MIC prevention in stainless

[ld25]

I have a cooling water loop that feeds a high temp furnace. 95% of the system is 316/304 SS including the furnace. What type of chemicals should I add to prevent MIC on the welds. The chlorides are about 2-7 ppm. Also, does Sodium Molybdate have any corrosion benifite for stainless cooling water systems?

Thanks

 

[unclesyd]

You have asked the $64 question.
I has been our experience that Molybdates aren't effective in preventing MIC on Austenitic SS.
Other than Chromates which will completely mitigate MIC, the only thing that has been effective is a low Chlorine dosage coupled with any other compatible treatment of the water being used. If MIC is established all bets are off.
This is based on our cooling water systems (5) used a giant pilot plants by the big three in the water treat business.

 

[bigc1304]

depending on the temps involved, a granular carbon bed may be used to trap a portion of the chlorine to keep levels in
check. The bed will have to be replaced or recharged at some point. I suggest you analyze for chlorine/chlorides
before and after the carbon bed and replace when effluent levels begin to rise above acceptable values.

 

[Metalguy]

Id25,
Do you have any idea about which bacteria are present in your water? Some species require different control methods.

 

[EdStainless]

oxidizing biocides are the ticket. I am not to fond of hypochlorates. The last thing that you need is more Cl in a closed system. Ozone is an option, as are some other additives. Your chloride levels aren't a concern, but biofouling could really mess your system up.
Talk to the water treatment guys.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[EMKWR]

Depending on the process ozone (can be operated in combination with UV) or low chlorine will do the job. It is however also important to look at the process circumstances which cause MIC. First of all there is the temperature. Unfortunately, this cannot be controlled since it's a process parameter of your cooling system. Cooling water systems usually have the right temperature for microbiology. The second one is flow. Microbes preferably settle in places with low flow speeds. Prevent your water system being stagnant. Third there is "food". Low TOC-levels (do you use RO-water in the cooling system ?) mean little biological growth. Finally there is oxygen. If there is dissolved oxygen, the anaerobic micro-organisms which cause MIC will not grow. Beware however of anaerobic spots in your system. If you have a completely closed cooling system, it probably will be completely anaerobic.

If biocides are used, it might be interesting to think about shock treatment. A shot of chlorine once a week has more effect than continuous dosage and is less expensive. However, do not use this in a system which already has a lot of biofouling, a lot of rubbish will detach from the metal surface and will enter your cooling system. There are interesting tools available to measure both active chlorine and biological activity on-line such as the Biotox of CESI. These can be used for process control when using bio-shock treatment.

Edwin Muller
KW2 Burau Veritas