[MA09633]

Greetings. 

I am a homeowner in South Florida.  This may be a bit unusual for this forum, but I've tried seven (7) contractors who have all come out and told me they have never seen anything like my issue before.  Some with 25 years experience.

So let me tell you about my issue, which I am starting to believe is a chemistry issue. 

First, the problem.

In South Florida, codes require homeowners to have hurricane protection.  For me, this means shutters that bolt onto brackets below and above the openings.  These brackets are made of powder coated aluminum.

Also, in South Florida, many people have pools, and patios with sliding glass doors ("sliders"). 

My brackets and slider tracks are corroding extensively.  But brackets on the windows that are elevated about 4 feet are not corroding.  Neither is anything else.

I've had what I now believe is an aggressive corrosion issue which I have a theory about.  I'm going to lay that theory out and I would like feedback on whether my theory is possible.  And if so, how to stop this!

My theory is that an open chlorine tablet (for the pool) was sitting outside on the patio for about 18 months.  These tablets are super concentrated (90% available chlorine).  Because the tablet over time emits gas (which is heavier than air), this gas settles near these brackets and the slider tracks.  These areas are about 2 1/2 to 3 inches off the deck, if that.  My theory is that the gas deposits chlorine ions on the surfaces of these aluminum areas and then when the window washer comes, the water that drips down combines to create hydrochloric acid.  This in turn eats away at the finishes on the aluminum brackets and eats the oxide on the bare aluminum slider door base and then reacts with the aluminum to form aluminum chloride.  I have noticed this clear oozing substance coming off the aluminum in areas and it later turns to a white powder (see pics).  My guess is that this is aqueous aluminum chloride.

So what I did was removed the chlorine tablet and cleaned all of these surfaces with a solution of baking soda and water.  Then rinsed.  But I noticed this was still happening, particularly in the cracks and crevices.  For example, where the bracket is bolted to the slab - in between the slab and the bracket the oozing is still happening. 

So I used a stronger solution of soda ash and water and rinsed these areas.  After that I flushed with the garden hose very thoroughly.   So I've repeated this three times and still see some of this oozing. 

Now I haven't had the substances tested, so I'm working on a theory here.

So any feedback on this theory?  Any thoughts on how to stop the further corrosion and the oozing?  Am I off base here? 

Thanks very much!!!

[Borek]

No idea what is going on, but your theory doesn't make much sense to me.

I wonder how the aluminum parts are mounted. If someone used iron or brass nails/screws somewhere, you can be a victim of galvanic corrosion.

[MA09633]

Actually, the installation of these items is very standard using Tapcon screws that bolt into the slab.  These are used routinely with Aluminum.  The other thing is that this setup has been here for 15 years, and the corrosion only started 12 - 24 months ago.

But thanks for taking a look.

[Arkcon]

There's a lot happening in your text block, and like Borek: said, some of it isn't likely.  Its a good thought experiment, on your part, regarding the height up the building that is responsible for different levels of chlorine exposure, but that doesn't have to be the only explanation.  It could be coincidence, or the lower levels are subject to more wear and tear, and this is ordinary environmental corrosion.  Or a bad batch of coating.  Or simply age.

Your chemistry is a little bit off.  Certainly free chlorine reacts with bare aluminum.  But the product, aluminum chloride, decomposes in moist air, reverting to aluminum oxide.  Also, free chlorine doesn't become hydrochloric acid readily.

I don't know why you went with mild base as a cleaning agent, that will attack bare aluminum too.  You should try to remove all of the corrosion, then maybe you can prime and paint.  There is corrosion resistant paint made specifically for aluminum.  Also, I've seen a formulation of naval jelly designed for stripping aluminum corrosion, I guess it's gentler than other formulations of naval jelly.  But it is definitely an acid based corrosion remover made for aluminum.

[vmelkon]

Your chemistry is a little bit off.  Certainly free chlorine reacts with bare aluminum.  But the product, aluminum chloride, decomposes in moist air, reverting to aluminum oxide.  Also, free chlorine doesn't become hydrochloric acid readily.

Are you sure it decomposes in moist air? Wikipedia says you can make AlCl3.6H2O with Al and hydrochloric acid.

[MA09633]

It gets a lot of moisture.  Humidity, pool, and rain.

[Arkcon]

Your chemistry is a little bit off.  Certainly free chlorine reacts with bare aluminum.  But the product, aluminum chloride, decomposes in moist air, reverting to aluminum oxide.  Also, free chlorine doesn't become hydrochloric acid readily.

Are you sure it decomposes in moist air? Wikipedia says you can make AlCl3.6H2O with Al and hydrochloric acid.

Hmmm...odd, we used to assume that aluminum chloride wasn't stable at all in aqueous solution.  But Wikipedia does quote solubility.  http://en.wikipedia.org/wiki/Aluminum_chloride#Synthesis  Hmm ... I 'll have to check some textbooks, and compare to whatever references Wikipedia is using.  At any rate, yes, the aluminum chloride might be stable at room temp, but should turn yellow and not stay scaly and white.  That's the pretty standard aluminum oxide corrosion.

[vmelkon]

It gets a lot of moisture.  Humidity, pool, and rain.

Chlorine does react with water and makes
Cl2 + H2O -> HClO + HCl
so what you said at the beginning about chlorine ion, you lost me.

I suggest that you do a simple acid test. Drop some HCl or citric acid or acetic acid or whatever you have available. If it is fizzes, it is likely CaCO3.
If the metal is galvanized steel, then maybe you have Zn(OH)2 or ZnCO3 on it. ZnCO3 will fizz with the acid.
If it is ZnCl2, dissolve in water. Also dissolve some NaOH in water. Add them together and Zn(OH)2 forms and eventually settles at the bottom.
If it is AlCl3 , dissolve in water. Also dissolve some NaOH in water. Add them together and Al(OH)3 forms and this stuff is gel like and tends to not settle at the bottom.