Pitting corrosion on piping 1

[mariolucas75]

Dear Forum,
What would be your thoughts on following deterioration (photo attached).
Piping is 2" (Wall thickness ~5mm) 316 SS in Marine environment for a long time under ~ 200 bars

 

[mariolucas75]

Dye-penetrant showed this type of indications, thus not knowing how deep they might me ... the piping is 10m long what would be the suggestion ? Dye-pen inspect the whole 10 m line ?

 

[EdStainless]

Yes, Pen the whole thing.
How did you clean it?
Internal or external?
You are going to have to cut out a piece and section it to see what you really have.
If this is typical Cl pitting, then it is likely much larger below the surface.

 

[TugboatEng]

If this is what you see after a very long time you have done very well. 316 can tolerate a marine environment provided many conditions are met. Did this pitting show up after a shutdown or layup?

 

[mariolucas75]

Th

If this is what you see after a very long time you have done very well. 316 can tolerate a marine environment provided many conditions are met. Did this pitting show up after a shutdown or layup?

Dear EdStainless / TugboatEng it is external surface of a pipe clean with a solvent (it was having stains on it). This line is in service for 10 years or so probably there were some shutdowns ....
Is that question related to susceptibility of SS develop cracks in intermittent service ?

 

[mariolucas75]

Does it look only as Chloride pitting or their elongated nature may dictate they are driven by stresses (pipe under 200 bar) thus it turns to be a Stress Corrosion Cracking?

 

[EdStainless]

CSCC would have propagated and been very clear.
And likely through wall.
The apparent orientation of the pits could be partly related to stress, but more likely related to either manufacturing or the installation.

 

[mariolucas75]

Ed

CSCC would have propagated and been very clear.
And likely through wall.
The apparent orientation of the pits could be partly related to stress, but more likely related to either manufacturing or the installation.

Stainless,
Could you kindly please a little bit in more details what do you mean under "but more likely related to either manufacturing or the installation"?

 

[EdStainless]

My first thought is surface contamination, either in manufacturing, handling, or installation.
This could be anything from dirt on the surface before annealing to surface Fe contamination picked up anywhere in the handling.
There is also the possibility of something resting against the surface while in service.
This would form a crevice and accelerate the corrosion.

 

[Louisiana Inspector]

It looks to be typical Chloride pitting caused by the breakdown of the protective oxide layer when exposed to high chloride concentrations found in seawater. Apply a good coating to mitigate.

 

[EdStainless]

There is no mitigation for pitted 316 in high Cl water.
You could grind the pits out, but I am sure that you be under min allowed wall.
There is no way to treat or coat pits that will stop them.
Aggressive cleaning and a very good coating will slow the pitting down.
But the slightest imperfection will allow pitting to accelerate from where it was.
Replace this with a non-metallic or a better alloy (AL-6XN).

 

[Dhurjati Sen]

Hope the dye penetrant used were chloride free.
As regards to the pits, it could be sensitization / chromium depletion that has taken place very slowly over the years (although the steel has never been used within the sensitization range).

 

[Louisiana Inspector]

There is no mitigation for pitted 316 in high Cl water.

Aggressive cleaning and a very good coating will slow the pitting down.

You lost me on this one.

 

[EdStainless]

You can't stop pitting unless you actually remove all of the effected metal, and I mean all.
The pitting process is autocatalytic, once it starts the conditions in the pit only make it grow faster.
The problem with grinding out the pitted material is that if one pit extends too deep then your material is scrap.
Cleaning is only so-so, even when you have ground the pits open (they are usually much larger below surface).
The very shape of them tends to trap impurities.
Then you try coating over them.
Have you ever seen a totally defect free coating?
And at any defect you will get accelerated crevice corrosion.
Coatings on SS is a bad idea.