cathodic protection of steel tube flying leads 1

[NickClark]

Corrosion professionals. For subsea super duplex steel tube bundles (flying leads) we do not consider that connection with the protected host structure provides any cathodic protection apart from the first few meters of tubing. I recognise that cp of pipelines can be achieved over hundreds of metres using equi spaced anodes. Is it possible therefore to similarly protect small bore tubing bundles from each end only over a couple of hundred meters? We are able to coat each tube which would help reduce demand and of course we would only need around 0.5v to ensure protection. I would appreciate opinions on this and any use full texts or papers . Does this work? if not why not? If it does work why is super duplex always used where 316 may be suitable.

Regards

Nick

 

[cloa]

Probably just reliability and maintenance issues for CP- its vulnerable to damage and needs some monitoring and you don't want to rely on CP where it might not have human involvement or its very costly.

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[SJones]

Think about the corrosion resistance of super duplex in seawater (if it got inside) and the yield strength difference to work out why it is preferred over 316L. The distribution of CP current is aided by coating, and kilometres of well coated pipe can be protected from a single anode location; but, would you need it for corrosion protection at the operating temperature of the application? Your biggest concern could be keeping the CP from cracking the SDSS rather than does it throw far enough.

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.

 

[NickClark]

5 degree water temperature and fluid temperature. No internal corrosion issue with 316 and we don't need the tensile strength. The only logical reason for choosing 25Cr was for external seawater resistance. I.e. the CP won't throw. Maybe the first post has it that simply its belt and braces. HISC not an issue really either with small bore tubing. One of the reasons I'm asking is the reported crevice corrosion limit of 20-30 degrees due to biofilm. I'm thinking coated tubes with cp provided from the termination. If the coating fails the cp should prevent crevice corrosion. Am I on shaky ground?

 

[SJones]

At that temperature - no.

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.

 

[NickClark]

Sorry. I'm getting confused between two issues. Then what about at 25-30 degree water temperature?

 

[SJones]

While it is generally agreed that the 22% Cr grades require painting and CP in order to avoid localised corrosion, especially at crevices, the 25% Cr grades do not strictly require CP if the operating temperatures are low, typically less than 45 deg C. However, it is not usually practicable to electrically isolate individual items.....hence components are also usually painted and cathodically protected

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.

 

[NickClark]

As good a spec as it is this is one of those occasions when I'm not prepared to trust eemua. There are too many papers and standards that conflict. The words strictly and typically suggest ( of course) that its not always the case.

Thanks for the replies

Nick

 

[SJones]

But it is only a 5 degree C operating temperature; surely that's typically less than 45, 30 and 20 degrees C? If you are so worried get BEASY, or a similar entity, to perform some CP modelling for you to give you an idea of the potential distribution that may be achieved. Otherwise, you are staring at 625.

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.

 

[NickClark]

Steve sorry I thought I clarified. Water temp is up to 30 degrees c. Note your recommendation thanks

Nick

 

[SJones]

OK Nick. It got a bit difficult to follow what temperature you were specifically dealing with. The Total paper at NACE 2012, Paper 1173, cites one operator in the South China Sea adopting 20 deg C as the critical crevice temperature. Your approach versus EEMUA appears to be more realistic bearing in mind that EEMUA is, essentially, North Sea orientated.

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.

 

[NickClark]

Thanks Steve I know the paper. And sorry for the earlier confusion!