Dimethylformamide & Triethylamine

[karlmet]

Basically this is the problem, A separation column has failed in service It is constructed with 316l the plates are 10mm thick, the welding process was stick, the vessel is used to seperate dimethly formamide (DMF) and tri-ethylamine (TEA). I cant give you any info on the welding procedure or post weld treatment The vessel in the last 6 months has had a new bottom reboiler fitted, again the materials is 316L The operating temp is 140 deg c the waste products from the process is formic acid 2-3% and water 12% "Black" products from reboiler tubes X-ray diffraction: NiO, traces Mo and Fe formate Energy dispersive analysis: Ni, Fe, Cr, Cl, S Bottom strip (waste) fluid (various analyses) Chromatography: 2-3% triethylamine, 2-3% formic acid, 10-12% water Ion chromatography: 20-120 ppm chloride; 10-50 ppm sulphate Atomic adsorption: Fe, Cr, Ni all < 5-10 ppm "Brown" products outside column X-ray diffraction: amorphous Energy dispersive analysis: Ni, Fe, Mo, Cl, S, Si, Al, Ca, Mg There has been a leak at the circ weld, (butt weld, back chipped out and re-welded) that connects the new bottom to the old column. What we have internall looks like excessive pitting, (not deep) either side of the weld around the full circumference. In addition to this there is pitting externally at the weld. The column isn't insulated or painted and open to the enviroment This is the first time we have had to deal with 316L so any help with the problem would be greatly appreciated. I have photos I can forward on that show the problems in greater detail.

A couple of questions on this:

Would the chemicals in the input flow be corrosive to the material?
Other than the obvious (chloride) would anything else cause problem?
Is the enviroment oxidision or reducing? I thought both had to occur together?

 

[StoneCold]

bkarlfound
Since no one else chimed in on this I will give it a shot.
Acids and Stainless do not do so well together.
Acids, chlorides and Stainless is probably worse.

If the base metal is holding up then your only problem is really at the weld.
You probably need to pickle the weld and the adjacent heat affected zone. (you can look up pickling here)

If you only have one problem spot you may be able to back grind the weld and then pickle the whole thing.

I am not clear on why you have pitting on the outside of the vessel. Is it really pitting or bad weld?
You probably could have welded it with 317L and been better off but it is too late now.

Please attach the pictures.

Thanks
StoneCold

 

[karlmet]

Thanks Stonecold, It basically looks like it has originated from the welding in the rebolier (possibly due to chloride in the water) and also in the bottom tray (waste) due to formic acid from the dimethyformamide? I will upload pictures soon.

 

[mrfailure]

This analysis really requires some additional lab work to understand what is happening. My first question would be whether the leak started on the OD or ID side. The issue may not have anything to do with chemicals but more mundane environmental exposure on the OD. Cross-sectioning through it would be really helpful. Pictures of the apparent pitting also would be really helpful. I know you said the base metal was 316L, but I would check chemistry to see if this was true and to help determine if sensitization is a possibility.