The most useful standard on this site for anyone sending us job work. Vent and drain holes, overlaps, weld sequence, and the design decisions that decide whether a piece comes out clean.
ASTM A385 is a practice, not a product specification. It tells the designer and the fabricator how to make an article that can actually be hot dip galvanized: where zinc and air need to get in and out, what to do about overlapping surfaces, and how to sequence welds so the piece does not distort.
Almost every galvanizing failure we see arrives as a design problem, not a coating problem. A sealed hollow section will not galvanize; it will build pressure and it can burst. The most valuable thing on this page is the venting rule below.
The rule most fabricators need and most drawings omit.
| Design feature | What happens in the kettle | What to do instead |
|---|---|---|
| Overlapping plates seal-welded on all edges | Cleaning solutions cannot enter or leave; residue bleeds out afterwards and stains the coating | Fully seal-weld the overlap, or leave a vent gap. Never do it half-way. |
| Large flat unstiffened plate | Uneven heating causes bowing that will not come back out | Add stiffeners, or accept and specify a straightening allowance |
| Asymmetric section, heavy on one side | One side reaches bath temperature before the other and the piece twists | Balance the section, or discuss the hanging orientation with us first |
| Weld slag and anti-spatter left on | Neither resists pickling and neither takes zinc — you get bare patches at every weld | Remove slag and spatter before despatch to the galvanizer |
| Paint, marking crayon or lacquer | Survives the degrease, resists the acid, leaves an uncoated area | Mark with tags or die stamps, not with paint |
None of this is exotic. It is the list of things that turn a clean job into a rework, and every item on it is decided at the drawing board rather than at the kettle.
Our kettle is 7.0 m × 1.2 m × 1.6 m, and the constraint on most job work is geometry rather than coating. We would far rather look at a drawing and tell you where to put the holes than take delivery of something we cannot dip safely.
That review costs you nothing and it is the single most useful thing a galvanizer can give a fabricator.
Answered from our own line, not from the standard's cover page.
ASTM A385 paragraph 12.4 calls for a minimum single vent and drain opening of 25% to 30% of the cross-sectional area, located in a corner. Larger is generally safer; the practical limit is what the structural design will accept.
Vents at the highest point and drains at the lowest point as the piece hangs during dipping, which is often not how it sits in service. Tell us the article and we will tell you how it will hang.
No, and neither should anyone else. Trapped air and moisture expand violently at 450–460 °C. A sealed section has to be vented before it goes anywhere near the kettle.
Usually differential heating on an unbalanced or unstiffened section. Large flat plates and asymmetric assemblies distort as one part reaches bath temperature ahead of another. A385 covers the design changes that prevent it; where a design cannot be changed, we agree a straightening allowance in advance.
Yes, and we would prefer to. Send the drawing with the enquiry. Reviewing venting and hanging before steel is cut costs nothing and prevents the two expensive outcomes: a piece we cannot dip, and a piece that comes out distorted.
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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