We have been asked to inspect a waste water treatment plant which contains above ground glass lined steel tanks which act as the aeration basins. Within the tanks are galvanised steel structures and components, including electric motors. All of the wetted galvanised structures have suffered from severe corrosion i.e. all of the galvanising has been removed in a short timeframe. We checked the influent and there was nothing unusual. The pumping station which feeds the plant is also ok. We wondered if the corrosion is electrolytic. There is contaminated land in close proximity and also buried live cables. We are not aware if there is a cathodic protection system in place.Has anyone had any experience of this?

There will likely be Galvanic corrosion if there are dissimilar metals connected by liquid and you will only need to check all the metals connected against the galvanic table. However when you say you have checked the effluent and found nothing unusual how do you know what is usual or not in relation to the metals within the system if you are not sure about the 'electrolytic corrosion'? Check the metals in the system against the table but also check the effluent for chemicals which can either attack the metals which are corroding or else which accellerate the galvanic corrosion.

We originally had lined concrete tanks and the 'nothing unusual' effluent went through it in next to no time at all. I would suggest that the sacrificial galvanised coating has been sacrificed by electrolytic corrosion which has been accellerated by having the wrong combination of metals and/or by something within the effluent.

Regards.

If a zinc anode (ie a bar of zinc) is bolted to a steel structure that is immersed, then an electrolytic current flows. This current protects the steel from corrosion while the zinc anode corrodes. The zinc anode needs to be replaced at regular intervals.

Thus, as westonpa has said, the galvanizing may be acting as a zinc anode and suffering corrosion.

The corrosion can be prevented by cathodic protection where a negative dc voltage is connected to the structure and the positive to an anode. The anode can be cheap metal that corrodes and is regularly replaced or it can be expensive exotic metal that does not corrode. The structure connected to the negative voltage supply does not corrode in fresh water or sea water provided the voltage is over about 0.8 volts relative to a half cell. If you go this route, you will need to establish that it works in your waste water.

John

Thanks Gents, its pointing towards the disimilar metals in this particular tank because according to photographs the upstream and downstream tanks show no signs of corrosion. I now have the opportunity to focus on some specific areas during my site inspection. I will also look at further analysis of the effluent and consultation with a specialist Galvanising company.

Regards


Geoff

Remembering a refurb I assisted with on an aeration digester, when I was still in the uk. We had to have an impressed current corrosion protection system

Yes I have met a similar situation. While investigating a leak a gunmetal union just crushed in my hand. Later the hot-water storage tank also sprang a leak. I t5hink the other electrode in this corrosion problem is the inside of your copper piping. Change it for plastic.

Ken Green