Black rust in WWTP

[LittleWheels]

Quite a lot of tanks and other infrastructure in wastewater treatment plants are built from reinforced concrete. I have found indications of macrocell corrosion in both bioreactors and final settling tanks/clarifiers in three WWTPs over the last couple of years.

Accepted wisdom is that macrocell corrosion usually requires high chloride levels and that WWTP concrete below water level is pretty much immune from reinforcement corrosion. Frankly, I'm not totally confident about my reasoning at one WWTP (possibility of high chlorides though) and I can't explain the observed corrosion at the other two WWTPs.

Is this behaviour widespread and has somebody studied this in-depth before? Any documentation specific to WWTPs out there?

 

[kclim]

Can you take your corrosion products to an SEM and have them analysed (EDS)? That may reveal more info on the corrosion process.

 

[LittleWheels]

Yes and I plan to do so, along with intrusive investigations.

Several experienced materials and corrosion engineers are reasonably sure that it is macrocell corrosion but can't explain why it is occurring. It is difficult to believe that somebody else hasn't already studied and written about the situation I'm seeing. So far, the tally is 6 tanks in 3 WWTP over a 1000 mile range.