Cathodic Pretection Criteria for burried pipelines 1

[09091960]

The criteria for the protection of a buried ferrous structure is to maintain a potential on all parts of the structure equal to, or more negative than, -850 mV with respect to a saturated copper/copper sulfate reference electrode.Further we limit the off potentials to -1150 mV to avoid the hydrogen evolution which will have direct impact on the coating of pipelines. As our pipeline is over 40 years old and with tape coating we are having few issues relates to maintaining the right potentials. Some areas of the pipeline we have to go as high as -1450mV On potentials to get the adequate off potentials to protect the pipeline. As the On potential is not considered a valid indicator of hydrogen evolution, is there a limit to ON potential? Any stress on the coating due to the On potential as well? Your feed back is higly appreciated.

 

[21121956]

Hello everybody:

Here are my two cents, suggesting to you to read these documents:

http://www.bushman.cc/pdf/corrosion_theory.pdf

and

http://www.usbr.gov/power/data/fist/fist2_8/vol2-8.pdf

For the second one, the Chapter 8.

El que no puede andar, se sienta.

 

[SJones]

Try researching 'pipeline coating rehabilitation' and 'pipeline external corrosion direct assessment, ECDA'

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

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

 

[09091960]

Hi Guys,
Happy new year to all.

The articles which I read so far are not directly addressing my point relates to the "ON" potentials. Can someone forward me the details directly adressing my point?
Many thanks in advance.

 

[brimmer]

Good question, it is the off potential that has been historically used as a measure of protection (or overprotection in this case), as it will be the current that actually reaching the pipe. -1450 mV is not that high for an on potential. The -1150 mV would be the max for the off potential (we use -1200 mV for example). The one thing I would mention is using potentials is not necesarily the best way to measure overprotection, mearly a convinience as it is easy to obtain, the current density would be perhaps more useful. If If you are a member of NACE or plugged into them, Task Group 285 is investigating cathodic overprotection and may have produced some papers or literature. As far as coatings and overprotection, we know about cathodic disbondment tests in the lab and how high potentials seem to accelarate coating disbondment, but in the field tests completed, tests show that this did not seem to be the case, there was no correlation between CP levels and coating disbondment. I beleive it was Worthingham that did a bunch of tests related to this. As far as CP damaging 40 year old coatings, it probably isn't going to do more damage than what is already done to a 40 year old coating. I would be more worried about excessive levels causing hard spots or HIC depending on your environment/steel properties. I don't know if there are any literature that addresses maximum on potentials and overprotection, just some stuff around off potentials and current densities.

 

[brimmer]

If you are worreid about overprotection and want to limit on potentials, I guess you could go to a different criteria rather than -850 off, you could use the 100 mV shift criteria. I don't know waht the regs/company rules you have, but this is described in NACE literature.

 

[09091960]

Thanks "brimmer" for the above two posts.We have already decided to follow the 100mV criteria for this pipeline. Having said that for certain areas(near compressor stations,foreign crossings etc) we are unable to use 100mV criteria. For my interest I will search into NACE reserch papers relates to this topic. Many thanks