physicsdude
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Concrete on galvanized metal deck
I hope this is the right place for this - if not, please forgive me.
Real life situation - we have a vented, galvanized metal deck with concrete poured directly on top of it - creating a pH at the interface of
approximately 13.
On the bottom of the deck, when the humidity in the room is very high, it condenses and creates a brownish, soapy liquid that is attacking the primer
on the metal joists.
No moisture is coming from above, only below due to humidity in the air.
Any ideas on what this reaction is creating (what the brownish liquid could be) ?
Just searching for ideas here...thanks!
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psychokinetic
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Hmm. Concrete and steel seems to be the usual way to build things, but over time the concrete expands and contracts etc; meantime water and oxygen get
in, rusting the metal.
If I recall correctly, zinc oxide is brown. And if it's seeping out with the water then it'll be liquid.
But hey, I'm a chemistry pleb - don't go tearing your deck apart until the masters have debated about it 
EDIT: Had a thought.... In order to get the zinc off (presuming it's zinc coated), it would need an acid. Do you recall what kind of concrete it was?
[Edited on 26-2-2010 by psychokinetic]
“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found
the object of his search.
I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
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JohnWW
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ZnO is white, not brown! It is also very opaque and absorbs UV light, hence its use in some sunscreen creams, and (along with TiO2) in white paints.
Any brown coloration would be due to contamination, one likely contaminant being hydrous Fe2O3 or FeO(OH).
A pH of 13 is likely to be sufficient, in the presence of water, to slowly dissolve Zn as zincate and Al as aluminate. It can also dissolve Fe2O3 as
ferrite(III). A strongly alkaline solution would also damage many paint compositions. But the under-side of the deck, if it is made of sheet
galvanized steel with concrete poured directly on top of it, should not be in contact with concrete at all, unless there is a leak, which seems
likely. So I think that the best thing to do would be to make the concrete top of the deck waterproof, by coating it with some sort of sealer followed
by coats of paving paint (or butynol or similar walk-on roof covering). At the same time, a better anti-corrosive paint could also be used on the
metal underside of the deck and its supporting joists.
[Edited on 27-2-10 by JohnWW]
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hissingnoise
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In the meantime a thermistor-controlled dehumidifier might be a short-term solution. . .
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psychokinetic
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That'll learn me to not check. Sorry physicsdude!
“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found
the object of his search.
I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
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Xenoid
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Hmmm.. your description doesn't make sense, can you elaborate?
Where exactly is this; "creating a pH at the interface of approximately 13"
The interface between the galvanised metal and the concrete will be the upper surface of the galvanised sheet not the lower surface!
Clearly, to get a pH of 13 and "soapy liquids", Ca(OH)2 (calcium hydroxide, slaked lime) is being leached out of the concrete. The brown colour will
be due to Fe+++ in various species (rust). Are there joints in the galvanised sheets, were the joints sealed, was the concrete waterproofed? Don't
underestimate capillary action and hygroscopic chemicals in situations like this.
Strongly alkaline solutions, will turn normal paints to jelly. Remember, NaOH (caustic soda) is used as a paint stripper, etc.
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chief
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Iron is passivated in the pH of concrete: Thats how at all it's possible to have steel within ..., as in many buildings ...
Also this was used in steel-Batteries (accumulators), which were a replacement for lead-acid batteries when it came to the use in railroad-waggons ...
because they are more shock-resistant ... (yes: plain steel in alkaline solution gives accumulators that rival the performance of lead-acid-batteries
... )
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Xenoid
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A "Google" search for "concrete corrosion galvanised steel" reveals many papers on this subject. In particular, in relation to the effect of chloride
de-icers on galvanised steel rebar in concrete structures such as bridges.
Here is a detailed study: http://www.che.sc.edu/faculty/popov/drbnp/WebSite/Research%2...
Could the presence of chloride (Cl-) or chlorine be a factor in your situation. In particular, are you situated close to the ocean or is there a
chlorinated swimming pool near the deck. Remember, all concrete slabs contain at least micro-cracks, where ponded water will be able to penetrate to
the concrete/galvanised steel interface by capillary action.
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