Corrosion in a clean water 316L break tank

[BigJay]

Hello All

I was wondering if anyone could help with a corrosion problem that we have encountered in some stainless steel (316L) Break tanks. The tanks are used in a water treatment process and the supply water is essentially potable with free chlorine levels of around 0.7 mg/L and a pH of between 7 and 8 (the water is derived from a surface water supply prior to treatment). The tanks have been in service for a couple of years and a recent inspection has found advanced corrosion (so advanced in places that there are holes in the 3mm plate) The corrosion is only in the air gap at the top of the tanks. A further ispection found a red sludge on the plate surface that was originally believed to be rust until it was wiped off to show a clean surface beneath.
We did not believe that the chlorine levels would be sufficient to cause any issues when we originally installed the tanks.
Does anyone have any ideas of what could have caused this?

Any thoughts would be greatly appreciated

 

[moltenmetal]

Could this be microbially-induced corrosion? Perhaps the free chlorine levels in these tanks isn't as high as in the bulk flow?

 

[EdStainless]

Is this localized at all? Along welds? In areas that were formed? What about right at the wet/dry interface?

The red surface could be iron that has been transprted from other parts of the system.

The tank has an open vent? Any scale or mud in the bottom of it? What is the external environment (indoors, outdoors...)?

Water may be splashing on the upper protion of the tank. As it drys off all of the impurities remain. Do you have a detailed water anaysis? Has it been done in various seasons?

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

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

 

[kenvlach]

The red stuff is rust coming from the pits; 316 stainless is still about 70% Fe.

Probably crevice corrosion under dried-on deposits mentioned by EdStainless. Evaporation from water wicking up the deposits increases impurities, especially chloride, Cl^-, which is higher than free chlorine to begin with.

Suggest testing the deposits for chloride. It's likely that 316L is suitable except for the splash zone, where you can apply a coating. Periodic maintenance is also necessary.

 

[BigJay]

Thanks guys for your advice.

Our investigations are ongoing and your pointers line up with are current assessment.

Ed Stainless

The tank is vented but only with a small 1" vent pipe. The external environment is a kiosk in which the tank is installed. The kiosk is outdoors.
We don't know if it is seasonal as the tanks have been in service for several years. It would appear that the intended maintenance schedule has not been implemented.

Regards