Corrosion Of Cooling Water System Due To Chlorination

[briand2]

I am responsible for several cooling tower water systems. Our standard procedures, etc require us to chlorinate the systems every six months. We used to get the measured Free Chlorine to 25 mg/l (equivalent to 25 ppm) and circulate for two hours. Recently, some of our colleagues have instead been maintaining 50 mg/l for 1 hour. The disinfection is more or less the same (twice as much free Chlorine, for half the time), but which is preferable with regard to minimising system corrosion? The systems typically contain stainless steel cooling towers, steel pipework and copper heat exchangers. Note; in both cases, we first add inhibited Hydrochloric Acid to bring the pH down to about 6.5, then add the Sodium Hypochlorite solution.

Thanks,

Brian

 

[Metalguy]

You shouldn't have a problem with either method, but there is a lot more involved. *If* this is an open-to-air system it should require much more frequent treatment. Therefore I'll assume it's closed. Since the treamtent is so infrequent, have you considered the use of a bromine/chlorine treatment? Cost more, but it works well at higher pH than Cl alone-even higher than 8.

What really counts for the SS (depending on which ones you have) is the residual *chloride* content of the water, and the avoidance of stagnant/low flow areas.

 

[YLWEA]

Aqueous corrosion of stainless in cooling tower or marine environments is a common subject of papers presented at the annual NACE conferences. You can search for those papers by key words on the NACE web page. I remember one particular paper from several years ago that contended that relatively brief excursions to higher than normal Cl- residuals were suficient to initiate pitting in 300 series stainless and that the pitting continued after return to "normal" chlorine levels. Sorry, but I can't remember the details.

 

[briand2]

Thanks to both of you for your comments.

Metalguy: Don't worry, the six-monthly disinfection is not the only way we have of controlling microbiological activity! We run with about 3-4 mg/l free Bromine all year, plus some "quat" biocide. The six-monthly events are in addition to our daily routines. The system is an open evaporative cooling tower system.

Regards,

Brian