Problems in Polyethlene-backed tape coating in gas pipe lines
Problems in Polyethlene-backed tape coating in gas pipe lines
(OP)
A vital factor to consider is coating type which has a profound effect on SCC(Stress corrossion cracking) in gas pipe lines. This is especially true when the coating has a tendency to disbond or forms a holidays. This is true for tape coatings, such as polyethylene-backed tapes used predominantly in early 1960s to 1980s.” Tenting occurs between the pipe surface and the tape along the ridge created by longitudinal, spiral, and girth welds. When the tape disbond from the pipe, moisture can accumulate beneath the tape surface. The tape itself has fairly high electrical insulation properties, thus preventing cathodic current from reaching entrapped moisture beneath the tape at the pipe surface. The cracks tend to occur at or near the toe of the seam weld where stress is concentrated and water has access, as well as where the coating has been damaged or disbonded.
Most of the gas pipe lines we are having got tape coating method and constructed in 1970. We are having lot of CP issues.W'll be having our Pigging run in couple of weeks time.What are the remidial action i should take in terms of improving the CP in this sort of conditions.
Most of the gas pipe lines we are having got tape coating method and constructed in 1970. We are having lot of CP issues.W'll be having our Pigging run in couple of weeks time.What are the remidial action i should take in terms of improving the CP in this sort of conditions.





RE: Problems in Polyethlene-backed tape coating in gas pipe lines
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdoweb/
RE: Problems in Polyethlene-backed tape coating in gas pipe lines
More specifically to your CP question, improve CP to meet NACE criteria as above, but shielding will still occur in places leaving pipe susceptible to corrosion/SCC, the only thing you can do is recoat the pipe with a better coating to ensure no shielding. Meeting CP criteria will not necessarily mitigate SCC/corrosion with this coating.
I don't know what stress level this line operates, but this also plays a large role in terms of SCC susceptibility, perhaps your line does not operate at stress levels conducive to SCC?
RE: Problems in Polyethlene-backed tape coating in gas pipe lines
I take it that the cracking experienced is predominantly along the longitudinal weld seam in the HAZ under the bridged location. This was always a weak spot when using the shop tape wrapped systems and there is a hard spot along the HAZ more susceptible to SCC. The HAZ is also most susceptible to developing fatigue cracks during rail shipment.
Is SCC occuring at specific temperature or soil conditions? SCC was most often assocaited with piping downstream from compressor stations. If so you may need to dig up the involved pipe and recoat if condition warrants.
My testing programs always showed poorest cathodic disbonding with the plyethylene cold applied, tape wrapped coatings.
RE: Problems in Polyethlene-backed tape coating in gas pipe lines