Pitting Corrosion On Inconel Overlay

[ASL92]

Gents, I want to ask about the cause of pitting corrosion test (G48). Our product is inconel manually overlaid on API X65QS. the specimen passed on the chemcial composition test as client specification (Fe is 8%, Cr is about 20% and Ni is about 59%). As our experience, these kind of result shows no issue for pitting corrosion although the Fe content is above 5%. The microstructure also doesn't shows any intermetallic phase.

Is there any other potentially cause of pitting corrosion than chemical composition for overlay products?

thx all.

 

[metengr]

Are you sure the Inconel overlay was free of porosity? Did you perform a liquid penetrant test prior to the G48 test? How did you qualify the corrosion resistant overlay?

 

[weldstan]

No Molybdenum? You can rest assured that a straight E/ERNiCr-X weld overlay will have a hard time passing.

 

[EdStainless]

I would suggest that you lightly grind a sample just to get it flat and then do a macro etch on it.
You would be looking for:
1. inclusions (usually oxides or slag)
2. porosity (see above comment)
3. significant variations in staining indicating significant amounts of segregation, that is local variations in chemistry (perhaps due to non-uniform feed or dilution)

Was this done in two passes? If not then it is hard to believe that you don't have carbides and other secondary phases.

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

Please clarify: when you say "cause of pitting corrosion test," do you mean 'cause of pitting corrosion test failure?' If yes, please elaborate upon the test conditions and the acceptance criteria.

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.

 

[ASL92]

thank you for the great response. We just perform LPT on the surface as-weld and it's free from porosity (no indication at all). But, i think lab doesn't do any LPT prior to G48 test.

Mo is about 8.13%. We do three passes actually. My first thought is the heat input and the interpass temperature is too high. But after refers to the chemical result, it shows nothing issue actually.

I think Ed was right, about local variations in chemistry. Due to manual overlay, it's difficult to reach uniform results.

Thx all for your great answers!

 

[samsam1973]

Please check the sample prep for G48 corrosion test. If the diluted zone on the bottom of the weld is not removed completely you are to find pitting on the bottom side. Since it is manual overlay you may have to make the samples thinner to reflect undiluted chemistry on top and bottom. Also samples from pipe could be tricky. In a similar case, i had to reduce the width of the G48 samples to get accurate results.