What temperature does stress corrosion cracking happen for stainless? 1

[donnajoe]

At what normal operating temperature would stress corrosion cracking become a concern for typ 304L and 316L stainless steel? What is the standard protection required to prevent stresss corrosion cracking of vessel heads at this temperature?

 

[stanweld]

There have been reported incidents, of which I am aware, of Chloride SCC at temperatures as low as 125 F in ASME Code design applications.

 

[mcguire]

That's a good number, even 120F is possible. To avoid SCC at this temperature you must choose a grade which is immune to pitting in the [Cl], pH, and temp at which you will operate. I suggest a duplex even, 2205. If you want to stay with austenitics, you're looking 130F for 316L and 140F for 317L. You would need a 6Mo grade to be perfectly safe.
You could use a ferritic like 439 or 444, but make sure it is annealed and not over Rb100.

 

[unclesyd]

I have investigated numerous failures from Cl induced SCC that happened at ambient temperature. Along the Gulf Coast (US) I see a lot failed bolts from Cl SCC and might have seen 100̊F Ambient. I've also investigated failures of SCC process vessels and lines from manufacturing areas where the temperature is keep at 72̊F 24/7/365.

I don't like to put a lower temperature limit on the onset of CL induced SCC. I think it gives a false sense of security. Chlorides and Austenitic S/S are not compatible in an industrial enviroment.

Our approach was to try to limit the S/S to exposure by chlorides. 95% of our process vessels (3000) and piping are S/S mainly 304L. We operated S/S from cryogenic to 850̊F and routinely process equipment at 1000̊F. We worked with coatings under insulation. insulation with low chlorides, low pressure steam condensate wash water, and probably the best leave it uninsulated if possible.

 

[mcguire]

Unclesyd
I respect your knowledge and experience. Tell me what alloy experienced SCC at 72F. If it's an austenitic, it must have been sensitized, or maybe it was ferritic?

 

[unclesyd]

As stated almost our entire plant was constructed of 304L, 304, 316, and 321 S/S. 304L was by far the most prevalent material of constructions.
The failure at 72°F was on 3 of 4 1 1/2" Sch 40 316L lines with all welded construction except at one point. The process material had a ph of 3-4 with 3-10 ppm Cl at the time of failure. It was pumped for distance of about 4000' through an air coditioning area with copious air movement. Prior to entering this area it went about 400' through a room that quite a bit cooler. I don’t know the temperature off hand but it was very noticeable 5-10°F. About 100' from the terminus the line changed in elevation and a set of staggered flanges were installed and the lines were stacked instead of running parallel. The top line leaked due gasket failure and dripped on the lower 3 lines. The leak was noted and diverted to catch basin, unknown to our group, for 4 months until the leak was so big it was noticeable. We found all three lines cracked and leaking. Metallurgical examination confirmed SCC originating from the outside, with only a few very shallow pits. The chloride level of the residue around the cracking varied between 40-100 ppm Cl. This was in the mid 80's so the Cl analysis wasn’t what it is today. The lines had never been steamed, only flushed 2 times with Demi-water.

 

[mcguire]

The critical pitting temperature on 316L pipe can be awsomely low because of the high levels of sulfur used in it to keep it easily weldable. This, however, must be the all-time worst. I wouls sure like to hear any other colleagues' experiences.
Congrats on being tipmaster of the week!

 

[stanweld]

unclesyd,
There have been many reported cases of SCC at welds in 304/316 SS while in out door storage, most notably in desert, marine environments like San Onofre, CA and in Saudi Arabia. It was believed that the surface temperature reached greater than 120 F during daytime exposure. Could the welds in your case have been precracked while being stored or installed but not in operation?

 

[unclesyd]

The cracking wasn't in the welds. In only one case was the cracking near > 2" the others were closer to 5". The pipe was fabricated in an adjoining area that is also air conditioned. Once the pipe was exposed to the process it stayed at 72°F as the area is 24/7/365.

In this same building, probably 10°F warmer, we had some 304L S/S vessels, 24" Sch 40 with pipe cap heads, holding Demin-water under 250 # N2 pressure. We lost two vessel heads due to a chilled water leak that dripped on the heads. I don’t have the Cl level of the chilled water. We determined that the failure mode was SCC, but went no further as the system was going to be replaced.

Being on the Gulf of Mexico Coast as mentioned I have seen numerous failures of boat hardware that could have seen intermittent 120°F temperatures, but I've seen others mainly bolting failures that probably saw ambient (95°F) but no more.

On reason we had the data/information is that any type equipment failure, no brain failures, was to be channeled through our group for analysis. No one touched a failure until the Lab was notified and signed off on the remedial action/repair plan. They were a few notable exceptions.

The irony of it all is the work that our group accomplished none of it was published. It was corporate policy not to publish anything considered operational. We had reams of reports, microstructural analysis, physical evidence, etc.

 

[stanweld]

unclesyd,
Thanks for the info!