Corrosion of steel adjacent to cooling tower 1

[huiaeko]

I am new to this forum, so forgive me if my question has already been answered.

Are there any corrosion effects to structural steel (A36) by biocides, algaecides, or chemicals normally used in air conditioning cooling towers?

 

[Metalguy]

Water and oxygen will obviously corrode steel if it isn't protected somehow. Is your CS painted/coated? The normal very low levels of biocides, etc. generally do not cause increased corrosion rates by themselves, but they can if concentration via evap. occurs.

 

[huiaeko]

Thanks for the response. The steel is coated with an penetrating epoxy primer/sealer, aluminized coating, then top-coated. Coating system has failed mainly in the area of the cooling tower.

 

[Metalguy]

I think that's a typical problem for such an application. Whatever salts are in the water are concentrated by evap., and you end up with a marine environment.

I'd try to get an established coating vendor/applicator to take a look and see if they'll clean and recoat--and give you a reasonable gaurantee period.

 

[kenvlach]

Metalguy is correct: any dissolved salts (ionic solids) will accumulate and create problems by absorbing moisture and causing galvanic/pitting corrosion. And, non-ionic solids can contribute to microbial attack.

Comments & questions:
1) What is the age of the coating?
2) What is your climate, T range & sunlight exposure?
3) Rain? sometimes rain-washed areas last much longer than where deposits accumulate. If so, washing helps.
4) What was the initial surface prep? Mill scale? Blasted to white metal and promptly coated? Zinc-coated grit is available if cannot coat promptly and prevents flash rusting.
5) Alternatively, treating the blasted metal with a dilute phosphoric acid-type wash primer is a good pretreatment. See

http://www.finishing.com/198/33.html

for references to MIL specs. that you can download from ASSIST:

http://assist2.daps.dla.mil/quicksearch/

6) Galvanic corrosion can occur between mill scale and steel, as well as between the constituent ferrite & carbide phases of the steel.
7) ‘Penetrating’ sealer is only meaningful if you have something porous, like concrete.
8) Aluminum paint as applied is only a barrier layer. It doesn’t give galvanic protection unless in direct contact with steel. You are better off with a zinc-rich epoxy primer.
9) Topcoat should be polyurethane, not epoxy, for UV resistance. Of course, must be compatible with primer.
10) Please describe the failure mode in more detail. Are the layers adhering to each other? Is the paint failing in spots & then corrosion spreading beneath?

 

[bwi]

We have seen significant corrosion around cooling towers when the towers had been treated with liquid bleach as the oxidizing biocide. Relatively new piping within approximately 50' of the tower had been corroded on their exterior surfaces, which had been painted when initially installed. Sodium hypochlorite (bleach) is cheap but very corrosive, plus higher dosages are required because cooling tower water pH values (8-9). I'd recommend checking with your water treatment provider to see if they are using sodium hydochlorite, and if so, have them switch to a tablet form that contains bromine and chlorine.