We have some "rust" at or near welds on SS304 tubing. The rust has formed right next to the weld and adjacent to the weld, both on the tube seam weld and the end plates we weld on. The material is all SS304 and we use SS308 weld wire. What is the easiest way of removing the "rust" in the field (China) and of keeping it from happening in the shop (USA)? Will wire brushing with SS wire brush do it? Is there some chemical tretment that will help? Many thanks!!
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SS304 Corrosion at Welds 16
- Thread starter IFRs
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The parts are seam welded tubing 24 feet long (max) that we weld end plates on to form buoyant pontoons. The tube is 0.050" thick, the end plates are 0.080" thick. We do not fabricate the tubing ourselves but we do weld the end plates on. Both welds have been seen to rust as well as random areas that were scratched during handling. I'm sorry, I don't know what the "C number" is or how to measure it. Is there a way to "de-sensitize" the metal that can be done inexpensively in the field? Would wiping the parts with an acid solution, then neutral water help? Are there cleaning solutions that work?
The C is the amount of carbon in the SS. Without getting too involved here, 304 SS CAN have enough C to leave the areas next to the welds susceptible to corrosion/rusting. IF you have CMTR's (certified mill test reports) on the parts you can determine how much C is in the SS.
Now, the important thing is how fast the weld areas cooled after welding, and the welding method has a major effect on this. The faster the weld cooled the better--unlike "regular" non-SS material. Torch (oxy-fuel) welding cools slow, because so much metal is heated. TIG/GTAW is next, while MIG/GMAW cools fastest. Given your thin parts, and the probablility they were MIG welded, they may have cooled fast enough so that the "rust" is merely cosmetic. If you don't want to wire brush them, you can use a strong solution of nitric acid (HNO3) on the areas. This will actually make the parts more resistant to future rusting. Just be careful and rinse it off after ~30 minutes. Make sure the acid is strong--not diluted much at all.
The other rust areas are most likely caused by contact with "regular" steel of some sort. They can be brushed with a SS brush or acid cleaned also.
Now, the important thing is how fast the weld areas cooled after welding, and the welding method has a major effect on this. The faster the weld cooled the better--unlike "regular" non-SS material. Torch (oxy-fuel) welding cools slow, because so much metal is heated. TIG/GTAW is next, while MIG/GMAW cools fastest. Given your thin parts, and the probablility they were MIG welded, they may have cooled fast enough so that the "rust" is merely cosmetic. If you don't want to wire brush them, you can use a strong solution of nitric acid (HNO3) on the areas. This will actually make the parts more resistant to future rusting. Just be careful and rinse it off after ~30 minutes. Make sure the acid is strong--not diluted much at all.
The other rust areas are most likely caused by contact with "regular" steel of some sort. They can be brushed with a SS brush or acid cleaned also.
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Metalguy - we probably have material certs. I will check on the C content if possible. What should I look for? Within the "spec" for SS304 there is enough variation in C content to affect the post-weld oxidation tendency? Should I change the material? If so, to what - 304L? We use the MIG process, I'm not sure of the longitudinal weld. Would a change in shielding gas help? I'll try the acid wash but in the field this will be messy and potentially hazardous. Is there an environmentally better choice? Please explain why wire brushing is not good. Many thanks and happy 4th!!
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You're probably better off using 304L or 316L. 316L is a little better if there are chlorides around-like from an ocean, etc. Without knowing more about the environment that's all I can suggest right now.
The max. C of regular 304/316 is 0.08%, while the L grades are ~.035 max. That small amount can make a huge difference during welding. Without getting too involved, the temp. the weld area must travel thru during cooling is very important around 1,200 deg. F. Above and below this temp zone, (~300 deg. wide) the cooling rate doesn't make much difference. If the weld area cools down thru there in a few seconds, then reg. 304/316 will be OK. As the C drops the critical time gets longer-several minutes if the C is below ~ 0.04%.
Brushing with a SS brush that has not been used on regular steels will remove surface tarnish, but if the weld zone (HAZ) has been sensitized (cooled too slowly) it won't resist future corrosion very well. If that is the case, it probably isn't practical to do anything about it in the field. The long. seam welds MAY have been heat-treated afterwards. Such a HT involves short times, but very high temps.-~1900 deg F (yellow hot). Then the parts are quenched in water-cold is preferred.
The max. C of regular 304/316 is 0.08%, while the L grades are ~.035 max. That small amount can make a huge difference during welding. Without getting too involved, the temp. the weld area must travel thru during cooling is very important around 1,200 deg. F. Above and below this temp zone, (~300 deg. wide) the cooling rate doesn't make much difference. If the weld area cools down thru there in a few seconds, then reg. 304/316 will be OK. As the C drops the critical time gets longer-several minutes if the C is below ~ 0.04%.
Brushing with a SS brush that has not been used on regular steels will remove surface tarnish, but if the weld zone (HAZ) has been sensitized (cooled too slowly) it won't resist future corrosion very well. If that is the case, it probably isn't practical to do anything about it in the field. The long. seam welds MAY have been heat-treated afterwards. Such a HT involves short times, but very high temps.-~1900 deg F (yellow hot). Then the parts are quenched in water-cold is preferred.
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We always use SS brushes - we are primarily an aluminum shop. The tubes are capped on both ends, pressure tested and sealed. They are used for buoyancy. Then they are assembled into a raft inside gasoline storage tanks where they rest on the liquid surface for years.
In the now sensitized weld areas, will the nitric acid treatment restore the chromium oxide layer and thence the corrosion resistance?
In the now sensitized weld areas, will the nitric acid treatment restore the chromium oxide layer and thence the corrosion resistance?
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Either a HNO3 or Citric acid bath will passivate the tubes.
I prefer the HNO3 despite its' hazards just from better results over the years.
Make sure the parts are cleaned/degreased prior to passivating.
20% HNO3 @ 50°/60°C for 30 minutes.
10% Citric Acid @ 66°C for 30 minutes
I prefer the HNO3 despite its' hazards just from better results over the years.
Make sure the parts are cleaned/degreased prior to passivating.
20% HNO3 @ 50°/60°C for 30 minutes.
10% Citric Acid @ 66°C for 30 minutes
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If your carbon is too high, such as above 0.04% or 0.05%, then the cooling from weld temperatures will cause regions near the weld to have low effective chromium contents. No amount of brushing or pickling will correct this. You must solution anneal and quench. Always require a low carbon grade to avoid this problem with welded tubing and tube sheets.
To encounter this problem, which is so easily avoided, makes me wonder if there isn't another factor. When you reply with your carbon level, tell us the sulfur level, too.
To encounter this problem, which is so easily avoided, makes me wonder if there isn't another factor. When you reply with your carbon level, tell us the sulfur level, too.
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This problem may be easily avoided but we were ignorant of it until now. Then again, we had not had this happen before, so maybe this batch of material was higher in Carbon than others. At this point we have product in China that needs remediation and a pile of raw material on hand for orders to be delivered this summer. I will get an analysis of the material soonest.
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The rate of the formation of the chromium carbides depends on the levels of other elements besides carbon. Nitrogen, nickel, managanese, silicon, and chromium, itself, can alter the rate.
After the fact the solution to sensitized material has to be heat treating to redissolve chromium carbides. Maybe someone out there has a technique they can recommend.
After the fact the solution to sensitized material has to be heat treating to redissolve chromium carbides. Maybe someone out there has a technique they can recommend.
I think Quench annealing this fabricated tubing is out of the question. There are terrible logistics problems to overcome.
i would suggest try a commercial H3PO4 cleaner/conditioner at full strength or very slightly diluted with distilled water, rubbed on with a Scotch Brite Pad, allowed to stand a few minutes then rinsed off.
If possible I would pickle it with the Citric Acid bath. This will require some sort of tank with a heater. HNO3 pickling could be done cold but I think too dangerous to personnel to handle and to disposed of.
If pickling is out of the question I would allow to dry thoroughly and coat it with the Shelia Shine or a rub with Semichrome polish to protect it until it gets in the tank.
I also would contact Cortec about an inhibitor to protect the overseas shipment.
i would suggest try a commercial H3PO4 cleaner/conditioner at full strength or very slightly diluted with distilled water, rubbed on with a Scotch Brite Pad, allowed to stand a few minutes then rinsed off.
If possible I would pickle it with the Citric Acid bath. This will require some sort of tank with a heater. HNO3 pickling could be done cold but I think too dangerous to personnel to handle and to disposed of.
If pickling is out of the question I would allow to dry thoroughly and coat it with the Shelia Shine or a rub with Semichrome polish to protect it until it gets in the tank.
I also would contact Cortec about an inhibitor to protect the overseas shipment.
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You should realize that cleaning off existing rust will do NOTHING to solve the underlying problem. The corrosion will recur whenever the structure isn't given unusual protection from the environment it is supposed to withstand.
It is much cheaper to own up to the fact that the structure is defective now, then it will be later when the customer sees what you have given himm. I would love to be his lawyer then.
Sensitization isn't a superficial problem to be treated with cleaners. This material is no longer stainless. If it can't be solution annealed, it is defective and shouldn't be used.
It is much cheaper to own up to the fact that the structure is defective now, then it will be later when the customer sees what you have given himm. I would love to be his lawyer then.
Sensitization isn't a superficial problem to be treated with cleaners. This material is no longer stainless. If it can't be solution annealed, it is defective and shouldn't be used.
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mcguire - are you saying that this is not just a surface phenomenon? Bear in mind that these parts are used for buoyancy for a lightweight raft inside a closed steel storage tank. They bear little live or dead loads. I was under the impression that the rust was a result of the chromium oxide layer being disturbed and in the heat affected zone a very thin layer now being chromium deficient. This chromium deficient layer now does not oxidize and become "stainless". Are you saying that removing the layer by wire brushing and passivating the surface with an acid bath and/or a phosphorizing solution will not prevent future rusting?
IFRs
I'm still working under the assumption that you have 304, not 304L, and therefore these weld HAF's may be sensitized. If that is incorrect, and you are merely dealing with heat tint, then it is superficial and can be removed as Unclesyd says and no lasting harm has been done.
If it's high carbon 304 and it is sensitized(it would look the same in both cases), then you have a non-superficial problem.
We can't tell you anything for certain without seeing the mill chemical analysis certification.
I'm still working under the assumption that you have 304, not 304L, and therefore these weld HAF's may be sensitized. If that is incorrect, and you are merely dealing with heat tint, then it is superficial and can be removed as Unclesyd says and no lasting harm has been done.
If it's high carbon 304 and it is sensitized(it would look the same in both cases), then you have a non-superficial problem.
We can't tell you anything for certain without seeing the mill chemical analysis certification.
To add to all this, we have to determine a few more things.
1. Is the "rust" formation merely a cosmetic concern, or are you concerned with deep pitting/corrosion that could go all the way thru?
2. Does the rust occur during transit and/or in the field?
3. Is water or water vapor present during use/layup?
4. Can you have a met. lab. perform a simple A-262 Part A test for sensitization? If you're real lucky, and your chem. isn't too bad, you may not have sensit. SS after all--because of how thin your parts are and the MIG welding process.
We all need more info from you!
1. Is the "rust" formation merely a cosmetic concern, or are you concerned with deep pitting/corrosion that could go all the way thru?
2. Does the rust occur during transit and/or in the field?
3. Is water or water vapor present during use/layup?
4. Can you have a met. lab. perform a simple A-262 Part A test for sensitization? If you're real lucky, and your chem. isn't too bad, you may not have sensit. SS after all--because of how thin your parts are and the MIG welding process.
We all need more info from you!
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