Pipeline cathodic over protected 2

[frank201]

Dear expert .
We work in new construction (12 inch) pipeline 3 km with 3LPE coating.
The method we are used in cathodic protection is Sacrificial anode.
We make 4 Test points .
The potential (CSE) is around -1600 mv ( pipe over protected)
Please advise if there some risk of coating disbanding?
Thanks in advance for your advise

 

[LittleInch]

What sacrificial anodes are you using?

How are you measuring the voltage? What reference cell are you using?

They seem very high / not possible with a sacrificial anode.

Remember - More details = better answers
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[frank201]

Cu/CuSO4 reference electrode

The number Mg anodes is 100 anodes (7kg) spacing approximately 30 metres

thanks

 

[SJones]

Sounds like someone got over enthusiastic with high potential magnesium anodes. Check the pipe coating specification, and the mill applied coating inspection records. The coating quality control testing should have involved cathodic disbonding testing at -1500 mv CSE, and, if lucky, -3500 mV CSE. Check the field joint coating specification, and test records, too. If the coating passed these tests satisfactorily, the risk of disbonding should be low. Your Level 3 CP Senior Technician, or Level 4 CP Specialist, might try a resistor in the test post connection terminal to restrict current output a little.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[LittleInch]

Sounds like someone got the design wrong.

Go back to the CP designer.

Or go and disconnect about 80% of the anodes.

I think 1300 or 1350 is the most you want.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SJones]

Disconnecting the anodes won’t help as the remainder will still have a closed circuit potential of around -1700 mV. Make sure that the pipe to soil potential measurements are being obtained correctly, and use resistors to choke back the anodes a little.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[LittleInch]

A short pipeline with 3LPE and good isolation has very little current demand so no idea how anyone calculated 700 kg of anodes.

Are these attached separately every 30m?

Basically your design sounds like it is massively over specifed and without easy access to each anode or bunch of anodes it will be difficult to adjust.

Normally you put 4 or 6 anodes in the same place and run them through a test post so that you can test and adjust. Sounds like someone more used to offshore pipelines designed this one....


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[frank201]

each 25 anode are branched together in one test point

 

[LittleInch]

So they're only an average of every 30m.

Well that will make it easier to adjust but 25 7kg anodes in one location for a new 12" pipeline is extraordinary. 4 or 5 would be more normal.

The current demand will be very low so you might end up with a high resistance the get the voltage down.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SJones]

Time to get a certified CP specialist on the job

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[alchemon]

Is your -1.6V IR free or not? If it is IR free, then how are you removing it?

 

[frank201]

for sacricficial anode it's difficult to measure the E OFF , i think is better to use corrosion coupon to mesure the E off