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Old 01-27-2020, 03:10 PM   #1
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Inox 316 tank with corrosion and leaks

Trawler family all:
I had change one of my stainless steel 316 tank of water, 125 gallons, after 22 years because of some leaks in the weld border. We install it last April. I see that it is loosing water. I dismount it and it has 4 points of leaks - all in the weld points. The water is rust red brown color.The manufacturer has used stainless steel 316 and he says that it is due to electrolytic corrosion due to the lack of sacrificial anode.The old one do not had also any anode of Zn and resist 22 year with little leaks 7 years ago.
Thank you very much for your comments about it.
I decided to put 2 news tank of polyethylene high density. No more stainless steel
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Old 01-27-2020, 05:22 PM   #2
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Since all the corrosion is in the weld, I'd say the builder used a cheaper alloy stainless steel filler rod. But stainless steel, even 316 corrodes in the absence of oxygen because it can't form a chromium oxide layer on the surface that protects it from further corrosion. That's why stainless steel hardware corrodes underwater.

Also 316L stainless is the normal alloy for welding. When 316 is welded, the heat causes the chromium to draw out the carbon weakening the corrosion resistance.
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Old 01-27-2020, 05:27 PM   #3
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Sorry to hear that. For what it’s worth, we had SS water tank leak repaired by welding 3/4” angle iron along a seam and two others.
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Old 01-27-2020, 06:06 PM   #4
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It sounds like poor weld bead, poor gas control, or wrong gas mixture. You shouldn't be seeing corrosion that quickly. It is repairable but eventually stainless will corrode. Poly will probably give you fewer headaches.
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Old 01-28-2020, 01:22 PM   #5
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Stainless steel electrolysis

Today we had a meeting with engineer specialized in boat electrolysis.
1 - The tank have suffer electrolysis due to:
Despite the filler rod was AWS E316/316H 16 special for SS316 the corrosion has attacked the seal and the weld points because due to the heat this is the weak point. May be because the water pipes are of cooper and there is no anodes inside the water tank like in a heater. May be because of the salty and humid aft place not enough ventilation.
Too many "may be".... Ok decided new tanks with polyethylene HD.
2 - My concern is now with the 4 tanks of 180 gallons each that I have done in SS 22 years ago. Fortunately diesel do not have good conductivity like the water and I use to keep it full in winter in order to avoid the condensation. But mmm...

Thank you very much SoWhat, Larry M, Lepke for your opinion that I really appreciate.
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Old 01-28-2020, 02:06 PM   #6
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Keeping your fuel tanks full will help prevent condensation on the inside. You will still be subject to condensation on the outside depending upon dew point. Moisture in warm moist air will condense on surface of cool tank. But you should be able to easily see and remove surface rust.

Top of fuel tanks around the fill tube is the other weak spot. The seal between the fill cap and the deck may not be perfect. Other than that, fuel tanks should last 40 years. Periodic inspection with a borescope of the hidden external sides of the tank are helpful.
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Old 01-28-2020, 02:25 PM   #7
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Yes,I have to buy a borescope and check it as I do not have easy access to remove rust and clean.
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Old 01-28-2020, 03:10 PM   #8
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Armando,

When the engineer says "due to heat" means that the weld has has chromium carbide precipitation in the heat affected zone of the weld. This is also know as sensitization, intergranular corrosion, or weld decay.

This occurs when the weld gets too hot, for too long, with higher carbon austenitic grades of SS metal and filler material.

316H is the wrong material for the job. 316L would be more appropriate.

Water quality and lack of oxygen are contributing factors too.

Just as previous posters have postulated.

I have seen fairly large (100,000 USG) 316 water tanks suffer this exact same failure mechanism in approximately five years.

Turns out the dual grade 316/316L wasn't dual grade after all. Just 316. My gut feeling is that the weld procedure wasn't tightly adhered to either, because "it's just a water tank".

Poly tanks are a good solution.
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Old 01-28-2020, 04:49 PM   #9
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I'd agree that it was probably due to not using 316L for both plate and filler, and possibly too much heat used in the weld. Both will increase the probability and size of the carbide precipitation zone. "Electrolysis" would not preferentially attack the welded area without it being different.
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