Hot-dip galvanizing
After-fabrication zinc coatings on steel: specification scope, fabrication allowances and inspection.
Tags: processes, finishing, coatings
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Five example readings of 75, 85, 90, 95 and 105 µm give a 90 µm specimen average. Under the illustrated rule it clears the 85 µm specimen minimum. The overall 100 µm coating grade still needs its own sampling and average. Apply the specified specimen and lot definitions.
Process and scope
After-fabrication galvanizing forms zinc–iron alloy layers, typically under outer zinc. Silicon and phosphorus in steel can increase growth and brittleness. Specify acceptance separately from brightness. American Galvanizers Association (AGA), coating metallurgy, updated July 2025
Electrogalvanizing and zinc plating instead deposit zinc electrochemically from a solution. They are separate process specifications, not alternate names for immersion galvanizing. Likewise, continuously coated sheet and batch-galvanized welded fabrications should not inherit each other's coating requirements simply because both are sold as galvanized steel. AGA zinc plating, updated September 2025, ISO 1461:2022 scope
Select the applicable specification
| Reference | Applicability and boundary |
|---|---|
| ASTM A123/A123M-24 | Hot-dip zinc coatings on iron and steel products, including many fabricated assemblies, shapes, plates, bars and castings. Identify the material categories and applicable thickness ranges. |
| ISO 1461:2022 | General properties and tests for fabricated iron and steel articles, including certain castings, dipped in a zinc melt. Its scope excludes continuously galvanized sheet/wire/mesh, automatically galvanized tube/pipe and products such as fasteners with their own specific standards. |
| Product-specific fastener requirements | Select the standard for the actual hardware and strength class; do not assume acceptance under the structural fabrication's coating specification also qualifies every bolt or nut. |
State the governing standard, edition and unit system on the order. ASTM's public scope requires its SI and inch-pound systems to be used independently, without combining values. ISO 1461 also excludes after-treatment and additional coatings from its scope: a paint or powder topcoat needs its own requirements. The two standards are not interchangeable inspection worksheets. ASTM A123/A123M-24, ISO 1461:2022
Fabrication decisions before galvanizing
Vents and drains let air escape and allow cleaning solutions and zinc to enter and leave a fabrication. Their locations depend on the actual immersion orientation: high points vent, low points drain. Sealed cavities or trapped liquid can create dangerous pressure during immersion. Obtain the galvanizer's approval of hollow-section openings, overlapping surfaces and lifting orientation before welding the assembly closed; do not use a universal hole size copied from an unrelated part. Poor drainage can also leave bare areas or excess zinc. AGA venting and draining, September 11, 2014
Thermal distortion is a fabrication concern, not merely a cosmetic coating defect. AGA identifies uneven section thickness, asymmetric weldments, progressive dipping and restricted drainage as relevant factors. More uniform sections, suitable stiffening, modular assemblies and single-dip processing can reduce risk, but do not guarantee final flatness. Agree final dimensional requirements and an acceptable correction method with the fabricator and galvanizer. AGA warpage and distortion, November 4, 2015
Normal galvanizing preparation does not remove every contaminant. Welding slag, unsuitable marking paints and coatings that resist chemical cleaning need removal by an appropriate mechanical method. Tell the galvanizer about existing coatings, weld residues and previous processing; specify cleaning responsibilities rather than assuming immersion will correct fabrication defects. Galvanizers Association of Australia, standard specification guidance
Threaded assemblies require a deliberate allowance. AGA recommends supplying bolted assemblies disassembled and describes standard-size male threads paired with nuts tapped oversize after galvanizing. Overtapping removes zinc from the female thread, so this is an engineered mating system rather than permission to chase every coated thread indiscriminately. Select the applicable thread and fastener requirements and verify assembly after coating. A smooth cylindrical diameter allowance is not a thread pitch-diameter allowance. AGA threaded assemblies, updated August 2021
Thickness and inspection
Coating requirements depend on the governing specification, material category and steel thickness. AGA's explanation of ASTM A123's 2024 revision emphasizes measured steel thickness and distinguishes rolled structural sections from similar-looking shapes fabricated from plate. Its example assigns a rolled beam with ½-inch measured thickness grade 100, while a beam fabricated from ½-inch plate receives grade 75. Shape appearance alone cannot determine the category. AGA, 2024 revision of ASTM A123, August 28, 2024
Distinguish an individual instrument reading, the average for a specimen and the required aggregate average. AGA's inspection guidance describes dispersed readings within specimens and sampling rules for single- and multi-specimen articles. An individual reading is not itself the specimen average. For ASTM A123, “one coating grade below” follows the sequence in Table 2: below grade 100 is grade 85, not whichever neighboring value appears in the material-category table. Use the full ordered specification for sampling and acceptance. AGA sampling protocols, AGA 2024 revision guidance
Original inspection arithmetic
Assume a specimen belongs to a category requiring overall grade 100, with individual-specimen minimum grade 85. Five illustrative dispersed readings are 75, 85, 90, 95 and 105 µm:
Specimen average = (75 + 85 + 90 + 95 + 105) / 5 = 90 µm.
That average exceeds 85 µm. The 75 µm reading alone does not establish specimen failure under this averaging rule, and the 90 µm specimen average alone does not establish that the overall 100 µm requirement is satisfied. This example checks arithmetic only; it does not replace the prescribed lot selection, specimen subdivision, visual inspection or other acceptance requirements. AGA sampling interpretation
Inspect finish and coverage as well as thickness. AGA identifies trapped contamination and welding slag among causes of bare spots. Define the permitted repair procedure and repair limits under the governing coating specification; ASTM A780 describes repair methods, including zinc-rich paint, zinc solder and metallizing. A repaired area is not a newly formed immersion coating. AGA finish inspection, AGA repair guidance
Exposure and release information
Atmospheric performance estimates depend on coating thickness and environmental conditions. AGA's time-to-first-maintenance chart is an estimate for the stated exposure, not a universal service-life guarantee. Its endpoint represents about 5% steel-surface rusting, rather than structural failure or complete loss of all zinc. Do not transfer an atmospheric estimate to immersion, soil or a different chemical exposure without a suitable assessment. AGA life-estimation guidance, AGA environmental guidance
Release the drawing with steel identity, fabrication condition, governing coating standard, vent/drain details, mating and final dimensional requirements, appearance expectations and inspection/repair responsibilities. Identify any subsequent welding, machining or topcoat operation so the completed assembly's protection can be specified.
Source editions: ASTM identifies A123/A123M-24 as active, updated July 17, 2024. ISO 1461:2022 is edition 4, August 2022. The dated AGA pages above are explanatory industry guidance; their historical publication dates do not supersede the ordered standards. No generic bath formulation or immersion cycle is specified here. ASTM edition record, ISO edition record