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23rd Mar 2019, 01:28 PM #1
Moisture inside metal SHS question .
Hi . I have some timber racks I have to weld back together . They are made from 50 x 50 x 3 SHS . I cut them in half for transport two years ago .
If steel is welded back together and the ends are sealed up as well, does the moisture inside get used up in the rusting process ?
Or does it continue to react and keep rusting ?
Rob
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23rd Mar 2019, 02:13 PM #2Senior Member
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The moisture inside isnt specifically the problem, its the oxygen. Moisture just acts as an electrolyte to assist the oxidation process. However, once the oxygen within the newly sealed tube is consumed, thats the end of it. The oxygen doesnt get used to corrode some steel, then freed to go and attack some more.
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23rd Mar 2019, 03:02 PM #3Member: Blue and white apron brigade
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Unless you have an airtight seal even a small hole will allow air to get in and out and this will continue the rusting process.
This happens because any container with a hole in it will breathe due to changes caused by atmospheric pressure (AP) .
When the pressure inside the SHS is lower than the AP air and humidity will seep into the SHS and continue the rusting process.
When the pressure inside the SHS is greater than AP air now containing reduced oxygen and water vapour will be expelled allowing room for fresh air (with greater levels of 02 and H20) to enter.
The smaller the hole the slower the whole process.
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23rd Mar 2019, 04:26 PM #4
Thanks that’s good to know . The info from the both of you reminds me being told something similar about fire . Take away one of the three components that make it happen and it won’t happen .
But Maybe that’s not so weith rust because the oxygen and humidity are in the one gas or come in at the same time ?
Surface rust is no big problem if it keeps happening with these .
I have just repaired a similar steel structure . A cattle crush I got for $100 . Same RHS as my timber racks . The top of the 50 x 50 is open to the rain . The bottom is sealed . This is pretty much why it was $100 .
Repairs to its rusted through base are not possible . I got around it by laying new 50 x 50 to the solid base bits on the outside .
The same sort of rot in my racks though would cause a collapse eventually .
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23rd Mar 2019, 06:11 PM #5Senior Member
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Seems to be some contradictory stuff there, with open ended and sealed up in that first post.
Wouldn't take "open ended' under a roof to be necessarily immune to rust though! As anyone with the usual tools on this forum knows, pretty normal to get a "sweat" on a cold lump of metal overnight with the right temperature and humidity conditions. In the aero industry, thin wall tube is oiled internally as a preventative.
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23rd Mar 2019, 10:56 PM #6
If internal corrosion is such a big deal,put a threaded plug in it and give it a squirt of Tectyl on a regular basis. If you are poor,sump oil it.
You can buy a spray gun with a long flexibe tube nozzle that can be pushed down a tube.
Grahame
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24th Mar 2019, 12:59 AM #7
I should have read it better before posting . Yes it did seem contradictory . I removed the second part.
Yep , I see that on my out door unpainted steel things . I could end up doing that with the oil . Sump oil as Grahame said above .
That sort of rust hasn't worried me so much in the past. The life of the structure will probably be shorter than it would take to rust through .
What Im doing as well is cutting the flat sealed end of the 50 x 50 leg off. What was the foot with a hole for screwing it to the top of some concrete at the old location. And Im adding around a meter of 50 x 50 x 3 Gal with some short side tags of angle Gal . That will be the new leg end that goes down the hole and gets back filled with concrete once I prop and true up the frames. So oil wont be a good idea if its going through to the concrete then the ground .
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24th Mar 2019, 01:19 AM #8Senior Member
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- Aug 2007
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- Saskatoon, Saskatchewan, Canada
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You can always get some desiccant bags and stuff one in before you weld it closed. What little moisture there is in the pipe will be absorbed.
Desiccant.com.au :: Moisture Control Specialists :: Home Page
Pete
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