[matlsc]

Deleted the bloody topic by accident!

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?

[discodermolide]

As I said before the triethyammonium formate is probably the problem, especially at the operating temperature.

[matlsc]

Thanks Discodermolide, is this the waste product from the input feed then? also would the chloride not cause problem?

[discodermolide]

Thanks Discodermolide, is this the waste product from the input feed then? also would the chloride not cause problem?

Chloride can also initiate corrosion.
But I would try and remove the formic acid.

[matlsc]

how is it possible to remove just the formic acid?

[discodermolide]

how is it possible to remove just the formic acid?

Why don't you wash the feed with water then the formate salts will remain in the aqueous as well as the chloride and other metal salts.