×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pitting in Duplex?

Pitting in Duplex?

Pitting in Duplex?

(OP)
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.
 

RE: Pitting in Duplex?

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?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Pitting in Duplex?

(OP)
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

RE: Pitting in Duplex?

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.
 

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Pitting in Duplex?

(OP)
Thank you smile

RE: Pitting in Duplex?

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
 

RE: Pitting in Duplex?

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
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources