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SS304 Corrosion at Welds
15

SS304 Corrosion at Welds

SS304 Corrosion at Welds

(OP)
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!!

RE: SS304 Corrosion at Welds

3
Yes, wire brushing will remove it, but it may "re-rust" because it is "sensitized" from welding. If you can post the thicknesses and C levels of the parts, we can assist you more.

RE: SS304 Corrosion at Welds

(OP)
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?

RE: SS304 Corrosion at Welds

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.

RE: SS304 Corrosion at Welds

(OP)
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!!

RE: SS304 Corrosion at Welds

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.

RE: SS304 Corrosion at Welds

Citric acid can be used (hot) and it's much safer than nitric, but it may take longer.  BTW, what runs inside the piping?  If it's water, does it flow constantly or can it sit stagnat for days at a time?

RE: SS304 Corrosion at Welds

Often times, some shops will use carbon steel brushes on stainless welds. You may want to start there as well....

The APIGUY....

RE: SS304 Corrosion at Welds

(OP)
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?

RE: SS304 Corrosion at Welds

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

RE: SS304 Corrosion at Welds

4
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.

RE: SS304 Corrosion at Welds

(OP)
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.

RE: SS304 Corrosion at Welds

(OP)
Metalguy - where can I read in depth about this phenomenon to understand it and be best able to deal with our present situation?

RE: SS304 Corrosion at Welds

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.

RE: SS304 Corrosion at Welds

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.
 
http://www.sheilashineinc.com/index.htm

http://www.winterbrookfarm.com/restore.htm

www.cortec.com

RE: SS304 Corrosion at Welds

(OP)
unclesyd - Sounds like I should:
1) Clean with an acid (nitric, citric or phosphatic)
2) Rinse and dry
3) Apply a cleaner or polisher
Questions:
1) Do I have the process correct?
2) Will it be necessary to wire brush to remove the rust?

RE: SS304 Corrosion at Welds

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.

RE: SS304 Corrosion at Welds

(OP)
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?

RE: SS304 Corrosion at Welds

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.

RE: SS304 Corrosion at Welds

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!

RE: SS304 Corrosion at Welds

IFRs, I would guess that sensitization is not the issue, not given the light gage of material involved.
It is more likely that the superficial rusting is from oxidation during welding.  You need to check part cleaning prior to weld and shield gas.
A pickling paste or acid bath will fix this.
Try this site http://www.oakite.com/
You are trying to remove any surface oxides, the Cr depeated layer below them and any surface imbedded Fe from handling.  And then leave a pasivated surface afterwards.

My big concern is that you haven't seen this before.  Have you not looked, or is there something different with the materials/process on this job?

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: SS304 Corrosion at Welds

(OP)
EdStainless - many thanks for your comments.  I found the material certs.  The affected material was C: 0.074%, Mn: 1.44%, P: 0.23%, Si: 0.35%, Cr: 18.31%, Ni: 8.20% and S: 0.011%.  It is possible that in the past we were unknowingly getting dual certified 304/304L.  On the other hand, this was an overseas shipment and the container may have had iron dust in it from a previous shipment, the door seals may have been wide open, the container may have been more subject to salt spray, etc.  I will check our MIG welding process to see if we changed any parameters ( gas, travel speed, wire alloy, voltage, etc ).  Our part cleaning prior to weld is wire brushing and wipe with an alcohol dampened rag.  I will check our welding gas mixture when the shop opens tomorrow.

RE: SS304 Corrosion at Welds

Watch using Alcohol, as it sits open it absorbs a lot of water and leaves it on the surface.
I think that you are on the right track though.  The shippemnt of these gave much more chance for this to happen.
You still need to come up with a real solution though.
Proper cleaning and a rust preventitive would be cheap insurance.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: SS304 Corrosion at Welds

  With 0.074%C you cannot rule out sensitization. It occurs in as little as 100 seconds at this composition, depending on nitrogen level.
  The sulfur level is also troubling. It, too, will lead to premature corrosion pits in the base metal adjacent to the weld where melting, but not mixing occur.
  There will quite possibly be a corrosion perforation of these parts.

RE: SS304 Corrosion at Welds

If you follow the procedure that I posted and use a 3M Pad,  available at any hardware or paint store and apply the H3PO4  with that allow to sit a few minutes the rinse then protect with either of the materials I mentioned you should be good to go. Shelia Shine is less work
I would use the H3PO4 as outlined due to being less hazardous.  
You can experiment with the H3PO4 and you might be able to spray it on with a plastic garden sprayer and maybe with a light rub be in good shape to rinse.  

RE: SS304 Corrosion at Welds

Syd, isn't the polish that truckers use on Al wheels phosphoric acid?  I agree, that would work nicely.

The S cuts both ways.  The material is much easier to weld with the S, it increases weld fluidity.  But if you overheat the welds it can lead to serious pitting issues.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: SS304 Corrosion at Welds

(OP)
More info:
We prepare to weld by wire brushing with SS brush and clean the parts with an alcohol soaked rag, then dry with a dry-to-damp rag.  The wire brushes are not new - they are used mostly for preparing aluminum for welding but never for preparing carbon steel for welding.  We MIG weld using 0.030 SS-308-L wire and gas that is 90% He, 7.5% Ar, 2.5% CO2.  After the weld is completed we wire brush then fill the tube with shop air to 20 psi and do a bubble test using soapy water ( 7:1 Ivory liquid ).  After testing, all but 5 psi is let out of the tube and it is held for shipment either in the shop or outside, stacked on other SS pontoons.

Pickling is pretty much out of the question.  I'm thinking that we should:
1) Find a substitute for alcohol
2) Use new wire brushes
3) Find a substitute for soapy water
4) After pressure testing, wipe on H3PO4, let sit for 5 minutes, wipe off with water, dry with rags then apply a polish.

Questions:
1) Where do I get H3PO4 ( what is it ? )
2) What is the best polish
3) Is a phosphoric based whell brightener a good final polish?

RE: SS304 Corrosion at Welds

Please don't use any liquid soap product.  You know how they are a gel, do you know how they control the thickness of the product?  THEY ADD SALT!! As the soap dries it becomes more concentrated.  In a high humidity environment the chlorides in the residue will eat through 304 or 316.

H3PO4 is phosphoric acid.  I would suggest that you contact a company that makes paste for cleaning after welding.

Yes, use clean brushes.  You can get all sorts of weld porosity and cracking issues from low melting metals in welds.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: SS304 Corrosion at Welds

3
I agree with mcguire. You cannot rule out sensitization. You probably have a serious problem and if you don't test for it you are sticking your head in the sand.
There are two factors that contribute. One is the material composition. 0.074% C is 50%~85% above the maximium.
The other is the shielding gas used. Helium is not the preferred shielding gas for stainless steel (great for ally) because more heat is generated in the arc plasma than argon based shielding gas. This translates into a hotter than neccessary weld.
In an earlier post it was mentioned the rusting problem has not previously been noticed. The fact that this time rust has been noticed and it is confirmed the specs are for 304 not 304L should send the alarm bells ringing.

RE: SS304 Corrosion at Welds

(OP)
rnd2 -
We never expeced to have 304L as we never ordered it - now or in the past.  We will in the future.  What shielding gas mixture would you recommend?  We had not had the rusting problem before but I looked at material certs for previous orders and the C% was the same.

RE: SS304 Corrosion at Welds

(OP)
Here is our plan:
1) Investigate welding gas mixture without He
2) Use new wire brushes or those only for SS
3) Find an alternative to soap
4) Find an alternative to alcohol
5) Dry the parts after welding
6) After drying, with a Scotch Brite Pad, rub Shelia Shine, Semichrome or some other Phosphate based cleaner / polisher
7) Wait 2-3 minutes then rinse off the polish with water and dry.  Or leave on if the polish directions allow.
8) Look for ways to reduce the heat input to the weld
9) Get a sample of the welded material tested to A-262 Part A test for sensitization
10) Change to SS304L when possible
 
Am I on the right track?

RE: SS304 Corrosion at Welds

There you go.
What is the best shield gas for MIG welding 304 SS?
Thread725-96369

RE: SS304 Corrosion at Welds

Your gas is fine.
GMAW is not too particular.
It would cost less to use 90%Ar-10%He.
If you go to no-He your torch will run hotter and your welds will be wider (less penetration).
You can still use Alcohol to clean.  But use small containers and keep them closed as much as possible.  Your guys doing weld prep should have to get fresh alcohol a few times per shift.  That will minimize moisture.
Steps 2,3,6-10 are the way to go.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: SS304 Corrosion at Welds

This is an extract from Ed Craigs 'WeldReality' that unclesyd posted in Thread725-96369 He sure hates gas salesmen!!
Gas Fact. Argon 3% CO2.
Range 2 - 4% CO2. (SG-AC-3)
Against the forces of aggressive gas salesmanship, fifteen years ago while working with AGA , I developed this gas mix for MIG stainless,

For the last 15 years I have been advising companies to use argon with 3% CO2. Instead of argon oxygen for stainless spray applications,
the argon 3 CO2 mix results in cleaner spray or pulsed welds with less weld porosity potential.

For the same period of time I have been advising people to forget the common, more costly 90 helium - 7.5 argon - 2.5 CO2 tri mix, and instead use the more effective argon 3% CO2 mix for those "thin gage" stainless short circuit applications.

In contrast to the costly helium tri mix recommended by all the major gas companies , there are six short circuit benefits attained from using the argon - 3 CO2 mix which provides lower weld energy with lower weld voltages; When used for "low carbon" stainless applications, the carbon content in the weld will be acceptable with this gas mix for both short circuit and spray applications.

In contrast to the more costly, higher energy, helium tri-mix, the argon - 3% CO2 mix when used on thin gage applications can provide;

[1] less part distortion,
[2] less weld burnthrough potential,
[3] less contact tip issues,
[4] improved arc stability,
[5] lower cost gas,
[6] more gas in the cylinders.

In today's North American weld industry in which gas marketing and gas sales strategy often has more in common with Disney Land than it has with MIG arc physics. It's been an uphill struggle to get the practical benefits of my two component CO2 message across. Try these mixes they works, are less costs and you get more gas from the cylinders.


RE: SS304 Corrosion at Welds

The rust at the welds is a result of Chromium depletion on the weld boundaries.  When heated (welded), chromium and carbon forms chromium carbide.  Therefor, there is a depletion of chromium and chromium is a great corrosion retardant.

RE: SS304 Corrosion at Welds

You can prevent the corossion by preventing chromium carbide formation.  How?  Add another element which is a stronger carbide former than chromium.  I believe Mo is one of them.  if you have any other questions or comments, let me know (manousa1@msu.edu).

RE: SS304 Corrosion at Welds

No, TakiM, Mo isn't one of them. They are Ti and Nb, and the addition of a sufficient amount of either makes them into grades 321 and 347 respectively. It's easier and cheaper to use an "L" grade.

RE: SS304 Corrosion at Welds

In one of the above statements rnd2 said,"One is the material composition. 0.074% C is 50%~85% above the maximium." I was under the assumption that the allowable Carbon content of 304 S.S. was 0.08%.  Just wondering if the standard has changed.  One of my current problems is corrosion at the weld joints on 304 S.S. products.  We use 308L welding wire. Is there any known problem using this combination that would create excess carbide?  

RE: SS304 Corrosion at Welds

0.08C is the max in the straight grade material.
The use of 308L will give you nice welds, but do nothing to prevent the formation of grain boundary carbides in the base metal near the welds.
I think that the comment about high carbon was related to using 0.025 or 0.030 as limits to help assure minimal sensitization.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: SS304 Corrosion at Welds

Tubing like IFR mentions is manufactured using HF induction heating of the mating edges to reach forge temperature, then pressed and held toegther as they cool.  This leaves a excess metal on the OD and ID of the tube's weld seam. While this excess metal is still soft (called scraf) it is remove with an scrafing tool of the OD and on the ID if the customer requests it.  This process allows welding with the introduction of an additional metal as in traditional welding.

Since the temperatures are near the melting temperature at just the mating edges, there is a HAZ (heat Affected Zone) about the seam which varies in microscopic structure depending on the cooling methods used and if the tube manufacturer anneals the tube after welding.  The steel goes thru all three phase that we all learned about when we studied steel in our materials class in engineering.

Besides annealing a tubing manufacturer can also metalize the OD with SS Spray metal deposition.  As you can see, you need to buy your tubing from a reliable tube distributor or manufacturer who can give you extactly what you want he they know the exact conditions that you will subject the tubing to.   

Tubing manufacturers do allow the process to drift out of control and get wider HAZ band than normal and get different steel structure by less than optimal cooling.  This ca happen when shift and operators change. The tubing you mention is typically made at a line speed of 200-300 feet/minute.

While and Eddy Current device can detect gross seam defects and Flux Leakage detector is the industry standard because it is calibrated to detect pin holes as small as 1/64".  The line is setup to automatically discard the tubing length if the hole is detected without shutting down the continuous operation of the line.  The tubing is cut to length at +200 FPM with a tolerance of +/- 1/32"



RE: SS304 Corrosion at Welds

jebrown:  you have an in-depth knowledge of tubing manufacture, that's for sure.  But I just re-read IFR's posts again and didn't see the grade of tubing mentioned anywhere, aside from the fact that it is welded seam versu seamless.  In fact I didn't remark the size of the tubing either, though I presume this is relatively large bore stuff due to its use.

A few questions for you:  is the HF induction method you've mentioned the same as the electrical resistance welding (ERW) method?  Are these usually fillerless welds, or is filler metal sometimes used?  Is scrafing (scarfing?) always used, or is the weld sometimes forged into shape?  If the tubing is annealed after welding, does the HAZ and associated problems disappear?

RE: SS304 Corrosion at Welds

(OP)
Our tubing is SS304, 12" OD, 18 gauge wall.
Future orders will be for SS304L.
Does that make a difference?

RE: SS304 Corrosion at Welds

High frequency welding is almost never used for fluid containing 304 pipe or tubing. This process is used for structural stainless pipe or exhaust pipe where perfect weld integrity is not very important. Most good quality stainless pipe is made by TIG or laser, where wall thickness permits. The weld is still scarfed to make it flush with the ID.

TIG welds must be annealed in 304 to minimize ( but not eliminate) the detrimental effect which as-solidified metal, as opposed to the wrought parent metal, has on corrosion resistance. In 316 this effect can be 20C on the critical pitting temperature depending largely on the metal's sulfur level.

Laser welded pipe is the best because the molten zone is miniscule and the resulting quench is so rapid that there is no diminution of corrosion resistance.

High frequency welded carbon steel pipe is common as is the lower quality butt welded.

RE: SS304 Corrosion at Welds

(OP)
Our tubing is seal welded on both ends with formed caps, pressure tested and used for flotation.

RE: SS304 Corrosion at Welds

IFRs,
If you intend to use these flotation devices in marine service, 304 and 304L SS are inappropriate choices. A very short service life must be expected.  You can easily determine if weld HAZ sensitization has occurred, as I and many others in the forum suspect, through metallographic examination, which I urge you to do.  

When welding thin stainless steels, heat is transferred away from the joint through conduction unidirectionally, which promotes slow cooling and subsequent chromium depletion through formation of chromium carbides adjacent to the grain boundaries in the HAZ. Convection cooling due to the gas shielding and purging during welding do not appear to be adequate to compensate. To aid in more rapid cooling, you can try copper or aluminum heat sinks adjacent to the weld as well as back purging with helium (an excellent heat transfer medium).  

Very likely, the only way to salvage the produced items is to paint/coat the welds if sensitization has indeed ocurred.      

RE: SS304 Corrosion at Welds

For superficial rust/iron contamination removal on stainless steels another economical alternative I have been told is to use deck cleaner from a place like Home Depot.  Most deck cleaners contain oxalic acid, which will remove the iron oxides on the surface, but not attack the stainless steel.

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