The most useful thing a galvanizer can give a fabricator. Decide this at the drawing board and almost every other galvanizing problem goes away.
Everything a hot dip kettle does depends on liquid getting in and air getting out. Degreaser, acid, rinse water, flux and finally molten zinc all have to reach every internal surface, and every one of them has to drain out again.
A sealed hollow section cannot be galvanized, and must not be dipped. Trapped moisture flashes to steam at 450 °C inside a closed volume. That is a safety issue before it is a quality one, and no galvanizer should accept the work.
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.
Two things about that rule are often missed. First, it is a minimum — larger openings drain faster, coat more evenly and are almost never a problem. Second, the corner location matters as much as the size, because a hole in the middle of a face leaves liquid trapped in the corners on either side of it.
Where a single large opening is not acceptable structurally, two or four smaller holes can be used instead, but the total open area still has to do the same job.
| Check | Why |
|---|---|
| Every hollow member has a vent and a drain | Sealed sections cannot be dipped, at all |
| Every internal compartment has its own pair | One vent does not serve two closed volumes |
| Openings total at least 25–30% of the cross-sectional area | ASTM A385 §12.4 |
| Holes are in corners, not mid-face | Corner liquid is what gets trapped |
| Corner gussets are clipped | A stiffener plate seals the triangle behind it |
| Overlapping surfaces are either fully seal-welded or left with a vent gap | A partly sealed overlap traps acid and bleeds it out later, staining the coating |
| Weld slag, spatter, paint and marking crayon removed | None of it survives to take zinc |
| Threads protected or agreed | Fasteners are galvanized to ASTM A153 and centrifuged, not dipped as structure |
Run this before the steel leaves your works, not when it arrives at ours. Every item on it is cheap at the drawing board and expensive at the kettle.
Our kettle is 7.0 m × 1.2 m × 1.6 m at 450–460 °C, and on job work the binding constraint is usually geometry rather than coating specification. We will look at a drawing and tell you where the vents and drains belong, how the piece will hang, and whether anything about the assembly is going to give trouble.
That review is free and it is the most useful thing we can give you. It is far better than taking delivery of something we then have to send back.
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, in a corner. It is a minimum — larger openings drain better and rarely cause a problem.
Vents at the highest point and drains at the lowest, as the piece hangs during dipping rather than as it sits in service. Diagonally opposite ends works well, because it lets liquid sweep the whole internal length.
No. Trapped air and moisture expand violently at 450 °C inside a closed volume. It is a safety hazard, not just a coating problem, and it must be vented before dipping.
A gusset welded across a corner creates a small sealed triangle behind it, so yes — clip the corner of the gusset. It costs nothing at the cutting stage and it is a common cause of rework.
Yes, free of charge, and we would prefer to. Send the drawing with the enquiry. Because our kettle is in-house, the people who will hang the piece are the ones who look at it.
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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