Soil corrosivity effect on corrosion beneath disbonded pipeline coatings

[RD76]

I am currently in the process of performing an assessment on an old underground pipe (approx 60 years) which has recently undergone In-Line-Inspection for the first time (High Res MFL).

With no previous inspection data to determine a corrosion rate, I have consulted API RP 581 (section 2.B.12 SOIL SIDE CORROSION) to estimate a corrosion rate and therefore determine which anomolies could reach critical size prior to the next inspection and therefore require repair, and to feed into the repair vs replace decision.

The pipeline is cathodically protected to an off potential more negative than -850 mV vs Cu/CuSO4, however the original coating was a coal tar enamel, and is known to be in poor condition. The on-potentials needed to maintain this level are such that further disbondment is expected. Therefore my main concern is areas where the CP is shielded by disbonded coating.

What i would like to understand is:

- How big a factor does soil corrosivity plays when the corrosion is occurring under disbonded coating?
- Can anyone suggest a factor I should use for those outlined in Table 2.B.12.3 – Soil Resistivity Adjustment of API RP 581 for this disbonded coating scenario
- Is anyone aware of research or guidance on estimating corrosion rates beneath disbonded CP coating

Thanks,

 

[SJones]

It may be helpful to clarify what form of disbondment you believe is present. Is it simply lifting of the coating away from the pipe surface allowing accumulation of permeating water, or is it coating perforation with subsequent lifting creating a crevice between coating and steel? If the former, soil corrosivity is not a direct factor, if the latter then see

S Y Li, et al, Statistical Approach To Corrosion Under Disbonded Coating On Cathodically Protected Line Pipe Steel, Corrosion, Vol 60, (11), pp 1058 - 1071

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.

 

[RD76]

Thanks Steve

I had been considering the former, but I would expect that there are instances of both. Thanks for the link I will obtain and review.

What I was essentially wondering with my original post was: is the local environment caused when corrosion is occurring beneath a disbonded coating with water permeation (i.e. the first scenario mentioned above), where the CP is shielded, more corrosive than would otherwise be obtained for a pipe without cathodic protection, and if so is there any research or guidance published on by how much?

 

[brimmer]

Not directly answering your questions, but for a corrosion rate why don't you use the ILI data? It will be more accurate the a subjective qualitative assessment where you are trying to make assumptions. Take a time weighted average of the corrosion, and assume years free of corrosion (coating in tact for the first years, normally 5 or 10 for asphalt). With high res MFL data, and sufficient correlation data from some excavations, you could also complete a POE to determine re-inspection interval.