The specification for the bolts, nuts, washers and small forgings that ASTM A123 does not cover. Class by class, with the ounces converted into the microns your gauge reads.
ASTM A153 covers hot dip galvanized hardware — castings, fasteners, rolled, pressed and forged products, and miscellaneous threaded items. The dividing line from ASTM A123 is not size but handling: A153 articles are centrifuged, spun or otherwise handled to fling off excess zinc so that threads still assemble.
That is why the coating weights are lower than A123's, and why a bolt galvanized to A123 will usually not thread. If a specification names A123 for the structure, it needs A153 for the fasteners that hold it together.
Minimum weight of zinc coating per square foot of surface, from ASTM A153.
| Class | Articles covered | Average of specimens | Any one specimen | Approx. metric |
|---|---|---|---|---|
| A | Castings — malleable iron, steel | 2.00 oz/ft² | 1.80 oz/ft² | ≈ 610 g/m² ≈ 85 µm |
| B-1 | Rolled, pressed and forged articles over 5/8 in thick and over 15 in long | 2.00 oz/ft² | 1.80 oz/ft² | ≈ 610 g/m² ≈ 85 µm |
| B-2 | Same, under 5/8 in thick and over 15 in long | 1.50 oz/ft² | 1.25 oz/ft² | ≈ 458 g/m² ≈ 64 µm |
| B-3 | Same, any thickness, under 15 in long | 1.30 oz/ft² | 1.10 oz/ft² | ≈ 397 g/m² ≈ 56 µm |
| C | Fasteners over 3/8 in diameter; washers over 3/16 in thick | 1.25 oz/ft² | 1.00 oz/ft² | ≈ 381 g/m² ≈ 53 µm |
| D | Fasteners under 3/8 in diameter; rivets, nails and similar | 1.00 oz/ft² | 0.85 oz/ft² | ≈ 305 g/m² ≈ 43 µm |
Conversions use 1 oz/ft² = 305.15 g/m² and 7.14 g/m² per micron of zinc, and are given to help you read a DFT gauge against an imperial specification — the standard itself states the requirement as a weight. Note that Class A and B-1 land on 610 g/m², the same figure IS 4759 and EN ISO 1461 arrive at independently for heavy sections.
Answered from our own line, not from the standard's cover page.
A123 covers structural products; A153 covers hardware and fasteners that are centrifuged after dipping to clear the threads. A153's coating weights are lower for that reason. A bolt galvanized to A123 thickness would generally not assemble.
For fasteners over 3/8 inch diameter, Class C: an average of 1.25 oz/ft², with no single specimen below 1.00 oz/ft². That is roughly 381 g/m², or about 53 microns. Fasteners under 3/8 inch fall in Class D at 1.00 oz/ft² average.
Because it was almost certainly tapped oversize after galvanizing, which is the accepted way of making room for the coating on the bolt thread. It is normal practice under A153, not a fault. Order the bolt and nut as an assembly so the fit is controlled.
Class 8.8 and below is routine. Above that, hydrogen embrittlement risk rises and A153 carries embrittlement requirements A123 does not — send the grade and the application and we will advise before the order rather than after the dip.
Yes. Foundation and anchor bolts in IS 1367 property classes 4.6, 5.6 and 8.8 are supplied with our substation, transmission and telecom structures and galvanized on our own line.
Tell us the standard, the coating requirement and the component, and we will confirm in writing what we can hold and how we will certify it. Galvanizing is on our own floor, so the answer comes from us and not from a sub-contractor.
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