Dear All,
I am a Engineer who is working for ship yard in Sri Lanka. We have under taken to fabricate a steem heating coils for a ship and the fabrication work is being attended. Pipe material is S/S/316 and as a filler mettel we are using S/S 316 L. When inspect the welds I saw there are some oxydizetion taken place. (very thin layer of rust only the weld area) Please help me to know the reason for it.
Thanks,
SYDNEY
Hello Sydney, from the brief description that you have given it is possible that the brushes or other metal preparation tools that were used during the welding of the joints were carbon steel or had been contaminated with carbon steel. Any grinding or sanding wheels, flap wheels, brushes, or other tools that have been used for carbon steel and then used on stainless steel will produce the rust that you have described. Generally the welders/fitters should only use new grinding wheels, flap wheels, or sanding discs for joint preparation. They should also only use new stainless steel brushes that haven't been used for brushing anything other than stainless steel. If the brushes are carbon steel or have been contaminated by use on carbon steel you can possibly see the conditions that you are describing. Do some checking with the welders or others who were present for the welding to try to determine if this is indeed what occurred. Best regards, aevald
aevald gives good advice, I would add that if it's on a ship, then your near the sea most likely. It would not be the first time the sea environment created the condition you've mentioned. A lack of a chromium rich oxide will cause the discoloration you speak of, even in that it takes time to create the layer of chromium oxide.
Remediation is typically easy. Avestas sells multiple chemical solutions for this, as do some others, in short, unless you've got embedded contamination via the wrong grinding disk as aevald has mentioned, then it shouldn't be to much of a problem. I would if I were you review the welding procedures and make sure they are sufficient and being held to.
Sydney, is it inside or outside of the pipe? Also is it truly rust or oxidation from a faulty purge? If you think it is from contaminated brushing/grinding you can have all discs, brushes, etc. color coded so they will only be used for stainless steel. Simply paint the back side of the discs white or color of choice.
Dear All,
Thanks for the quick respond. Now I got the point. It is a very thin layer of rust. We have fabricated the pipes in steel related work shop and we used commen grinders and brushes to polish the weld area to get good appearance and finaly ends up with this kind of situation. Once again thanks all of you for the valuable advices.
SYDNEY
Hello Sydney, from the reply that you just gave here you may have some bigger issues than simply changing the way that you do things in the future. I believe that the conditions that you are seeing on the outside of the pipe may have something to do with some internal issues as well. After your description of the welders and fitters possibly using contaminated grinding wheels and such, you may end up with some internal corrosion issues that could compromise the integrity of the welds themselves. There are many others on the forum here who are much better equipped to address this issue than myself and I would hope that they might do so at this point. I am just raising the red flag, you need to do some additional questioning and searching to determine if this could be an issue that you will need to address further. Best regard, aevald
Hello Syda;
I don't dispute any of the advice provided. Cross contamination from abrasives or carbon steel (or stainless steel) wire brushes used on carbon steel prior to being used on stainless steel is always a concern. However, often overlooked is that the ferrite in the weld deposit can also cause "rust" to appear on the exterior surfaces of the weld once they have been exposed to surface moisture. You may be able to treat the surface of the completed welds with an acid that will etch (remove) the ferrite and prevent "rusting" on the surface. I believe there are citric or nitric acid treatments available commercially to do that.
Aevald also asks the important question, is the oxidation on the pipe ID or OD? If the oxidation is on the ID, insufficient purge (or root shielding) is most likely the culprit. Another concern is whether the pipe ID was properly cleaned before welding. Any materials or contaminates on the ID that are volatized by the heat of welding can be introduced into the liquid weld puddle. The end result can be undesirable.
I do have one other concern you may like to consider, that is one of allowable stress. You stated that the base metal, i.e., pipe is type 316 alloy while the filler metal used is 316L. Please note that the allowable stress at high temperatures is lower for the low carbon grades of austenitic stainless steels. You may want to consult with your construction code to see if the operating temperature of the system will be high enough that you should be using 316 filler metal rather than 316L so that the weld will have the same allowable stress "rating" as the pipe.
Best regards - Al
Interesting point regarding the temperature. Isn't 316Ti a better choice for high temperatures than 316L?
My remarks were restricted to the use of 316 or 316L.
This application appears to be for steam (high temperature) and I assume in a chloride free environment where stress corrosion cracking due to sensitization isn't an issue.
There's a lot of details that we were not provided with, so the answer is limited to the information available.
Best regards - Al
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