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  1. #1
    Join Date
    Aug 2008
    Location
    Tennant Creek, Aust
    Age
    61
    Posts
    596

    Default Shed Wall Rusting

    I just noticed at the side where I rarely go it had started to rust on the bottom.
    Would rust converter kill the rust, then paint it with hydroseal paint, be the best way to fix the problem?



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  2. #2
    Join Date
    Jan 2004
    Location
    Mackay North Qld
    Posts
    6,446

    Default

    To be brutally honest I think your problem is a bit more complicated than straight rust.

    I believe what you have going on there is electrolytic corrosion. While the sheetmetal is in active connection with the damp cement an electric current, albeit ever to small, flows and is responsible for that creeping corrosion that is occurring.

    In the course of my work I have visited many, many sheds in my city of Mackay and what I noticed is that where the sheet metal physically contacts the the concrete. In every case where the corrugated iron or colourbond /trimdeck contacts the cement rust occurs. Put a steel locker on concrete for any length of time and watch where the rust begins.

    To fix it properly the sheet must be removed from contact with the concrete.
    When I built my 9x 6m shed in 2014, I selected a design where the sheets do do touch the concrete. The slab is stepped and the sheets continue below the main slab level and finish below the floor level into the stepped area. Six years later despite living in rust city there is no corrossion on the base of the shed walls.

    Sure you can kill the visible rust and coat it with some proprietary product but it will reoccur while those sheets are embedded .

    Here is but link to understanding what happens, there are many if you want to look for them.
    https://www.cement.org/learn/concret...dded-materials

    To answer the " how to fix it part of the post, I would lean to removing the lower portion of the wall and replacing it with timber or fibre sheet replacement.

    This is probably not what you want to hear, but I can't envisage anything else at the moment which will be permanent and effective.

    Grahame

  3. #3
    Join Date
    Sep 2009
    Location
    Newcastle
    Posts
    341

    Default

    Concrete should not be in contact with the steel like that.

    My shed the Colourbond was only resting against the edge of the slab (not slab poured up to the sheets) and it was rusting. I installed bitumen impregnated foam between the concrete and Colourbond and that fixed the issue (and stopped the pests).

  4. #4
    Join Date
    Dec 2018
    Location
    NSW
    Posts
    586

    Default

    Electrolytic rust, or just the presence of lime and all the other alkalis in the concrete? The latter is my bet.

  5. #5
    Join Date
    Aug 2011
    Location
    Melbourne
    Posts
    4,779

    Default

    When I built my shed, the concreter told me that if I wanted to be a bit fussy, I should coat the bottom of the colorbond sheets with a coating before doing the slab. So I coated the bottom ends of the sheets with a gripset bitumen based paint.

    8 years on and no corrosion. I always assumed the corrosion is from that fact that concrete is caustic.

    Simon

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  6. #6
    Join Date
    Dec 2013
    Age
    56
    Posts
    1,416

    Default

    You could use a 9inch grinder to go around and cut 10mm off, clean and paint as you said.
    Then buy that bitument stuff they put between concrete sections cut into strip's laid under it to fill the gap to stop vernem coming in.
    There would be a sealer available to stick to them steel and bitumen to hold it in place.
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  7. #7
    Join Date
    Oct 2008
    Location
    N.W.Tasmania
    Posts
    1,407

    Default

    I'm not sure what is going on here, because concrete is alkaline and that should prevent steel rusting and not promote it. It could however, strip the protective zinc or zincalume layer off the steel leaving it at the mercy of the environment. Having read the link Graeme provided I am wondering if the concrete was mixed with more water than it should have had, and bearing in mind your location at Tennant Creek, maybe the water was bore water with a fairly high salt content, perhaps that would explain your issue. Tennant Creek is not usually too wet but winter night times can be cold and condensation may be the water source to make the rust happen. Are any foot plates on the columns showing rust signs where they bear on the concrete?
    I don't really like the idea of cutting the lower section off as has been suggested, because in any heavy rain event, possibly not too frequent in your neck of the woods, would make for much easier water ingress than it has now. I think that I would try your idea with rust converter, possibly some epoxy and then hydro seal paint. Denso tape may also be worth considering, it will give long term protection if it is protected from mechanical damage. Good Luck
    Rob

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