Inconel 718 and chlorides 1

[JasonLouie]

I have been reading several threads regarding Inconel 718 and chlorides. Sometimes this combination is not recommended and sometimes it is. Would anyone be able to help me clarify why?

A vendor is offering us a 718 spring instead of X750. The process stream is mostly oil with a few % water (the water contains chlorides - mostly sodium chloride, although some magnesium and calcium chlorides are probably present). Operating temperature is 240F. Since 718 has 50% nickel content, and the chlorides should be neutral (not acidic), would 718 be suitable for this service? I do not understand why 718 would be susceptible to chloride SCC.

Thank you very much!

 

[EdStainless]

I have done slow strain rate testing of 718 in high Cl and found a reduction in elongation to failure. There is some acceleration of the cracking by the chlorides.
I think that it has to do with the precipitation hardening mechanism.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[TVP]

As Ed mentioned, grain boundary precipitation causes susceptibility to SCC in this alloy. In general, Alloy 718 is considered to be suitable for this type of service (see references below), but it really depends on the exact thermomechanical process history, the service temperature, the concentration of chlorides, the presence of crevices or any other concentration source, etc.

NASA-STD-P025

http://standards.gsfc.nasa.gov/reviews/xxx-011/01-108.pdf

High Performance Age-Hardenable Nickel Alloys Solve
Problems in Sour Oil and Gas Service

http://www.specialmetals.com/docume... Solve Problems in Sour Oil & Gas Service.pdf

 

[strider6]

The Alloy 718 has a low resistance to crevice corrosion in seawater and chlorides containing waters, respect other nickel alloys and has a relatively low PREN. If you look in the paper cited by TVP of Special Metals you can se a comparison between several Nickel Alloys. In your conditions at 240F i don't know if Alloy 718 is the right solutions.


S.

http://www.corrosionist.com