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Old 01-29-2013
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Stainless water tank conditioning?

I have a fairly new custom stainless fresh water tank that is out to have a leaking seam re-welded. Is there anything that I might consider doing to the inside of the tank to aid in keeping the water fresh? I have seen a couple of references to "commissioning" a new tank, but I haven't had any luck searching that. It is only about 20 gallons, all that will fit into my 22' cat, but the water can stay in it for up to a month.

Thanks for ideas.
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Old 01-29-2013
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Re: Stainless water tank conditioning?

Mike,

Do you happen to know what alloy of s/s was used for your tank?
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Re: Stainless water tank conditioning?

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Originally Posted by PorFin View Post
Mike,

Do you happen to know what alloy of s/s was used for your tank?
---------------
I don't have any idea. It was made for the prior owner, although he didn't actually get to use it before selling the boat. The tank seems to be very nicely made to exactly fit and maximize the volume, but one long seam failed and looks to me as if it was almost rust along that seam. As a result, not sure just how knowledgeable the maker was about SS and metalurgy, if he used the wrong material for the welding. It is now in the hands of a guy that does welding on nuclear subs, for a makeover as needed.
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Re: Stainless water tank conditioning?

Weld quality is the basis for long term service in SS tanks - no pin holes, no 'weld-laps', no 'gas blowouts', full penetration welding, etc. ... pretty much impossible for 'hand welding'.

The ultimate for service life extension is by grinding all the welds flat followed by surface polishing ..... and that will increase the cost of a tank replacement by about 5 to 10 times. Best economy is to simply have the failing weld seams 'repaired' on a repetitive periodic basis.

Standard shock sanitization for a new or cleaned* potable water tank by NSF recommendations is: (for 20 gallon tank) - 8 oz. Clorox per 20 gallons, let soak for 1-2+ hours, then fully empty, flush/rinse 2-3X, etc.

*cleaned = no internal 'slimes' or other indications of rampant bacterial growth.
Best way to lessen bacterial growth- empty the tank when not on board (just like you do with your water jugs).
If the dock water is already chlorinated as from a 'municipal' or 'city' supply source, then you probably wont need to 'chlorinate' for maintenance purposes.
Otherwise (for 'well water', etc.), add laundry grade clorox at a rate of .4 oz per 10 gallons of water that you add to the tank .... the object is to dose the water so that you 'just barely' can perceive the 'absolute faintest chlorine smell possible' in the spiggot water. Your wife should do the 'sniffing' so you dont 'overdose'.
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Re: Stainless water tank conditioning?

It should have a clean out hatch. If not, have your guy install one, you will want/need it at some point.

It can't hurt to manually wash it out with dish detergent (I prefer dishwasher detergent, as it doesn't foam much), by reaching through the cleanout. Rinse well and air dry.
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Re: Stainless water tank conditioning?

If the alloy used was not a low carbon stainless (i.e., 316L) -- and if it's showing signs of rust already, it probably isn't -- then you should read up on heat affected zone corrosion. To generalize, the heat generated by welding alters the surface composition and makes those areas fairly prone to rust.

From my reading, I also seem to recall that the chlorides in bleach will do a serious number on these areas and significantly hasten the weld decay.

Recommend you do some internet searching on the topic...
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Re: Stainless water tank conditioning?

re: chlorides in bleach affecting corrosion of stainless steel.....

Although true, a commissioning sanitation using the 8 ounces of chlorox per 20 gallons of water is adding a very low level of chlorides and chlorine, plus the contact time of a few hours is really a non-issue as far as corrosion goes.

As for long-term protection of the tank, oxygen is a corrosion inhibitor for stainless steel. Keeping the tank free of debris and slime is a great benefit. Keeping it empty and dry is ideal when you aren't there. Otherwise, turning over the water and adding a small amount of chlorine (or peroxide) is a good idea to keep the slimes down.

The idea of adding just enough chlorine to barely detect it at the tap is perfect.
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Re: Stainless water tank conditioning?

You'd have to totally fill a tank with 'drugstore' hydrogen peroxide to do any good vs. microorganism control.
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Re: Stainless water tank conditioning?

be sure that the welding is done with a inert gas purge in the tank during welding ( argon or nitrogen) or the welding is done in a vacuum chamber. Passivation should be done to any weld repair on a stainless steel tank to remove surface iron and form chromium oxide on the surface. after that you can sanitize the tank with chlorinated cleaner
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