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.

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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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.

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

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.

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."

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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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.

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.

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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Is any reference for this topic?

Quote (Meysamsh)

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."

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-Stainles... 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.

@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

• https://files.engineering.com/getfile.aspx?folder=eaa20d75-f5b3-4499-bf71-3

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.

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Do not use SS with clorides above 50 ppm.
luis

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

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

There are correct SS alloys for service up to concentrated brine, but you have to careful, and ready to pay the price.
What let this application down is the weld quality. Making fine, smooth, and clean welds in SS is not the same as making sound strong ones. Shielding and pickling are critical.
The other thing is that when you are in environments like this it is wise to over alloy the filler, 317L wire would have made sense.
130ppm Cl with neutral pH, ambient temp, and no biological activity should work for 304L. But we are back to the welds....
The biggest issue with SS isn't the Cl level, it is temp, pH, and biological activity.Two pH more acidic or 10C hotter is roughly the same as 10x the Cl level in impact.

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@Edstainless
You say the building-up the welds, didn't works?

@0707
As i know, we can not use hydro test limits for in service condition. Because we have many tensions in hydro test.

Meysamsh

please clarify I don´t understand your question.

luis

The difficulty is that larger welds will cool slower and have more segregation in them. This lowers their corrosion resistance.
And the shielding needs to be perfect. I watch welders turn out welds where I have to strain to see the ripples and there is zero discoloration on either side. On the backside the weld bead width is less than the material thickness and very flat.
Design plays a role also, no corner welds, no traps or dead spots, and if you can do it no welds in the very bottom.

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I just saw the photograph.

I suspect MIC. By chance, is there little to no flow where the leaks occurred? By chance are the leaks close to, but adjacent to the weld in what we call the HAZ? Did this happen after the tank was newly installed and leak tested using the "fresh" water? Was the water completely drained after the leak test or was the tank allowed to set flooded with water while the piping, pumps, etc. were installed?

The iridescent green color is what makes me suspect Microbiologically Induced Corrosion (MIC).

If you have access to the inside of the tank, look for small mounds of "brown jello" that look like small ant hills along the welded seams. The mounds are excrement from the bacteria that I suspect is the culprit and cause of your problem. You will find that "clean" water isn't all that clean, even if it is chlorinated.

Attached is a photograph of MIC I discovered on one of my failure investigations. Doesn't it look familiar? I will post another photograph to look at.

Best regards - Al

Here's a photograph of the "mound" typical on the wet side of the tank. It is the excrement from the bacteria and microbes causing the leakage.

I could see the mounds of excrement through a few feet of standing "clean" water.

Best regards - Al

• https://files.engineering.com/getfile.aspx?folder=cf20d9f5-31dd-4a0f-8d48-5

I seem to be having a problem uploading a photograph of the "dry" side of a leak caused by MIC, so I'll give it another try.

Best regards - Al

• https://files.engineering.com/getfile.aspx?folder=66a74655-d58e-4a8f-9127-5

The tank involved was a brand new installation that was leak tested by the owner. The tank was allowed to sit with the water in it for about 90-days when they noticed the tanks dripping water. At that point, the plant was still under construction. They wanted to blame the fabricator for the leaks, but it wasn't the fabricator's fault the owner tested it with "dirty" water and allowed it to stagnate.

Best regards - Al

• https://files.engineering.com/getfile.aspx?folder=66a74655-d58e-4a8f-9127-5

Just thought I'd add to this discussion. We have recently experienced issues with MIC on newly installed process pipework in 316 stainless. This was using UV treated towns water that was left in the pipework for only a few weeks during commissioning. Almost all of the corrosion sites were on or around the heat effected zone. We are now having to replace a large quantity of this pipework.

From now on I will be ensuring heat tint oxidation is kept to an absolute minimum and welds are pickled and passivated wherever possible. Also wherever water is to be left stagnant for a prolonged period employ use of a biocide.

Treating the water with sodium nitrite prior to shut down will prevent the issues associated with stagnation. In fact, it will prevent corrosion in all piping systems of all materials.

The problem with water standing stagnant for a few weeks is twofold.
First the biocide is not persistent enough. We deliberately use biocides that decompose so that they do not impact the environment (we can't use the Cr and Mo compounds any more for this reason).
So hyprchlorite or hydrogen peroxide work fine (peroxide is nice because you can't use too much) work, but are not persistent. Hypo will last for a week or so, H2O2 will only last a day or two.
The other issue is that when you have no flow if you do start getting local corrosion you will likely also have local oxygen depletion. Since SS relies on CrO2 for its passive film the low O2 will make re-passivation more difficult.
In SS the problem with bio growth isn't the growth itself, the bacteria does not eat the SS. It is the fact that it is forming a crevice condition which is more severe, and that it will die and release decay products which are often corrosive.

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I'm just curious as I'm more of a general fleet operations guy but is sodium nitrite used for offline systems? It's the corrosion inhibitor in engine cooling systems. At 3000-6000 ppm it seems to prevent nearly all corrosion and microbe growth even in systems that may sit for periods of time. It's a film forming additive so it will not work on dirty surfaces. Disposal may or may not be an issue.

I know that sodium nitrite is used in curing meat, so it does have some biocide effects.
And it is very effective at inhibiting corrosion on carbon steels. I have seen it used for treating freshly sand blasted to prevent rust blush. It was also the primary inhibitor used in cross country gasoline pipelines.
I know that it also has some inhibition effect with SS and Cl solutions, but the effects seem to be limited to a range of pH, temp, and concentrations.

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@gtaw
Those pics were amazing.
It seem we have a same problem. I'll send more photo from inside of the tank. Before and after cleaning.
There are many mounds of brown jello.

Now i have little problem with sending photo. I'll be send them as soon as i can.
Best regards.

My client refused to believe the issue was caused by microbes, "The water is so clean!" I even included a few articles on MIC, but it wasn't what they wanted to hear. They wanted new tanks from the manufacturer.

In my younger years a consultant told me, "To be successful, you have to learn to tell the client what they want to hear."

I was a poor student, told him he was a whore and I would find a different line of work if that was the case. He's gone, but I'm still here. I've never been accused of being diplomatic. I did have a client tell me, "I don't always like hearing what you have to say, but I know what you say is your honest opinion and I can take it to the bank."

@gtaw
May i have your email address?

We can't give it out on this Forum.

Go to the website for the American Welding Society and find the library for Inspection Trends, you can find a number of articles I've written and my email there if you are serious.

Best regards - Al

Yes Al.
I just got done giving a deposition in a damages case. "You know if they had washed the dirt off this equipment once in a while it would corrode less" was not the diplomatic response, but it is the truth.
I used to work in Hygienic service applications. Their problem is that they forget that the cleaning solutions are also corrosive.
I have never had a shortage of consulting work. For some reason users don't get much smarter as a group.

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Quote (EdStainless)

For some reason users don't get much smarter as a group.

That jibes with my observation-based theory that on aggregate, humanity is getting stupider just slightly faster than it is getting smarter. The differential seems to have widened since 2016 though.

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

@gtaw
Sry i couldn't finde your email add.
This is the photo of inside the tank

• https://files.engineering.com/getfile.aspx?folder=385584df-437a-48dd-a4f0-6

Photo of small mounts

• https://files.engineering.com/getfile.aspx?folder=22032fb9-a1a3-4808-b6bc-9

After cleaning the mounts

• https://files.engineering.com/getfile.aspx?folder=9a316b30-2aff-4044-a7ad-e

While these images are consistent with MIC you can't blame the corrosion on MIC until you rule out everything else, it is a fact of life that microbes are everywhere. In this case the root cause is the poor weld condition.
It wouldn't hurt to also have a plant to minimize the MIC opportunities.

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@Edstainles
As you see, this mounds are every where. On weld and plate.
How we can considered that's the weld problem?

I agree the morphology is consistent with MIC, but good luck capturing any microbes. A conclusion of MIC is usually reached by inference, after clearing all other possibilities off the table, as EdS stated.

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

Meysamsh, The question isn't 'are there any microbes', you will always have them in systems. If you can limit their numbers and limit the nutrients then they are not a huge problem. The serious attack that you see is along welds so it calls out to me as primarily a weld issue. Fix the welding and treat the water, then you will just have some surface stains.
Now here is a tip, microbes will always grow preferentially along welds. The welds have a different microstructure and even in good cases have some secondary phases, this results in micro-currents. Under normal conditions these currents are below the levels that causes corrosion but they are attractive to microbes.
This is why in general 304 and 316 are considered to have virtually no resistance to MIC. It because of how week the corrosion resistance of the welds is.

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Meysamsh

SSTP304 cannot be used in potable water with 130 ppm chlorides storage tank. The solution is to change the storage tank material, you can paint, you can cladding, but soon or later you have to change your tank.

luis

@Edstainless
Thanks for your great answer.

@0707
We want to apply PTFE lining inside the tank

Meysamsh

I hope you will be successeful.

luis

With any coating system the three most important items are:
1, Surface preparation
2. Surface preparation
3. Coating application

You might also look at the two and three layer 'paint' systems used inside of potable water towers.
I am not enamored with PTFE linings, getting good adhesion is a real issue without shop baked treatments.

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Contact Belzona if you want the best coating system.

Belzona is a well known coating system.

luis

TBE, one option. But theirs pale when compared to 3 layer systems designed for continuous immersion service. I have seen Belzona and a German epoxy system used in waterboxed and such and they do very well. But for the inside of a tank I would look for a more specialized material.

You also have to be cautious coating SS. Be aware that any failure of the coating (blister, disbond, holiday) will likely lead to rapid localized corrosion of the SS.

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P.E. Metallurgy, consulting work welcomed

I should have said "has" coatings systems. Belzona has a full product range including specialized tank coatings for continuous immersion. They have a rubber coating, Belzona 2121, that I think would be ideal for tank interiors as it resists chipping and cracking. Of course you may find their price unreasonable as you pay a premium for the one stop shop. There are many other rubber tank membrane systems.

I see that this is for a potable water application so that will of course limit your choice of materials.

I'm so grateful for your great advices. I'll be considered them