304 SS jacketed tank VS. well water 2

[oompaloompa]

Greetings all. I work for a company that manufactures water jacketed holding tanks for 50+ years. These tanks are made from 304 SS and have various fittings for water inlet/outlet. They are non-pressurized, and have a filling elbow near the top that is open to the atmosphere. The water in the jacket is circulated (open loop) and is heated using cartridges. The temperature fluctuates between 88-140 deg F.

I have a customer that has repeated failure of our tanks. He is using well water in the tank jacket. In the first instance, the internal threads of a 1/2" NPT fitting looked like they rusted out, like a galling almost. In the second case, the welded seam where the tank wall meets the bottom plate, failed almost all around the perimeter. The inside of the jacket walls have a thin, brownish film on them.

I'm using pretty standard, off the shelf certified steel. I used steel from a separate supplier and mill the second time around to eliminate the "bad batch theory." The fact that this happened twice in less than six months with this guy leads me to believe it is Iron in his water or the ph level. This water is in compliance with state drinking water guidelines.

What can be causing this?

One engineer here suggested a "sacrificial rod" of Magnese or something that is inserted and replaced periodically to attract contaminants. Any thoughts on this approach?

Thanks in advance,

Michael

 

[jehan17718]

Michael,

I am not a "materials" guy and hence not an expert on this subject but I am presuming this to be a case of a chloride attack on SS 304.

Would you be having data regarding the chloride ion content (typically expressed in ppm) of the circulating water (well water) used in the SS 304 jacket? You mentioned in your post, that it is well water (implying it the same is not treated). Your post did not mention about the Cl ion concentration in the well water.

From a pressure vessels background, I can confirm that our specifications explicitly specify the maximum permissible chloride content for the hydrotest water. This is to ensure that Cl- ions in the hydrotest water does not initiate a chloride attack (pitting corrosion) in the future. If I recollect correctly, 316 has superior pitting corrosion properties than 304.

Regarding the proposal to use sacrificial anodes I am in no position to comment.... as mentioned in the first line of my reply ;-)

Hope this helps.

-jehan

 

[metengr]

The leaks are most probably chloride pitting or stress corrosion cracking associated with the 304 stainless steel, given the service temperature.

For this service condition (water and temperature), your best bet is to upgrade to a duplex stainless steel.

 

[MJCronin]

oompa.....

I agree with the statements above.

When there is a failure of 304/316 SS materials, the fundamental cause in 75 -90% of the cases is chloride pitting.

It is always your primary suspect....

-MJC

 

[unclesyd]

I agree with the above posts concerning chloride induced cracking and possibly pitting. There is another possibility that may enter into the picture, that is MIC (Microbiological Induced Corrosion)which can greatly increase the potential for the chloride induced pitting or cracking. It needs to be determined whether or not MIC is present. If MIC is involved it precludes the use of a lot of materials.

 

[oompaloompa]

There are a couple things going on here. 1. I noticed a brownish film coating inside the jacket walls. After some research, I'm convinced this is evidence of "rouging." 2. The corrosion near the tank bottom is rust-like in appearance and pitted.

The customer is not cooperating with our request for a water sample. I have one public report showing they failed a water alkalinity test during this time. I repaired his first tank once, the pitting continued. We sent them a new second tank, and the pitting attacked that one as well. The cost of these tanks are generally less than the cost of an outside failure analysis test. But obviously, tank replacement under warranty can't go on forever.

The duplex steels are probably the way to go but are cost prohibitive in tanks this size (40gal on avg). The sacrificial anode seems to delay the inevitable, and may not account for every nuance in their water.

I think we are going to recommend a closed loop water circulating system using distilled water. This may be the route necessary for customers with well water.

If you have any further ideas, I'm still open to them.

Thanks,
Michael

 

[waskillywabbit]

"The customer is not cooperating with our request for a water sample. "

That speaks volumes.

I would stop cooperating with fixing their tank under warranty.

Brian

 

[oompaloompa]

Brian - Easier said than done sometimes. Ultimately it will come down to that though.

Can someone direct me to chart showing acceptable chloride ion content at various temperatures? I've researched that certain sodium chloride solutions begin attacking 304 at 131 deg F and are very strong at 140 deg F but I need some basis to compare with what's going on here.

Thanks again!

Michael

 

[waskillywabbit]

http://www.azom.com/details.asp?ArticleID=1177

Brian

 

[metengr]

thread1135-190330
thread307-178690

 

[oompaloompa]

I found the first thread when I did a search before posting.

The second thread I had not seen and is extremely helpful.

Thank you all.

-Michael

 

[HEC]

The sandvic site is also a good reference for stainless steels

http://www2.sandvik.com/sandvik/014...0a4c288c2017a643c12569430020d70e!OpenDocument

Mark Hutton

 

[sreid]

Is there any chance that there is an electric current flowing through the tank? I've seen threads rapidly eaten away through electrolosis (SP). Also, I'm told this can happen at weld beads (also dissimilar metals).