CATHODIC PROTECTION FOR DUPLEX

[mcisajp]

What are the considerations that justified the use of CATHODIC PROTECTION and /or external paint on a DUPLEX buried gas pipeline? Can I get some hint on that?... Also if this is totally necessary, what if the efficiency of the paint system needed and how much costly is it compare to a C/S pipe one, the CATHODIC protection needed?

Thank for you attention to this matter.

mcijp

 

[gr2vessels]

mcisajp,

In agressive soil, the duplex piping can/should be protected with additional coating. The coating can be paint (suitable for the soil also) or hot metallic (eg. aluminium)coating and topped with an additional silicone coating.
Any coating placed on a pipe can have flaws, like pinhole defects (detectable with spark test), that can generate accelerated localised corrosion points.The Duplex is not totally impervious to corrosion, in fact is quite vulnerable to those pinhole concentrated galvanic corrosion or agressive soil induced stress corrosion cracking. Generally, if you elect to include an additional protection of your expensive duplex pipe by coating/painting it, you'd like to protect it also from agressive pinhole concentrated corrosion, installing adequate type and amount of say, sacrificial anode or other type of cathodic protection, including monolithic joints and spark arresters. However, if you can afford to replace the duplex piping every few years, instead of pipe coating which includes cathodic protection and systematic monitoring/testing of the integrity of your cathodic protection, all the above is not required. I cannot comment on the suitability of C/S piping against your duplex material buried ina perticular soil, but typically the well treated and protected C/S piping should last you decades, compared with the similarly protected duplex which should easily last a couple of hundred of years.

Cheers,

gr2vessels

 

[mcguire]

I find it rather amazing that duplex alloys as a group would be considered vulnerable to corrosion in soil. What soil chemistry and which duplex alloy would such cause this situation?

2205 is cathodically protected for subsea conditions because of crevice corrosion, not pitting. 2507 needs no protection subsea. It's difficult to imagine salinity and temperature conditions as severe underground.

Are you referring to lean duplex such as 2101 or 2001, both of which are as good or better than 316L? 2003 beats 317L. I just can't see these alloys as vulnerable as buried pipelines. Someone set me straight if I'm mistaken.


Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[rconner]

While certainly not claiming expertise in this specific buried material application, I noticed that about 5 years ago Mr. mcguire in a thread on this same forum had made a post I think concerning duplexes and chloride environment to the effect, "Duplex stainless can, like any other stainless alloy, pit if the pH, chloride concentration and temperature are sufficiently adverse. The subsequent risk of SCC is minimal, however, because cracks which form in austenitic grains cannot propagate through surrounding ferrite grains. Hence, the resistance to SCC failure. Martensitic stainless can't come close to the SCC resistance of duplexes, nor can austenitics."
When one uses the term "buried", this general term of course can conceivably be applied to many substantially different specific environments and locations, including varied amounts of and variable exposure to the substances chloride, water, and of course MANY other things! I noticed some of these things are addressed in a paper at

http://www.euro-inox.org/pdf/map/CorrResist_SoilsConcrete_EN.pdf.

While this paper makes the generalized statement in its abstract, "In principle, stainless steels should be in the passive state in soils, but the presence of water and aggressive chemical species such as chloride ions, sulphates and as well as types of bacteria and stray current, can cause localised corrosion.
In such aggressive environments, stainless steel piping buried in soils should therefore be coated...", I think they may go on later in the paper to indicate that some very specific alloys can be used uncoated in some specific buried environments. One aspect I don't see discussed directly in this paper however is potential mechanical effect e.g. of expansive/contracting soils, thermal movements, etc. of various soils/backfills.
While I guess I understand the experience behind the statement, "...well treated and protected C/S piping should last you decades", and I while I realize the perception regarding stainless, I was however just curious as to what specific experience would necessarily lead one to a much superlative conclusion regarding e.g. duplex or other stainless steels that they would last a lot more in buried service or as stated, " ...a couple of hundred of years"?

 

[gr2vessels]

OK, it was a silly exageration the mention of hundreds of years. The intention was to elevate the idea of durability in service of a well protected investment item made of Duplex material and that the additional cathodic protection cannot be excluded in certain circumstances. The mentioned circumstances could be the subject of an other thread. The topic above is about the feasibility of additional protection for the buried Duplex pipe, since many believe that the duplex stainless steel is impervious to any corrosion attack.
Yes, Duplex stainless steel vessels in marine environment are being aluminium sprayed AND epoxy coated to last 20+ years. Some Duplex buried piping can also be coated and cathodically protected, depending on the service requirements, location, difficulty of access for inspection and maintenance, etc. Depends also how much one is prepared to invest and protect his investment.
Cheers,

gr2vessels

 

[kenvlach]

From the paper cited by rconner, some duplex SS will require protection in some soils, e.g., > 0.2 wt% chloride.

mcisajp, a free sample chapter, External Corrosion of Oil and Natural Gas Pipelines is available from the newest ASM Handbook, Vol. 13C. see
thread338-163571

Note: the sample chapter is only a small portion of an entire section entitled 'Corrosion in the Petroleum and Petrochemical Industry' on pages 922-1053. There are other sections on corrosion in marine & underground environments.
The Table of Contents of this volume is available at

http://www.asminternational.org/Tem...isplay.cfm&file=05145_Handbook13C_TOC_WEB.pdf

ASM members: Oct. 31 is the final day to purchase Vol. 13C at the pre-publication member price ($159).