Interference on foreign structures

[nimrod123]

Can anyone shed light on the following:

I have recently applied ICCP to an old underground sewage tank, which is adjacent to farm buildings.
The system is basically vertically mounted MMO anodes at tank invert level, and coupons are used for monitoring purposes.

Once the system was installed, the natural potentials were obtained for the following:

1) sewage tank
2) RSJ of farm building
3) Metal fence
4) Lamp post

All of these structures had a natural potential of –450mV to –550mV. Also the water tank is isolated from all other structures due to the pipes (filling, and decant pipe) being plastic.

The system was energised, and calibrated via the coupons to obtain a protected potential of around –1250mV, with an instant off of –950mV

However the problem is this:
The RSJ, metal fence, and lamppost have incurred interference, which is in the form of a potential reading of –320mV. This means that these structures have become slightly more anodic, and theoretically will also be protecting the water tank.

I am certain the sewage tank is isolated from all of these structures

I think this is a case of EMI, as I have installed a 10A 48V switch mode TR, can anyone concur? Or suggest another hypothesis?

Regards
Nimrod123

 

[CathodicGuy]

Have you done a continuity test versus local and remote half-cells?

If there is an anodic (DC discharge) area, there will be a local pickup (cathodic) area. Find these by P/S reads, run a temporaty drainage cable and see if you can mitigate the depressed reads.