Welded Hollow Steel Forms Outside?

I make steel sculptures that are mostly hollow forms and wonder if they should have venting to allow the interiors to "breath" when situated in the outdoors. At a former job where we fabricated much larger hollow forms, the structures usually had a vent(s) to allow free exchange of air and water vapor from the inside to the outside but I always wondered if this was necessary. Would the water and oxygen inside a welded closed structure eventually get used up in creating iron oxide at which point the process would stop? I have made a few that had a bad or porous weld that did fill with water and those had to be modified to drain or the bad weld repaired if I can find it. What is the general consensus about venting fabrications like this?

I always detail a drain hole, at least 1/2" dia.

I have seen rusty & crusty near the base bad enough, that smaller holes "rust up shut".

Vent and drain holes are required if the tubular structure will be galvanized. There are a number of industry guides on the internet describing the location and size of the required holes.

Zinc coatings require continuous exposure to carbon dioxide present in the air to form a protective layer of zinc carbonate.

There are other concerns about water leaking into a tube and freezing causing the tube to bulge.

Fabrication/Erection - Hollow Structural Sections

Sealing the tubular structure to prevent internal corrosion would be worthwhile if the structure was pressure tested to verify that it was air tight. This could be accomplished with a welded threaded bushing and screw in plug to allow safe welding and periodic inspection. The same arrangement is used in compressed air tanks.

There is a vented galvanized tubular steel forest service bridge on one of the local trails that has rusted through on the lower surface of the railings. I suspect this is caused by ice sitting in the tubes during the winter. The decision on what to do will depend on the installation site weather, soil, and air.

I would vote first for a nitrogen purge to eliminate moisture, only have to do it maybe five minutes then seal off the vent and fill.

I have never seen a drain hole stop rust. In stainless steel the ice blows it apart where water accumulates so you are screwed regardless of a drain hole.

Expandable foam not likely to work as a filler as most boat builders will tell you all that foam sucks up water eventually.

Second, I did a car rebuild (66 Mustang Coupe)with all kinds of hidden places that would rust first day out of garage. My auto body friend told be to use a paint that would be sprayed into voids and let dry, after welding up the new metal the "paint" or whatever it is was unharmed by the heat from the weld and kept the rusting to a minimum. I think 3M made it. That was 20+ years ago but autobody people must know what it is I used.

Pathogen said:

Second, I did a car rebuild (66 Mustang Coupe)with all kinds of hidden places that would rust first day out of garage. My auto body friend told be to use a paint that would be sprayed into voids and let dry, after welding up the new metal the "paint" or whatever it is was unharmed by the heat from the weld and kept the rusting to a minimum. I think 3M made it. That was 20+ years ago but autobody people must know what it is I used.

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"Weldable Primer" I think is the name.

digger doug said:

"Weldable Primer" I think is the name.

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yep its a zinc weldable primer, usually used while doing bodywork.

Thanks for all the comments. My understanding of primer is that it is not something to protect a surface from rust but a porous undercoat for the finish paint which does seal the surface. Maybe the weldable primer is different though. One thing I want to avoid is making the form an attractive place for hornets and such so whatever venting I use must be able to exclude them.