Holes in welded and galvanised structures

Items that are hot dip galvanised have to be vented in order to prevent dangerous pressure build-up, I've had many items dipped and it is a good

build up .

Am Freitag, 6. Oktober 2017 02:00:24 UTC+2 schrieb David Billington:

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Makes sense. Not what I guessed, though. I was guessing it was to prevent s tagnant water from building up inside the tubes and causing corrosion.

Even if it were not galvanized, there is another reason to do it. When I was welding up some tubing to make a support for a corner notcher I started with a top rectangular frame with three sides milled off each end of two of the four sides, leaving a flap which covered the open end of the other two tubes. I drilled holes in each of the un-flapped tubes to vent into the flapped ones, plus holes to vent into the following upright legs. The reason for this was I expected expansion of the air trapped in the tubes, and when welding that could blow the fill material (Tig welding, FWIW) out of some of the joints. Give it vents, and you won't have expanding air pushing out the melted metal and weakening the joints.

Later tubes joining the legs part way down were also drilled on the underside to avoid similar air induced holes.

Enjoy, DoN.

Here is a venting guidance video from Wedge Galvanising

, it include brief footage of what can happen when an unvented items is dipped and it bursts.

Am Freitag, 6. Oktober 2017 12:25:31 UTC+2 schrieb David Billington:

Great video. It explains a lot of small structural features on fabricated s tructures. Thanks to everyone for the responses. Interesting thread.

By the way, does anyone know why the cut-out on this excavator dipper is pr ovided? I figure it might be something to do with stress distribution and f atigue resistance, but I can't work out why this cut-out location would hel p. Maybe it has another purpose. Any ideas?

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Am Freitag, 6. Oktober 2017 12:25:31 UTC+2 schrieb David Billington:

By the way, I was told lately that galvanising sticks better if the steel surface is slightly corroded. Anyone know if this is true?

Possibly the corrosion leads to a rougher surface which results in a thicker zinc coating. Items are intentional roughened by sand blasting for that reason.

Am Freitag, 6. Oktober 2017 13:17:30 UTC+2 schrieb David Billington:

That makes sense, especially among guys who are keen to save money.

I don't know if it would save money as my understanding is you pay by the weight of zinc added. It's a very effective bulk process and my neighbour priced up galvanising versus painting in his former job at

system if used would have required sandblasting the steelwork anyway. The galvanising didn't require sandblasting but it may have been done for extra protection, the initial acid dip removes any rust and mill scale anyway.

Am Freitag, 6. Oktober 2017 13:45:23 UTC+2 schrieb David Billington:

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Maybe it's just a way to save on the cost of the sandblasting to roughen th e surface.

In high-volume commercial galzanizing, the steel is prepped with a zinc ammonium chloride bath. Chemistry is not my thing but I think it's actually a conversion coating that cleans the surface and leaves a protective film of the conversion product on the steel.

Even slight oxidation of the steel weakens adhesion of the zinc.

As I think about it, that "high-volume" treatment I mentioned may apply only to galvanizing steel strip in a continuous process. That's where I learned about it, when I was reporting on making car body panels.

I see that the American Galvanizer's Assn. has much more info, which probably applies to fabricated structures and parts:

For the stuff I've had done the first stage is the acid dip to remove mill scale and rust, the 2nd bath is as you mention, and the 3rd stage is the dip in the molten zinc.

I've been told that the start and stop of a weld is where fractures are most likely to start and that on equipment such as that the welds are intentionally started and stopped in a low stress region so it may be that the hole in the centre of the weldment is where that happens.

If a closed segment had some water left in it by accident, and then you welded on it, it could explode. Some people have apparently been killed in such explosions, or horribly burned by flying molten metal.

Theoretically, this could happen just from trapped air, too.

Jon

I hope they had the customer sign that he had drilled vent holes, so he can pay for it instead of them. Messy! 840F/449C expands air a bit, I'm guessing.

I suspect they missed that one as the galvanisers I've used will drill holes at their discretion if they feel it isn't appropriately vented.

Interesting. How about these holes and weld tails (see below)? These must be for additional strength and fatigue resistance, right?

Chris