certificate applies.
To avoid surprises, ask for the mill test report, flag high-silicon heats, and talk to the galvanizer before fabrication if a project mixes plates from several mills.
Preparing steel so it passes
Most failed galvanizing inspections trace back to fabrication, not to the zinc kettle. Four habits prevent most of them:
- Vent and drain holes. Hollow sections and closed assemblies need openings so air, flux and zinc can move. Detail them on the drawings.
- Surface contaminants. Paint, heavy oil, weld slag and marker paint do not come off in the pickling steps. Remove them first.
- Weld quality. Clean welds and the right filler material help the coating form evenly. Our guide to steel fabrication quality control covers the phases from material receipt to final inspection.
- Distortion. Large flat or thin-walled assemblies can warp in the bath. See our notes on welding distortion control.
European galvanizers make a related point: written procedures between fabricators and galvanizers support quality control when galvanizing is a subcontracted activity, including preparation such as venting and drainage, according to EGGA guidance on EN ISO 1461 and CE marking. The same logic works in the US.
Welding on galvanized steel after the fact is its own subject. Zinc fumes require ventilation and the right protective equipment, and a guide on welding galvanized steel safely covers the precautions. Plan welding before galvanizing wherever you can.
Our perspective: write the clause before you buy the steel
We make highway guardrails, PV mounting structures and other custom steel products, and galvanizing is a standing part of how we think about corrosion. Our experience is that the standard itself rarely causes a problem. The problems come from unclear ownership. A buyer cites A123, a fabricator assumes the galvanizer will fix venting, the galvanizer assumes the drawings were final, and the dispute begins at inspection.
Our advice to procurement and engineering teams is simple. Name the standard and the edition. Name the hardware standard separately. Decide in advance who handles vent holes and who handles touch-up. Ask for thickness readings with the delivery documents. For long-life infrastructure, the coating clause deserves as much attention as the steel grade, and our guide on guardrail corrosion requirements for procurement shows how we approach it for 50 to 100 year service goals.
We also think weathering steel and galvanized steel should be chosen deliberately, not by habit. Our note on weathering steel guardrails explains where that choice changes the answer.
— Hu
Galvanized guardrails and steel supply from Gq-international
If your project needs galvanized steel for roads and bridges, our galvanized steel guardrail systems page describes the product line. We supply guardrails, PV mounting systems, energy storage containers and custom steel infrastructure products to governments, EPC contractors and infrastructure firms. Send your drawings and coating requirement through our main site and our team will take it from there. Pricing is project-based and is not published.
FAQ
What is the ASTM standard for hot-dip galvanizing?
The main one is ASTM A123/A123M, which covers zinc coatings on fabricated iron and steel products such as structural steel. Hardware and fasteners fall under ASTM A153/A153M. Cite both when a project has structural members and bolts.
What is the ISO standard for hot-dip galvanizing?
ISO 1461 covers hot-dip galvanized coatings on fabricated iron and steel articles. ISO 10684 covers fasteners, and ISO 14713 gives design guidance. ISO 1461:2022 is the current edition cited here.
How thick is a hot-dip galvanized coating under A123?
It depends on material category and measured steel thickness. In the 2024 edition the minimum average grades range from 35 to 100 µm, and all values are minimums. Check Table 1 of the current edition for the exact figure for your steel.
Is ASTM A36 galvanized?
No, A36 is a structural steel grade and does not by itself mean the steel is coated. A36 can be hot-dip galvanized after fabrication under A123. Silicon and phosphorus levels in the heat influence how the coating forms, so share mill certificates with the galvanizer.
Sources
- Advisory Note 28: Hot-dip galvanizing standards
- ASTM A123/A123M-24 text
- ASTM A123/A123M-13 text
- Overview of ASTM A123/A123M
- ASTM specifications, AGA
- ASTM A153 for hardware
- ISO 1461 vs. ASTM A123
- ISO 1461:2022
- Hot-dip galvanized components specification
- EGGA: Hot dip galvanizing to EN ISO 1461 and CE marking