How to resolve white rust issues inside ductwork?

[PagoMitch]

I am helping a client commission some medium sized (34,000 CFM) chilled water AHU's that were installed several years ago, but never started. This is in a hospital, on an island with lots of rain and high humidity.
In cutting into a large (10 ft x 3ft) vertical SA plenum in order to get to some control dampers (long story), I discovered the duct interior is maybe 80% covered by what I believe is white rust.
It ...appears that the galvanized sheets were wet while being stored, corrosion occurred, and then they were cut up and formed and installed. In a few areas, iron oxide rust has started to form.
Note that in a hospital, internal duct lining/insulation is not allowed. Over time, internal lining breaks down, becomes friable, and (typically) black liner specs end up in the airstream, deposited on all horizontal spaces within the occupied space. Ironically, these AHUs and downstream ductwork were intended to replace the systems that had those exact issues.

Most of the sites I can find detailing solutions are aimed at cooling tower basins, inside heat exchangers, etc.

Anybody know the best/easiest/cheapest way to deal with this?
1. Ignore it? I would think it will only get worse, and then (I assume) start flaking off into the airstream. As this is in a hospital, that would not be a good thing.
2. Vinegar wash? I have seen this mentioned on several sites, but the acidic process is intended to REMOVE the zinc, then apply "neutralizing chemicals", which would then require re-galvanizing?
3. Wire brush and the apply an epoxy or spray galvanizing coating on top of the existing deteriorating surfaces?
4. Some other type of...PVC-based but Flame and smoke rated interior covering for the deteriorating galvanizing?
5. ???

TIA.

 

[Heaviside1925]

1. Ignore it? I would think it will only get worse, and then (I assume) start flaking off into the airstream. As this is in a hospital, that would not be a good thing.
I've had projects where this occurred, new and in retrofits. None of them were a hospital setting, so this thinking may be incorrect for such an occupancy. The white rust was brush off (non metal brush), wiped down and put it into service, if there were no serious surface deformations observed. The thinking being that the surface would only see conditioned air in the future, therefore the initial environment that caused the corrosion would not be present. Unless this is for outdoor air makeup, I would assume these ducts will just be seeing conditioned air, so I doubt it would get worse. Will this affect the life expectancy? likely some but consider they were allowed to sit for several years as well. Unless the owner or AHJ is requiring remediation, I would not see any additional driver other than cosmetic. I'm fairly sure that ASHRAE and SMACNA do not offer any further recommendations other than cleaning.

2. Vinegar wash? I have seen this mentioned on several sites, but the acidic process is intended to REMOVE the zinc, then apply "neutralizing chemicals", which would then require re-galvanizing? I'd be concerned an acid wash making the situation worse.

3. Wire brush and the apply an epoxy or spray galvanizing coating on top of the existing deteriorating surfaces? Possibly but performing this work in a functioning hospital may add a lot of complications and unless you are able to remove the smaller ducts, I don't see how you remediate these in situ.
4. Some other type of...PVC-based but Flame and smoke rated interior covering for the deteriorating galvanizing? I am not convinced you would see future deterioration. Whether it is white or red rust, consider that these oxides layers provide a level of protection themselves unless an actual environment conducive for corrosion exists.

 

[PagoMitch]

Thanks Heaviside1925 -
These ducts indeed just see conditioned air - hopefully 55F@ < 55% RH. My client is the facility, which in this part of the world, is also the AHJ. Yes, it's weird.

The vinegar wash/scrub would indeed be a mess. I also do not know of any spray galvanizing that is suitable for inside ductwork; I usually use this stuff on welds on the hitch under my car...

The (really) good news is that this only appears to be on this Plenum ductwork. The connecting plenum above appears nice and shiny; which leads me to believe this occurred prior to installation.

I wish I had a better feel for "worsening". On one hand, we have HEPA (MERV 14) filters directly upstream, and due to the high price of power, this ductwork is designed at about 1000 FPM to minimize air DP. So there is not much velocity to contend with. On the other hand, these units are 30% OA, and that outside air is drawn through a wall mounted louver, at 17 feet above sea level and 0.25 miles from the wave crashing shore. I lived/worked there for several years, and the salt laden wind-blown air will deposit a thin layer of salt on a freshly washed car (enough to clearly write your name in with your finger) in about 8 hours.

Funny story: When I was there, I replaced a pair of old Cooling Towers with A BAC galvanized model, with a 30 HP motor turning a centrifugal fan. These (we had 2) run 24/7. After about 4 years, one of the staff found a 12" rusty piece of metal on the sidewalk near the cooling towers. WTH? Put it aside. A couple days later another one. Hmm. I finally went over to look. Found another piece of 12" length steel in a small angle configuration. Turned out the cooling tower fan wheel was throwing the blades into the air, and landing on the surrounding grass and sidewalk. Had to switch to a snow-cone type fiberglass cooling tower with a low velocity prop fan. Bottom line - corrosion there is the worst I have seen anywhere.

 

[Heaviside1925]

I understand your concern with future corrosion, especially with a marine environment. My counter argument would be, how much more time are you truly adding to life expectancy of the ductwork?
As far as remediation, I can't recall ever field applying a coating to the interior of ductwork in this use type. You bring up valid concerns about flame and smoke spread depending on the coating type. I'd also be concerned with getting the required surface prep. SMACNA may have some guidance, but I would expect it involve a factory application in a controlled environment with prescribed thicknesses. Maybe my concerns are unwarranted and it's a simple wire brush and clean, then prime and paint but since I haven't done it, I'm just expressing the concerns I would have.
I appreciate your cooling tower story. Had a funny visual of people picking these funny looking pieces off the ground then looking around for the source.
I was assisting a client trying to convince the parent company that the site cooling tower needed to be replaced since it was in risk of collapse. We did our due diligence, taking measurements and elevations, structural evaluation, reports...etc. The parent company was still not convinced but asked for a picture be taken of a level placed on the cooling tower to "show them" that it was really in risk of collapse. We did and the parent company said they couldn't see the level bubble in the picture, our response...exactly!! Our client received the required funds after that.

 

[PagoMitch]

Good story! Amazing what you have to do sometimes to convince folks you know your job...

I talked with another guy at our firm who retired a couple years ago; he had not seen anything like this either. He had a better idea for a liner though - literally cut to fit and screw on another coat of galvanized steel. Problem solved; hopefully something with a bona fide US standards G90 pedigree. That may have been the problem here; some suppliers on island have been ...allegedly suspected of substituting NZ or AU or Asia materials for similar US Specified products. While many items are similar, some are just... slightly different. Like the sizes between US sourced and NZ sourced block. Close... but the wall will look funny at the joint.

 

[3DDave]

You might investigate laser cleaning, which will require use of fume extraction during the process but should remove the surface contamination and leave the underlying metal oxide free. If nothing else getting a demo will be interesting.

 

[HVAC-Novice]

Don't add acid or other corrosives... that just makes it worse.

Don't plan on treatment that relies on proper preparation and application. Whoever has to climb into that duct and perform the work, will do a half-ass job most the time to get out of there. I know I would. And without proper preparation and treatment by the book, smearing any material on may make things worse and will flake off. Try t be realistic here. Real humans have to do that, not a text book.

Anything you plan to apply, consider smoke/fire properties.

How and when would any work performed? Can the duct be turned off for days or weeks this work will take? The answer to this may exclude most options.

How do you provide 55°F air that has 55% RH? That doesn't make sense unless there is some special drying equipment.

 

[Heaviside1925]

HVAC-Novice What are you getting at? 55F @ 55%RH seem reasonable with a CW AHU.

 

[HVAC-Novice]

What is the discharge air temperature of that unit? If it is 55D dry bulb, and the air going in was humid, RH is 100%. RH is temperature dependent. RH=RELATIVE humidity. It will be 55% once it heated up to over 70F.
Check our psychrometric charts to analyze cooling and heating of air.

 

[Heaviside1925]

HVAC-Novice, Consider CW ~40F DB, CW reheat 55F - 55F 55%RH. It is typical to have reheat as part of a CW AHU for the exact reason you are noting. Some even have a second stage reheat. PagoMitch's past posting indicates a competent MEP, so I am taking face value when noting

55F@ < 55% RH

. Not sure that's at issue here.

 

[BronYrAur]

Air leaving a cooling coil is close to 100% RH (unless it is a dry climate). There seems to be confusion about that in some of these posts. You most likely do not have a reheat coil in the air handler and probably have them out in the ductwork (e.g. VAV boxes). So the air leaving the AHU is very humid.

 

[PagoMitch]

All,

First, my bad on the LAT conditions. This project has several AHU's, and the 55DB/55%RH were from memory. The unit in question has a 51DB/51WB 100%RH LAT, while the adjacent OR AHU has a 58DB/50WB/55% LAT. 38F CHW temp and Heat Pipes in the OR AHU - pretty cool tech. There is also reheat in the downstream Terminal Units - although at this point in time the HHW system has not been completed... Sigh. Thanks for the callout.

Back to the issue... The good news - at least for this problem - is that the units have yet to be started; this is what I am trying to get done. they are trying to get this system up by the end of Sep; but I don't think that is realistic.

At this point I am leaning towards either:
1) Sand off the obviously iron oxide rust in the few areas they occur and recoat with spray galvanizing - assuming I can find something listed for use inside a duct; then hope the problem never gets worse, or
2) Require the Contractor to line the problem ducts with new sheets of G90 galvanized duct, literally screwed to the interior walls of the deteriorating ductwork. This would be the best, long term fix, without having to worry about the existing conditions getting worse.

 

[HVAC-Novice]

" adjacent OR AHU has a 58DB/50WB/55% LAT. "

Unless you are in really dry climate, there has to be a re-heat coil in that AHU to raise the DB from 50°F to 58°F (that would be a LOT of fan heat added if you get an 8°F rise)). I would verify those data and make sure what is measured actually is correct.

Instead of RH, it is better to measure and use dew-point temperature since that directly relates to actual water content.

I don't know if an RH of much less than 100% would be helpful here. I assume it would limit corrosion some what compared to 100%. But probably not something you want to rely on. maybe the reduction in corrosion rate would be too small to measure.

Have you talked to a contractor what they think is feasible? Like your idea to add sheet metal, is that actually constructible here?

I doubt there is a good "spray on galvanizing". There is a reason everyone does the more complex hot-dip or galvanizing. I know there is spray paint that claims to be galvanizing, but it isn't actually for the long term.

 

[PagoMitch]

Finally found what looks like an authoritative source of white rust info - from the grain silo guys!

https://scafcograin.com/wp-content/uploads/2019/07/19-7-White-Rust.pdf

An interesting read. It makes a lot more sense than a couple Contracttor sites I found with advice to basically "leave it alone, it will heal itself".

Mine appear to be in the start of the red iron rust. I intend to call them Monday for their opinion re: if the white rust will get worse, and if so, will it flake off in the airstream. It is supposed to be MERV 14 filtered air leaving this AHU.