Rusting 304 SS tank. 2

[Alex4305]

I'm new to this bulletin board but from what I can see already it looks like a great source of technical info.

Well, to my issue. We are having problems with a water tank that we make in house for coffee/tea brewers. The tank is made from 20 GA. 304 SS sheet metal that we stamp, bend and weld in house. The tank has a rectangular shape and we first spot weld the overlapping seams for a secure fit and then we fuse weld the seams to make it water tight.
We use a TIG machine with Argon as shield gas and NO filler metal. The welding is done by a person and not a machine.

The problem is that after a period of time (2 to 4 months)an area around the welds starts to rust and subsequently leak. The area that rusts corresponds to the "hot zone" induced by the welding process. I'm assuming the stainless is loosing it's properties due to the heat induced change in the grain structure of the stainless.

We are getting a lot of returns due to this problem and I would like to find a simple and cost effective solution. I have thought of using food grade silicone at the seams to prevent the "heat zone" from seeing the water in the tank, but I think this will just delay the inevitable rather than prevent the problem from happening.

Has anybody come across a similar problem and been able to succesfully solve it? Any input would be appreciated.

 

[metengr]

Corrosion along the seam weld is indeed the result of heat from welding that has locally changed the corrosion resistance of the 304 ss. Basically, the heat from welding allows the carbon in the stainless steel to combine with chromium and migrate to the metal grain boundaries in the heat affected zone of the weld. Unfortunately, this locally depletes the chromium in the stainless steel and renders it susceptible to corrosion attack.

To minimize the corrosion problem, I would recommend switching to a 304L grade of ss. The L grade is a low carbon version of 304 ss that will have less susceptibility to local corrosion after welding. The lower carbon content results in less chromium depletion. During the welding of the seam, you should take care to avoid heating the surrounding base material of the tank higher than 350 deg F.

 

[deanc]

You rust because you have lost the protective oxide layer.

Do you have salt anywhere in the process? ie:bare hands at prep.

You may try a back purge or trailing gas.

 

[Alex4305]

metengr-switching to 304L sounds like it would be the best solution.

I was reading in another section of the forum about rapid quenching in cold water. If I keep using regular 304SS and I use the rapid quenching technique will it minimize the chromium depletion to the heat affected zone? Just a thought.

Also what would be a good way to ensure the temp of the base material to be less than 350F other than to tell the welder to be quick and to lower the heat setting? How can I determine when it goes past 350F? Are those temperature sensitive markers a good way to determine that?

Thanks for your response.

 

[Alex4305]

deanc-there are bare hands handling the metal prior to welding. I'm assuming a good cleaning would be in order prior to welding. Can you recomend any good cleaning solutions that won't be adversive to the welding process.

Another question, how will purging prevent the carbon from migrating to the heat zone and combine with the chromium?

 

[metengr]

Alex4305;
Considering the amount of welding and the thickness of the material, you would have no real benefit gained by quenching the tank after welding. Instead, you would have to heat the entire tank, after welding is completed, and subject it to a solution heat treatment (heat at 1900-1950 deg F and quench in water), which would add cost and result in potential distortion.

As far as temperature measurement of the base material during welding, you could use temperature indicating crayons or a portable contact pyrometer, if you produce enough of these tanks to justify the cost.

 

[deanc]

I also agree with metengr. A material change would be of great help. However SS still needs special handling.

The salt in a persons hands will cause corrosion. Wear clean cotton gloves. They are good for GTAW welding anyway.

Try acetone for cleaning. Trich. works best but you can't get it anymore. Scotchbrite for the bad areas. Don't use a carbon steel wire brush.

A purge will give you a cleaner finished weld(sugaring) with better mechanicals. A trailing gas may help you while the weld cools.

 

[Alex4305]

Switching to 304L and a keeping the area clear are changes we will implement. Now, do you think that passivating would be necessary even if we change to 304L.

Alex Canale
Manufacturing Engineer
Cecilware Corporation

 

[Rich2001]

Alex,

Based on my personnel experiences of having designed and fabricated many 304L water jacketed appliance type of equipment for the life science industry, I believe that it would behoove you to perform a solvent rinse, to remove salts and oils, followed by a rinse with nitric acid to passivate the 304L stainless steel.

All employees should wore white cotten gloves when they handled the stainless steel in the shop, unles the SS is protected by other means.

The heat input should be minimized. By using a filler ER308L rather than 'no filler' one can lower the heat input.

If you ar concerned about the discoloration of the weld add a training gas purge to the weld until it cools. this prevent a thick layer of oxides to form around the weld area.

 

[arunmrao]

One common shop floor source of contamination is grinding dust, if it is being carried out near the weld shop. Please sand blast the surface and avoid any such contamination. Also post welding are you grinding the weld surfaces,if so please ensure adequate cleanliness. Also ensure that ths wheel has not been used for any mild steel fabrication work.

 

[EdStainless]

304L is a must.
If there is any dicoloration darker than a light yellow tint after welding you must remove it. A pickling paste followed by a very good water wash should work. The heat tint also ties up Cr in the structure and prevents it from helping the corrosion resistance.
A finish passivation is needed also. Any place that steel touched the tank is a possible source of pitting initiation. If you don't want to handle nitric acid you can use phosphoric or citric, it will just take longer.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[mcguire]

Alex
You got a lot of good advice from some real experts and it may be hard to put it in perspective. In order of priority the things you need to do are
1.change to 304L to eliminate sensitization, your primary problem.
2. Remove post-weld oxide if it is darker than yellow color.

You mention no weld integrity problem so you don't need to change any welding preparation or execution parameters.

 

[Alex4305]

Yes, thank you all for the advice given. We will start implementing these new ideas right away.