Pitting corrosion in oil and gas 2

[MaxCarb]

Good morning to all (Italian time).
I would like to talk about pitting corrosion in oil and gas applications.
Taking in consideration 316SS and a wet stream with an important amount of salinity (dissolved chlorides),
from my academic studies I know that without oxigen, and the relevant cathodic reaction, pitting corrosion doesn't take place.
these are my questions:
1. On a material selection report, is it correct to state that, also in presence of high quantity of chlorides, pitting corrosion doesn't take place?
2. In oil and gas application, which are, if any, other species (except oxigen) that can generate a cathodic reaction in order to have pitting corrosion?

Thanks in advance for your answers

 

[moltenmetal]

You can definitely initiate pitting in the absence of oxygen, but you do need another cathodic reaction to take up the liberated electrons for it to proceed. Fe+3 is often a culprit if already present in the stream and if solvated (i.e. if water is acidic enough), but there are others.

http://corrosion-doctors.org/Electrochemistry-of-Corrosion/Cathodic-processes.htm

 

[EdStainless]

As MM said, any oxidizer will do the trick.
And even then the amount of oxygen that it takes to for these reactions is very low.
Using 316 in this environment is a fools errand.

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Plymouth Tube

 

[SJones]

1. No - there will always be some varying degree of likelihood of pitting. Maybe the question should be framed in terms of 'stable, propagating pitting'
2. Hydrogen ions generated by dissolved acid gases

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.

 

[MaxCarb]

Thanks all for your replies.
Could I apply the same considerations to the chlorides stress corrosion cracking?
I guess yes!

Thanks and regards

 

[weldstan]

Yes. As Ed stated the use of 316 in your service is a "fools' errand."

 

[moltenmetal]

Let's not go overboard. Austenitic stainless steels are used successfully in all sorts of services where there's a theoretical risk of pitting or stress corrosion cracking, and have been for a long time. You don't need to jump onto duplexes the second there's a little chloride, especially in a deoxygenated environment. It's a judgment call, requiring a lot more information than you're giving us.

 

[EdStainless]

Let's see;
I move from 316L to a lean duplex
1. Better pitting resistance
2. significantly better CSCC resistance
3. Higher strength, so I use thinner walls and less metal
4. Lower overall cost (if I take advantage of the higher strength

Not moving to a lean duplex from 316 is bad decision.

Unless you are operating over 600F or under -40, or need to do severe forming I don't see any reason to use 316L.

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Plymouth Tube

 

[Guest102023]

Agree with the advice against 316, but you haven't given us the makeup of the fluid or (most importantly) the temperature. Never assume there is no oxygen present; as mentioned it doesn't take much. Lean duplex is a nice step up from 316 in pitting resistance; the next step up is 2205.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"