Corroding pool ductwork 1

[emalsyd]

Last year we installed new underground pool duct at a long-time customers residence. Pool is indoors, at the same time we installed a new Seresco dehumification unit. Pool is salt water vs chlorine.

The main underground duct was PCD coated spiral. Branch duct up to the floor grilles was aluminum. Dirt/sand mixture was backfilled in around the new ductwork and a new concrete slab was poured.

The system has been running for just under a year. We received a call from the owner last month, said they were getting dirt/dust blowing out of the floor grilles. We discovered there were holes corroded through the aluminum ducts in various areas. Some holes were about a quarter sized holes, other smaller. The dirt packed in around the ducts was falling through the holes and some was getting blown out the floor grilles. We thought whoever installed the concrete had used acid wash, which got down the ducts and corroded them.
The pool guy, who handled all the new pool plumbing/piping either hired or was responsible for the concrete work. He claims the aluminum is corroding from the concentration of pool salt in the air going through the ductwork. I'm finding that hard to believe.

We haven't done a natatorium in a number of years but I could have sworn we always used aluminum duct in the past. My other thought is even if aluminum was the wrong choice, I find it hard to believe it would corrode so badly in under a year. Any thougts would be appreciated.

 

[HVAC-Novice]

What are the ground temperatures the duct experiences and is that heating, or cooling, or both? what climate?
You will have moisture condensing in the duct and you have salt in the air.

Dehumidification will be impossible. when you dehumidify the air, you will just increase the evaporation rate from the pool, especial if it is warm. So that alone doesn't protect you from corrosion unless you install an industrial dehumidifier.

 

[SAK9]

It was not very wise to use a metal duct for burying underground. FRP or HDPE would have been better choices.

Aluminium is not well known for strength so typically I have always used a gauge higher than recommended when I needed to use Aluminium in places like MRI rooms etc. So it is worth checking what gauge was used. Since the duct is pressurised how is dirt getting into the duct?
Typically for pools I have always used stainless steel if the ductwork was exposed.

 

[emalsyd]

Dirt is getting into the duct because the duct has corroded to the point where there are holes through the duct. The dirt outside of the duct is dropping into the duct and some is getting blown out the floor supply grilles. We used PCD coated spiral for the main trunk. We couldn't get sheets of PCD coated in time so used aluminum for the branch ducts up to the floor grilles. I believe the branch duct is 18x6. Duct is 24ga. The ducts aren't caving in, they are corroding/being eaten in certain areas. I would think worst case the aluminum would begin to pit vs corrode, especailly after only running for 10-12 months.

 

[LittleInch]

Is there any possibility you're getting galvanic corrosion here between the aluminium and any steel components or the pipe?



Remember - More details = better answers
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[emalsyd]

No,the corrosion is mostly up top, near the floor grilles, which are aluminum. The aluminum drops to the PCD spiral are about 18"-24" long. Most of the corrosion is maybe 12" and up. There is just dirt from the top of the PCD duct up to the bottom of the concrete slab surrounding all the aluminum duct. And some pool water piping the pool guy installed just under the slab. But that was all PVC.
I was just talking to one of my guys, the concrete deck is a stamped concrete. One of my other guys also has that for his outdoor patio and he was saying they use acid wash on it after they stamp it, then use a sealer on it. I'm assuming they did the same here on this concrete, which is what we think corroded the aluminum so quickly but the pool guy is saying that's not the cause.

 

[SAK9]

The only way to prove acid wash caused it is to send a sample to a lab for analysis and get their opinion on the reasons for corrosion

 

[Socrates5]

If the duct is in contact with the soil, galvanic corrosion most likely

 

[LittleInch]

Is the soil permanently wet from the pool water? AL doesn't like wet salty water on it all the time.

Seems like the acid might have started the corrosion, but something else is finishing it off.

Any pictures or diagrams of the set up?

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

 

[emalsyd]

No pictures. Will try to get some when we go back.

The soil doesn't appear wet at all, at least the soil falling into the ductwork is dry.

I could maybe see galvanic corrosion where the duct contacts the soil but I find it hard to believe it would corrode through the duct in under a year. But I might be wrong about that.

If I send in a sample, should it be where one of the holes has corroded through or where there are drip marks down the aluminum?

 

[HVAC-Novice]

Any soil has some moisture, it has air pockets (oxygen) and it has minerals (ions). So you have an electrolyte and oxygen. Soil also can be acidic (naturally, or whatever got into the backfill).

The soil that fell in may have dried out from the airflow. And soil moisture content changes with varying conditions. A desert area probably is better than a rainy area.

There is a reason metal water pipes get covered/sleeved before backfilling.

Burying air ducts is a bad idea for many reasons (mold due to condensation etc.). Corrosion is just another.

 

[TugboatEng]

Aluminum is subject to crevice corrosion. To be corrosion resistant it must be clean and exposed.

 

[HVAC-Novice]

Based on what was described, sounds like the designer didn't consider the special needs of underground duct and especially pool conditions. Probably have to rip it all out, re-design (ideally not duct under ground) and install properly.