Welding 304 and corrosion issues

[paintmaker]

I have a question regarding corrosion in welded 304L SS pipe. We installed 3" Sch 10 pipe last year to transport latex emulsion materials through the plant to manufacture latex based paints. The emulsion is nearly neutral pH, usually non-agressive, but due to the water present will rust CS pipe. Zinc can promote some undesireable reactions with the material, so galvanized pipe is not an option and 304 is usually used for this material quite sucessfully.
However, now a year later, we are seeing pin hole leaks at the welds. All joints are butt welded, some TIG some stick, depending upon fittings attached.
A new contractor who is adding more pipe found the leaks and has suggested that the previous work was done with 309 rod, not 308 and is the cause of the leaks. Visual inspection of the exterior of the welds is the basis of the speculation.
Any opinions/guidance on what to do next? Is this a reasonable issue requiring further analysis- ie: should we consider cutting out a section of pipe with weld and sending it to a lab for (what test?)analysis?
Or is it just a bad initial weld job? Pipe is elevated and not readily observable, latex emulsion dries clear, pipes are full all the time, and leaks are sticking to pipe now, no drips, so no idea when this started.
Thanks for you patience and help.

 

[unclesyd]

I would not look to the 309 SS filler metal as being the culprit.

A few questions.

Does your emulsion have any chlorides?

Were any of the welds inspected prior to commissioning?

Can you cut out failed weld for a visual inspection and possibly a picture?

 

[Metalguy]

Sounds like it could be a case of MIC (microbiologically induced/influenced corrosion). The attack is usually worse at the weld metal because welds solidify as ferrite and austenite. The ferrite phase resists oxidizing conditions, whereas the aust. resists reducing conds.

MIC can be either or both.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[metengr]

Absolutely agree with removing a leaking weld with pipe section for metallurgical analysis (send it to a metallurgical lab). This is the only way you will find out the root cause.

 

[paintmaker]

Thanks for the responses.
Chlorides are not usually present in these latex emulsions, so Cl stress corrosion has not been seen in other older equipment (tanks, piping) when inspected.
The suggestions of microbiological activity initiating the corrosion is intersting however. Latex is "bug food" for these critters, and paint is formulated with biocides to control their growth both during processing and in the can. However, water used in process is from the tap.
We have seen this happen in lines that transport the clay slurry- mined materials with minimal bio control additives in water, used as an inexpensive opacifier. A coating forms where flow is lazy, biofilm is produced underneath, and pitting is found when the system is cleaned (usually at or near welds). We spike those materials with more biocide to help control this, but the bugs are relentless and you never get them all. We have not seen this be much of an issue with latex emulsions however, so this may be material dependednt with these specific new materials.
It sounds like sampling and testing is in order. Again, thanks for your help.

 

[EdStainless]

I don't see that swapping 308/309 would matter. All have similar Cr levels and similar pitting resistance. I suspect that the welds were heavily oxidized in the inside. There may also have been crevices and gaps on the inside. It would be easy for pitting to take off under those conditions.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[strider6]

i agree with EdStainless, i think that the problem is that during the welding operation the welder didn't use a backing gas or the quantity was not enough to prevent the oxydation of the materials.
SS materials need always a backing gas to prevent heavy oxydation and heat tint that decrease the corrosion resistance due to the depletion of chromium immediately under the tint.

The corrosion resistance acn be restored by removing the oxyde and chromium depleted layer.

NECESSITY OF REMOVAL OF HEAT TINTS ON STAINLESS STEELS TO
AVOID OR MINIMIZE CORROSION
"During welding of stainless steels, coloured surface oxide films may be developed in case a perfect inert gas protection (such as Argon) is missing.

The presence of heat tints makes stainless steels ulnerable to several types of localized forms of corrosion due to the lack of a coherent passive oxide film. Possible forms of localized attack are:
- Pitting corrosion
- Stress corrosion cracking
- Crevice corrosion
- Microbiologically induced corrosion (MIC)
To avoid these forms of corrosion it is essential that heat tints are properly removed before the stainless steel equipment or piping is exposed to aggressive or aqueous environments."

http://www.vecom.nl/english/documentation/tb/TB-2005-11-eng.pdf

S.

Corrosion Protection & Corrosion Control