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How do you stop the inside of a hollow iron joist from rusting?
4

How do you stop the inside of a hollow iron joist from rusting?

How do you stop the inside of a hollow iron joist from rusting?

(OP)
I'm having a storage floor (raft) installed beneath the roof of my garage, consisting of plywood sheets resting on mild-steel hollow square joists. The joists have welded right-angle flanges at the ends. The end flanges don't completely seal off the hollow section at the extremities, so air can still enter the hollow core of the joists. The flanges will rest in slots cut into the concrete/limestone walls of the garage. Each joist is in two parts, one sliding inside the other, so it will be extended in-situ to span the exact gap from one wall to the other, and a central bolt on the outer joist will be tightened to press onto the inner joist to stop it from sliding after installation.

The builder just delivered the material and it looks like mild steel, but seems to have a black surface to it. Pics here - https://imgur.com/a/RhP8ruD . I've been advised to paint the outside of the steel to stop it rusting. My concern is the inside of the square hollow section - since it will be exposed to air, is it at risk of rusting? I am unable to paint the inside, the beams are 3 meters long and section is about 10cm x 5cm so barely big enough to insert a hand with a paintbrush.

Wall thickness is about 5mm - any idea how many years it would take for rust to appreciably weaken it? I live in Malta, the relative humidity reaches a peak of about 75% in winter. FWIW I'd estimate storing about 300kg of stuff on this structure. There are 4 joists in total. My structural calculations are a little rusty (pardon the pun) so I haven't calculated the factor of safety on this (I'm leaving it to the actual builder)

So, is there anything I can do to protect the interior? Should I try sealing the ends to the concrete walls with silicone caulk (once installed) and put extra paint or silicone on the joint between the inner and outer beams to prevent air entering as much as possible? Or does that trap moisture and make the problem worse? I enquired about galvanising but that's not an option in my region since the companies that do it have discontinued the service.

RE: How do you stop the inside of a hollow iron joist from rusting?

A buddy painted the entire tubular frame of a car - covered all but one hole with tap or plugs and poured in a suitable amount of paint and then sloshed it around. I would start with a rust conversion treatment and let that dry - put a fan to push air through it to hasten the drying; it may require a cardboard shroud to force the air in.

I don't know how concerned one should be - condensation from moist air circulating will mainly happen on the outside of the tube, but go for internal coating if you want to be sure. Add a pad to the ends that telescopes to deter scraping the paint - they make self adhesive slippery tapes.

RE: How do you stop the inside of a hollow iron joist from rusting?

Without any draft through the beam there should be very little air exchange to bring moisture in and out of it. I would expect any corrosion inside to be minimal in an interior environment. The corrosion will also be concentrated around the areas of ingress. Can you not seal the inside of the beam by closing of all gaps and holes with an adhesive urethane or silicone caulk?

RE: How do you stop the inside of a hollow iron joist from rusting?

(OP)
That's what I was thinking - I could caulk the area where it joins the wall on both ends, and caulk or paint the sliding joint in the middle as well as the bolt (since it won't need to slide ever again after installation). Would that arrest rust development, or make things worse due to keeping moisture inside?

Someone else suggested a zinc-based primer (for the outside, before painting). Would that help protect the inside?

RE: How do you stop the inside of a hollow iron joist from rusting?

I would be much more concerned by that crappy looking weld to the angles and the small bearing surface of that angle on the wall. I would simply call that an angle not a "flange". Any more pictures of the end connection?

Is that last picture showing the beams in their "minimum" length state one inside the other?

but in answer to your question is it at risk - Yes, but will be a low risk if this is inside and not getting wet inside.

But if you want to be sure try something like this https://www.lanoguard.co.uk/products/lanoguard-mot...
Just search "metal corrosion protection spray liquid"

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: How do you stop the inside of a hollow iron joist from rusting?

Yes the only real risk is if rainwater gets into them.
You can buy sponges soaked with a vapor phase corrosion inhibitor.
They are meant to be used in toolboxes, gun cabinets, and the like.
Toss a couple of them into each end before they are put together.
Then after install caulk up all of the gaps.
Paint the outside with a good rust conversion primer.
The black is an oxide and the primer will bond to it.
With swings in temperature the outside will sometimes be cooler and end up with condensation on it.
If you don't use a good paint you will get drips of rust water across the garage.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: How do you stop the inside of a hollow iron joist from rusting?

(OP)
You have a point, those angles might look a tad small. I don't have exact dimensions, I trusted the builder on this one. They will rest on pockets in the walls of the garage. The walls are a combination of solid limestone and hollow concrete blocks.

Right now the beams are stacked vertically at the garage and I won't be back there until the weekend so I can't take other photos, but the side you're seeing is the side that extends out, so the welds aren't actually welded to the outer metal in the picture but to the inner sliding section that you can't see in the pic. You are correct that they're in the fully retracted position in the pic.

I expect 300kg of books to be stored on this structure. There's four of the metal beams, the size is about 3.2m wide by 1.8m long (so one beam every 60cm). The plywood sheets are the ones used in Malta to prop up roofs when they place rebar and pour concrete - they are long enough that each plywood sheet will span the length of all four beams to help distribute the load a little.

If you're concerned that this might be marginal for the loads involved I could look into bolting angles to the wall beneath the beams to help further support the beams. I realize that the entire load is going to be held by loading the welds in shear and the angle will act as a cantilever in bending, though my intuition tells me the stone of the garage walls will likely fail before the metal structure, unless rust makes the angle corrode through.

To answer the original corrosion question, I've purchased a few tins of this - https://www.hammerite.co.uk/product/direct-to-rust... . I plan to start by painting the angles and the cut ends very well on the ground, then painting the rest of the exterior once the beams are in place. Then finally I will apply silicone caulk to the gap between the beam and the walls to seal off the inside as much as possible. Then again, since stone is porous, it might allow moisture to diffuse in via the stone itself?

RE: How do you stop the inside of a hollow iron joist from rusting?

(OP)
Update - I've done some quick calculations (I haven't touched this in over a decade so correct me if I'm wrong). The compressive strength of brick, hollow concrete blocks, and limestone are of the order of 5, 8 and 10 MPa respectively. That's 50, 80 or 100 kilograms per square cm. So my projected load of 300kg could theoretically be borne by 300/50 = 6 square centimeters of brick. Or an area of 6cm x 1cm. In other words, an area about the size of your thumb.

I am of course ignoring any tensile stresses induced in the brick from imperfect angles and imperfect installation, and any resulting stress concentration from badly-angled angles. I would estimate the total area of the angles to be something like 5cm x 5cm x 8 angles = 200 square cm, so a factor-of-safety of about 33 (again, assuming the idealised case of purely compressive loading on the masonry and neglecting the unequal stress distribution due to bending deflection of the angles under load). Seems to be ok, right?

The 300kg is a conservative figure taking into account the weight of the plywood, the beams themselves, and the couple of files and textbooks we're taking out of the bookshelves and transferring to this structure.

RE: How do you stop the inside of a hollow iron joist from rusting?

Weather-resistant condoms

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: How do you stop the inside of a hollow iron joist from rusting?

Take a tip from the aircraft makers, and install tube oil. This is a drying oil you slosh on the inside of the tube similar to linseed oil.
B.E.

You are judged not by what you know, but by what you can do.

RE: How do you stop the inside of a hollow iron joist from rusting?

You don't mention whether the garage air is conditioned or not. If the humidity in the garage is maintained less than 40%, than the steel will not rust. Caulking of the tubing ends will eliminate the rusting on the interior side. I would be most concerned about the bearing surface on the tubing ends. I would use galvanized shims where the tubing rests on the walls.

RE: How do you stop the inside of a hollow iron joist from rusting?

(OP)
Relative humidity inside the garage is 65%. Not air-conditioned, electricity is eye-wateringly expensive in Europe, I don't know of anyone using it for a garage. It wouldn't be possible either, because the area above the door has very large open air vents installed for copious air exchange with the outside world (some building code requirement). This is a garage inside a block of garages at basement level.

I painted the whole thing in two coats of white Hammerite paint. The bearing surfaces too. Hope that's enough. The builders may have scratched them a bit when they inserted them into the brick walls. I did plaster around them with instant plaster. To my knowledge plaster is alkaline, as should be concrete and limestone, so it should retard rusting somewhat right?

RE: How do you stop the inside of a hollow iron joist from rusting?

I have a steel WF beam in an unconditioned shed here in Tidewater Virginia RH here often gets to 90% summer night time. The beam is mill finish, no paint. 20 years, no significant corrosion.

With the paint, the beam should last the life of your garage.

RE: How do you stop the inside of a hollow iron joist from rusting?

Relative humidity is usually highest at the coolest time of day anywhere in the world. That is not why water collects inside tubes. 90% RH at the warmest time of day is the problem, combined with sunshine warming the steel. When the sun sets, the tube cools and and draws-in humid air. Further cooling at night causes condensation inside the tube which drains to the bottom and accumulates there because it cannot evaporate.

The tube should either be completely sealed, or have a drain hole at the bottom. Sealed means bubble-tight, to prevent any breathing. It is difficult to seal weld a closed volume because the heat of welding expands the air inside. It is usually more reliable to leave a vent hole open while welding and then seal the hole.

RE: How do you stop the inside of a hollow iron joist from rusting?

Quote (Compositepro)

It is difficult to seal weld a closed volume because the heat of welding expands the air inside.

???

Weld the thing closed. The fuel for corrosion inside is very limited, and the corrosion rate will soon drop to zero with no measurable loss of wall.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: How do you stop the inside of a hollow iron joist from rusting?

IM, as you're welding the last but shut the expansion of gas will cause pin holes in the weld. Best to drill holes to allow the enclosed space to breath during welding and thread pipe plug later. This also appeases your marine chemist when you need a hot work permit.

One could use a Gore (of Gore-Tex) style vent. They have ones nowadays that are literally a sticker but there are also a variety of threaded options. The sticker style are routinely used for automotive headlight enclosures and they seem to work quite well.

RE: How do you stop the inside of a hollow iron joist from rusting?

Weld one pass. There might be pinholes (but that is always a risk; single pass welds don't tend to be helium-tight).

Weld a second pass/layer and the box is sealed for good. End of.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

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