Stainless steel in chloride environment 9

[Meysamsh]

Hello everyone.
I have little problem.
We have a potable storage tank that hase been made by stainless steel 304.
But the concentration of chloride is about 130 ppm. Corrosion is start in near the welding.
That was design problem, and we cannot change the material.
What is the solution? Can we paint the inside the tank? Or can we use the cladding?
Thank a lot..

 

[TugboatEng]

Depending on temperature, that chloride level could be acceptable. The fact that the corrosion is concentrated at the welds indicates that the welds themselves are the problem. Carbide precipitation, contamination, etc... All come to mind. An acid pickling or even an electro-polish may resolve the problems. If either of those fail, treat it as a mild steel tank and sandblast and coat the interior.

 

[EdStainless]

Clean the welds with new abrasive flap wheels, start coarse and work to finer grits.
The use pickling paste to clean all of the areas that you cleaned.
The scrub and wash well to remove all acid.
If your temperature isn't too high and your pH is not acidic then you might make it.
Otherwise TBE has the right answer. Have an inexpensive tank made from steel and have it blasted and coated (with immersion rated coating system like they use inside water towers) in the shop.

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P.E. Metallurgy, consulting work welcomed

 

[moltenmetal]

If it's a small tank, and internal work is not possible, it's not clear how you're detecting the corrosion, so I'm assuming it's a large tank. If so, TBE and EdStainless have given you the best advice.

 

[Meysamsh]

Thanks for your tips.
All of the welds have been tested by RT and they were acceptable.
But now welds have pinholes and we have leakage from those

 

[moltenmetal]

If your tank already has pitting which has gone through the wall, you're in serious trouble. It will not be sufficient to merely make repairs where there is leakage right now, because pits are autocatalytic and once started, they can grow quite fast.

 

[Meysamsh]

Yeah pitting already started.
I suggest first of all, building-up the weld and then use a suitable liner or paint on inside the tank

 

Once pitting has started, corrosion is off to the races. In the very short term your best mitigation is to smooth the surface by progressive grinding and polishing.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

The highest reliability and lowest cost option is to have a steel tank made and coated.
Trying to save a pitted SS tank is a loosing proposition.
It will continue to corrode. You can't clean it well enough unless you actually grind the pits out.
And then you will still miss some small ones.
And whatever you do they will continue to grow.
The welds in SS pit for 2 reasons.
First the structure is bad. It is basically a raw casting with heavy segregation and lots of residual delta ferrite. Both of these lead to very low corrosion resistance.
Secondly the heat tint that is left (even just the slight gold color) sucks more Cr out of the alloy and into an inert oxide layer. This makes the surface under the tint lower in Cr and less corrosion resistant.

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P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Hang on a minute.

130ppm from what I can see would be easily classified as "fresh water".

So either the ppm is or has been at some point a LOT higher or there's something you're not telling us.

There's no way unless you're at some elevated temperature that a stainless steel tank should pit to this extent.

So the questions are:

Did you make this tank? If so how did they weld it?
If you bought it tell the vendor you want a new one or your money back
I'm no welding expert but I do know Stainless steels need careful welding - those welds don't look very good and at the very least have surface carbon steel contamination from a grinding wheel or similar wire brush.

How thick is this tank material?

We don't know how big this tank is, can you get man entry?
You will need to drain it to do repairs which could include weld repairs and possibly internal lining / high build epoxy. "Painting" won't do anything for you.

So you need I think to do a metallurgical examination of the weld to find out what in fact you have there. Only then can you make a sensible judgement as to whether you need to grind out the welds and re-weld it or just patch it up.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Meysamsh]

We make this tank. All welders were qualified. All the tank has been tested by RT.
Water came from the river, and we treated the water, some part of the water go to the DM unit and some part of the water go through the active carbon and come into this tank.
This case have been happened after about 6 months.
The ppm of chloride is 130 ppm and the temp. Is ambient.

 

[EdStainless]

304 SS, welds in a tank in the sun with 130ppm Cl is very likely to pit.
In HX service (say 80C) with good material and no welds and 200ppm will pit every time.

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P.E. Metallurgy, consulting work welcomed

 

[Meysamsh]

Is any reference for this topic?

 

Is any reference for this topic?

Several lifetimes of experience.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[LittleInch]

I wasn't asking if the welders were qualified, I was asking for the welding spec, weld material and type of welding.

Also how thick is this vessel.

It might be RT, but what was the acceptance level, especially of inclusions.

Ed, I will bow to your greater knowledege, but this

https://www.bssa.org.uk/cms/File/SSAS4.92-Stainless Steels in Supply Waste Waters.pdf

seems to say pitting below 200ppm is "rare".

HOWEVER, we don't have a temperature from @Meysamsh. So what is the temperature of the water / tank surface??

Also "and we treated the water,".. Treated it with what or how exactly??

What are the Oxygen levels?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Meysamsh]

@littlench
I said that. Temp. Is ambient. Between 30-50°C.
Based on API 571 clause 4.5.1 all of the s. S. 300 series has been affected from from Cl scc.
Now i dont have the doc. But tommorow i can say to you the thickness.
Diameter of the tank is 12m.
Water analys is attached.
COD=24 ppm
BOD= 12 PPM

 

[EdStainless]

The water isn't very bad, the high pH helps.
If the tank had been made from 316L and the welds shielded enough so that there was no heat tint it would have worked fine.
In fact if these welds had been very small and very clean it would have worked.
The combination of heat tint, weld segregation, and delta ferrite make the welds much less corrosion resistant than the base metal.

I have seen well annealed and pickled 304 handle 500ppm Cl at 85C, but I would never bet money on it working. The slightest crevice or surface defect and it is all over.
500ppm as an upper limit for 304 appeared in a lot of papers in the 70's, but as SS was used in more product forms and applications they came to realize that that level was not realistic.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[0707]

Do not use SS with clorides above 50 ppm.
luis

 

[Meysamsh]

@0707 is it your answer based on the hydro test limits?

 

[0707]

Yes in SS the water for testing should have less than 50ppm. If your water has 130ppm SS wouldn´t be used in my point of view you have to opt for another type of material I think in your operating conditions even a CS material would be better than SS.

luis