We have experienced some problems with corrosion on the inside of stainless steel pipe, specifically at or near the weld affected area. We have taken steps to minimize any corrosion contamination from carbon steel. We are checking the quality of the stainless steel itself but I do not suspect this as the main culprit. How would an insufficient purge affect the stainless steel around the weld area? Would an inadequate purge cause a rust bloom on the inside of the pipe? Thank you for the feedback.
What kind of stainless are you welding and what filler wire?
The pipe is A312-TP316L stainless steel and the filler wire we generally use is Avesta Type ER316LSI.
What r u utilizing the pipe for? What size pipe is it? Yes an insuficient purge can allow carbon and other impurities from the air to tranfer into your inner puddle. U really need to run a purge thru all your passes. Also, make double sure that u don't use a carbon file to dress your fit or a grinding wheel that was used in a carbon application prior to the stainless application. One mistake I made once was using what I thought was a stainless wire wheel when in fact it was a carbon wire wheel. Also, if you are G.T.A.W. Try taking a piece of emmery cloth and quickly cleaning your filler rod. This will remove any surface impuritys. One last thing, be sure that your gloves r clean to prevent transfering potential contaminates from your glove to your piece thru your rod.
p.s. when your weld is done your root should be bright gold to silver in apperance. Hope this helps.
Thanks for the reply. We use different brushes and flapper wheels for carbon steel and stainless steel so that there is no contamination from these sources. We also use separate pipe stands and if there is any question about contamination, the pipe is "insulated" with covering the stand with clean duct tape. The welders do clean their rods before using. The pipe is exhibiting rust before being put into use so the medium is not the concern. I will add that this is not a common situation, it happens just when we seem to get complacent about corrosion contamination control.
Usually stainless steel welds should be pickled/passivated. It is known for general corrosion applications grinding/brushing/blasting is not sufficient to remove the oxidised layer after welding, even if the weld is purged during the welding process.
Further, if possible the last weld should be made at the corrosive side of the weld. If not, example small diameter pipe, a thick root should be made using correct filler metal.
Never forget purging : before you start welding, during and after, until the temperature drops below 250°Celcius.
What kind of corrosion are you experiencing? Is it a generalised corrosion, pitting corrosion or cracking? In addition you say it is at or near the weld affected zone. Are you trying to say that the corrosion is not actually in the weld metal, but adjacent to it in the heat affected zone?
If we knew the answer to these questions, we could give a better answer.
Regards
Niekie Jooste
The corrosion is a generalized covering, generally next to the weld more than on the weld itself. This is not a normal occurrence as we have taken numerous steps to control contamination of the stainless steel, refer to my earlier reply. My feeling is that it is a isolated incident with one particular welder.
This does sound a little puzzling. Here are my comments:
1) If the corrosion was caused by a contamination on the filler materials, then the problem would lie in the weld, as the contaminants would be mixed in with the moulten metal. As your corrosion was not in the weld, this does not appear to be the case.
2) If the corrosion was due to contamination on the base material with residues from carbon steel, then the corrosion would typically be of a pitting type. Seeing as the corrosion was a general corrosion, this does not seem likely. (Just make sure that it is general corrosion and not a very fine dispersion of pitting.)
3) If you had a purging problem, then you should have worse problems on the weld metal than the base metal next to it (Unless you have a very strange configuration at the weld.) because the weld is the hottest part, leading to the greatest oxidation.
4) You say that the corrosion was generally next to the weld. Does this suggest that there was some corrosion seen on the weld metal as well? If the weld metal was really shiny while the base metal next to it was corroding, then it suggests galvanic corrosion. You say that you are using a 316 filler, so this should not be. If it is so that the weld is shiny, I would perform an analysis on the weld. Possibly you used the wrong wire, or the wire was marked incorrectly. (It would have to have a significantly different composition. Not something like a 304, rather a high Ni alloy.) If on the other hand there was some corrosion on the weld as well as in the base metal then I believe that it is probably due to incorrect or inadequate pickling and passivation procedures, or possibly not cleaning the pipe adequately after these treatments.
I hope this helps. - Let us know if you come to a final conclussion.
Regards
Niekie Jooste
P.S.
You seem to be suggesting that one of your welders was at fault due to "sloppy workmanship". What exactly could a welder do to make this type of corrosion happen?
Only purging or prep. work. Both of these appear not to be the problem. The other possibility is a mix-up with filler wire. This is rather a shop control issue.
Regards
Niekie Jooste
My first question: is the pipe in service presently, if so what is it conveying?
In prepping the weld joint, if you used a stainless steel wire brush to descale, what type of stainless steel bristle, i.e. 302ss?
When you say "rust bloom" it sounds like you have general corrosion, not pitting, crevice corrosion, or stress corrosion cracking?
Forward a little more information and we can solve this problem.
Regards, Brad
The pipe where we recently noticed rust was just completed in fabrication. It had not been put into service. We do use stainless steel brushes (I do not know exactly what type of stainless the bristles are made from) and separate flapper wheels than the ones used for carbon steel or any "non-stainless" material.
The corrosion noted is a slight general covering, not a pit or crack. I believe it may have been the result of sloppy work by one of the welders. We instituted a corrosion contamination control system in our shop and generally the results have been good. We also passivate the stainless steel after fabrication with WEDAC.
We have also starting instituting a standard purge system shop wide for consistency between welders. Hopefully these steps will eliminate the problem.
Thanks to everyone answering.
If the welder performed a powered brushing operation with a stainless steel bristle made of 302 or 304 you will get this kind of "rust bloom" just from humid air exposure. If the welder did a hand brushing operation it's almost impossible, to get the same form of contamination.
I'm not familiar with WEDAC, but if it is not a pickling solution or paste it will not remove the contamination left from power brushing with either of the two types of bristles. Nitric Acid by itself will not remove this, you need to have the addition of HydroFlouric acid, i.e. ammonium bi-flouride for the contamination to be removed. I strongly recommend 316L bristles or Scotch-brite abrasive materials for any cleaning of 316L materials, this will solve your problem. Purging until the weld is completed will minimize any crevice corrosion when the pipe is put into service.
Best regards, Brad
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