Pitting in Duplex? 1

[Nomismum]

I have been given conflicting information on the time that Duplex may be resistant to pitting: one source says 7 days, and the other several months.

My pipes may be exposed to some seawater content for 2 to 3 months (most of the content will be treated with oxygen scavengers, but the distribution cannot be guarenteed).

My valve body is carbon steel and will need CP and so isolation from the CP system would be imposible, ruling out super Duplex.

If the 7 days is correct then I need to find another material solution, but if I can resist pitting for a couple of months then Duplex will be suitable.

 

[EdStainless]

I assume that you are talking about 2205, the generic mid-chemistry duplex.
Pitting by what solution? How high is the Cl? What about biocides?

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Plymouth Tube

 

[Nomismum]

I am assuming Duplex 2205, but this hasn't yet been finalised and can be changed.

Pitting by seawater.

There will be biocides and oxygen scavenger, but the flooding is carried out during installation of the flowline so the distribution is not guarenteed. Also the flowline will opened temporarily subsea at one end which will allow some small untreated water ingress.

I'm not sure about the CI

Thanks

 

[EdStainless]

With good biocides and such you will get as much time as they persist. With no treatment (or when the treatment ages) you will get biological growth and pitting in a couple of weeks.


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Plymouth Tube

 

[Nomismum]

Thank you

 

[strider6]

In seawater you've to consider also crevice corrosion which occour at lower temp than pitting and has higher likelihood in stagnant water.
Pitting is influenced by many factors, like temperature, ..
Usually in seawater the choice is for 2507 which has higher PREN but the final selection depnds also on factors like criticality of the service, and possibility to make maintenance or replacement in case of failure..

"In a very extensive programme different piping system components were tested for 6-12 months in natural and chlorinated (0.5-0.8 ppm) seawater of ambient temperature (31). The programme included several test loops containing flanged pipes, valves, pipe branching and deadlegs. Pumps were tested in separate non-metallic systems. In all tests, components made of 316L and S31803 were attacked by severe crevice corrosion after a short time."
taken from :
Corrosion of Duplex Stainless Steels in Seawater

http://www.outokumpu.com/applications/upload/acom_10292653.pdf?docid=681

This is a guide of outokumpu:

http://www.outokumpu.com/pages/Page____26793.aspx

hoep this help

S

Corrosion Prevention & Corrosion Control

 

[SJones]

I'm not following the logic for a carbon steel valve in a duplex ss subsea system, nor am I following the logic for CP of the carbon steel ruling out the use of super duplex. You will need an oxy scav and a biocide package at a sufficient concentration to cover the intial line fill and account for the top up (which may not be practical at 8 ppm O2 in the raw seawater). Best to talk it through with a chemical supplier and a chemist. Watch out for the dewatering and drying operations and any oxygen ingress.

Steve Jones
Materials & Corrosion Engineer

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