In the nearly 400 indoor pools we have personally visited and evaluated, one of the more common conditions we have seen is a crusty white substance underneath air ducts. It looks almost like a type of salt, and it appears to come from the inside of the duct, and seep out, collecting at the bottom of the duct. This article explains what it is.
If you are new here, our firm exists for one purpose: to eradicate bad air quality in indoor pools worldwide. Bad air quality is a preventable problem with proper HVACD design, water chemistry and facility maintenance/operation.
Bad air quality is caused by airborne chemicals that off-gas from the pool and wet deck, and these chemicals are generally referred to as chloramines. Chloramines, however, are only three variations of inorganic byproducts (monochloramine, dichloramine and trichloramine). Trichloramine off-gasses into the air, creating the "pool smell", and causing rampant corrosion in natatorium spaces. But it is only one of several hundred variants of harmful disinfection byproducts (DBPs) in the air.
Chloramines in the air contain chlorine atoms in various states, including chlorides. When combined with the warm, moist air in a natatorium, these chemicals then condense on metal surfaces that are colder than the dew point. Mixed with water, sitting on the surface, these chemicals attack and cause corrosion. Worse yet, they stay behind after the water evaporates away. More and more moisture continues to condense, creating a vicious cycle of corrosion. Once the outer layer of the metal is compromised, the damage works its way into the metal and does not stop.
Stainless steel is often specified for indoor pools because it "stains less", and indeed is more robust than galvanized or carbon steel. However, chloramines will still destroy it over time, unless it is submerged in the water (i.e. stainless steel gutters, ladders, main drains, etc.). Look at this stainless steel eyehook that the air duct hangs from. This pool is less than 2 years old:
Mind you, chloramines are heavier than oxygen and tend to stay low in the room. This eye hook is in the ceiling, about 18 feet above the pool. You can imagine how bad the chloramines were toward the floor of the space.
And while stainless steel gets oxidized into chromium oxide and ferrous oxide and hydroxide (rust), not all metals in pools are stainless steel. In fact, most of them are aluminum or galvanized.
Galvanized steel means it is zinc-coated (usually). Zinc is a weaker metal, and a better reductant (meaning it loses electrons more easily than steel). The idea behind galvanized steel is to last longer by allowing the zinc coating to take the brunt of corrosion before the steel itself does.
Using galvanized steel is a very bad idea in an indoor pool environment. Treat chloramines and other corrosive chemicals in the air as inevitable conditions in a natatorium. Metals that are easily corroded (like zinc) have no place in such an environment. They corrode quickly. When they do, they create a "white rust", which is primarily zinc hydroxide.
To make matters worse, galvanized items tend to be screws, bolts, eye hooks, and other important hardware that supports other things. Look at the brackets clamping this steel cable:
Most pool dehumidifier manufacturers are keenly aware of the corrosive nature of indoor pool air. They coat their coils to prevent such corrosion on the coils, and coat as much of the insides of these expensive machines as possible.
First, let's look at the comparison of a non-coated evaporator coil next to a coated coil. Both systems are approximately seven years old.
As you can see on the left, uncoated coils do not fare well against the constant chloramine-laden air attacking it. Whereas the coated coils on the right look unaffected. you will notice some rust in the cabinet at the bottom left (of the right image), showing it is indeed very corrosive air flowing through there, yet the coatings hold up well to protect the coils themselves.
It would be nice if everything could be coated inside the dehumidifier, but alas, there are components that cannot be. Below are two examples from the same machine. The first shows a pump with a cast iron volute, showing typical rust, ferric hydroxide (Fe(OH)3), and an aluminum component above it that is showing the chalky-white aluminum corrosion product, aluminum hydroxide (Al(OH)3).
And here are brazed copper tubes, which have been coated, and yet the chloramines and other aggressive airborne chemicals are still able to penetrate the coating and create copper chloride hydroxide corrosion:
Manufacturers do the best they can with the technology available. The reality is, indoor pool air is corrosive. Coatings help, but they don't address the root of the problem.
While we almost never see carbon steel duct, we do see carbon steel support structures (I-beams, roof supports, etc.) in natatoriums and pump rooms. These metals easily corrode, even without chloramines. Paints and epoxy coatings can help, but none of them stop corrosion indefinitely. The only product we have seen that truly prevents corrosion is a 2-part spray-on material called EonCoat®.
If you are specifying or installing carbon steel components in the natatorium, pump room or chemical storage room(s), we strongly recommend coating them with EonCoat® before the pool is commissioned.1
Metals corrode quickly in natatoriums because metals tend to be cooler to the touch than other surfaces. Oftentimes, metals are colder than the dew point, causing moisture to condense on them. This moisture, as discussed earlier, is loaded with chloramines, which contain chlorine atoms of different states (like chlorides). Chloramines + moisture = rapid corrosion. These two substances catalyze oxidation of metals.
This is why stainless steel looks like the rust was "sprayed on" in little water droplets. It was how water condensed on the surface:
Now let's shift gears. We began this article talking about various forms of corrosion, including Aluminum Hydroxide, which looks like a white, chalky substance on aluminum. It looks like dried salts.
Now we'll show you what dried salts actually look like. In these photos, you know these salts are not corrosion because they are on PVC pipes and other non-metallic surfaces. When a PVC pipe has a small leak, water may travel through it slowly (but consistently), and evaporate as it comes out, leaving behind its minerals and other dissolved solids. It looks like this:
As you can see, this material clearly originates from within the schedule-80 PVC pipe, not metal. This is not corrosion.
We have been in hundreds of commercial pool pump rooms, and we see this issue in about a third (1/3) of them. It's more common than you might think. And in full disclosure, we have only had one sample collected and tested. It was comprised mostly of salt (sodium chloride) and calcium.
Similar white deposits area also common around chlorine feeders (both liquid chlorine and cal hypo) and muriatic acid feeders. All three of which are rich in chlorides.
So far, we have covered the basics of chloramine corrosion, condensation, and salt/mineral deposits oozing out of pool plumbing. With air ducts, any combination of these variables can cause a white substance to form underneath the duct.
Here are some quick things to think about, especially if you are a facility owner or mechanical engineer reading this. In natatoriums...
Air ducts move moist air, laden with high concentrations of trichloramines and other corrosive chemicals.
Antimicrobial fabric duct is preferred in a natatorium, but the branch ducts to connect it to the dehumidifier need to be rigid. In those sections, we recommend coated or aluminum ducts. Do NOT use stainless or galvanized ducts. 304L Stainless Steel fails rapidly:
Why does this "white rust" appear around the seams of the ducts? Because as the warm, moist air flows through the ducts in the warm natatorium, as it passes through the colder mechanical room (or outside during cold weather), the duct itself may be colder than the dew point, causing condensation inside the duct. That condensate then follows gravity and seeps out of the joints of the ducts, which are not always water-tight.
Air ducts are not often specified to be water-tight, because they are designed to move air...not water. They are not plumbing lines. And when that condensate drips out of the duct, you get instances like the above photo, showing aluminum duct corrosion.
(Airborne chloramines + moisture) + condensation on metals = corrosion. Corrosion differs for each metal, but generally speaking, chloramines and other airborne disinfection byproducts (DBPs) will attack just about all of them.
Coatings do help, as shown in modern day pool dehumidifier coils. But any cut or abrasion of the coating creates a vulnerability area. For carbon steel, EonCoat® works better than any coating we have seen.
If you do see corrosion beginning, it is just a symptom of a much bigger issue, and our firm exists to help you eradicate the real problem. We have personally visited nearly 400 indoor pools, and almost all of them faced frustrating corrosion issues. And with proper chloramine management on both the air and water side of things, these issues can be mitigated almost completely.
1 We only recommend products that we have personal experience with, and have seen work. We are an independent consultant, and do not have any financial incentive for recommending EonCoat or other products.