Cathodic Protection - 100mV Shift 1

[varri79]

Dear Gentlemen,
I am new for this forum.
In impressed current CP, what is the significance of 100mV shift?
I knew that reference potential will be measured after immediate switching of Anode current.
Advanced thanks...

 

[CathodicGuy]

In some cases, the 850mV criterion cannot or should not be used (that's a whole 'nother story).

The 100mV shift is usually measured as the arithmetic difference between the instant off and the static potential (if you have that data). You could also measure the decay in potential from the instant off over a period of time (e.g. 4 hours for steel in concrete).

It's not practical in areas w/stray DC ground voltages and cannot be interpreted properly unless you simultaneously interrupt all DC current sources.

 

[varri79]

Thank you.
pl. explain about which areas 850 mV criterion cannot/should not be used.

 

[CathodicGuy]

The 850 mV criterion is applicable to coated ferrous structures. It would be inappropriate to use it, for example, on bare steel or iron pipe, where complete polarization is unlikely and it would be an overly conservative criterion. IR drops in the soil must be considered when measuring the data, and this is probably its biggest weakness. Also, it is not valid when connected to metals that are more anodic (Al or ZN), but can be used when connected to copper.