SS304 Corrosion at Welds 16

[IFRs]

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!!

 

[EdStainless]

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

 

[IFRs]

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.

 

[EdStainless]

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

 

[mcguire]

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.

 

[unclesyd]

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.

 

[EdStainless]

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

 

[IFRs]

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?

 

[EdStainless]

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

 

[rnd2]

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.

 

[IFRs]

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.

 

[IFRs]

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?

 

[rnd2]

There you go.
What is the best shield gas for MIG welding 304 SS?
thread725-96369

 

[EdStainless]

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

 

[rnd2]

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.

 

[TakiM]

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.

 

[TakiM]

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).

 

[mcguire]

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.

 

[Indeng64]

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?

 

[EdStainless]

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

 

[jebrown]

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"