Holding tank corrosion mechanism

[roger]

I posted recently about mechanisms for the cotrrosion of stainless steel holding tanks. I'm rather less sure now. We have various possible means of chemical attack. Urine when it "matures" creates a lot of ammonia and gets alkaline.
Fecal matter decays and absorbs oxygen, It then starts to produce hydrogen sulphide (bad egg gas). There is also in the tank salt water containing chloride ions.
The high oxygen demand means that stainless steel will not self heal by oxydising to give a protective oxide layer.
One reference talks of chloride and sulphide corrosion of stressed stainless steel. That would fit particularly for the welds on a stainless steel tank. Are there any metallurgists out there?
I do have a particular interest, My yard in Sweden can fit a holing tank at a reasonable price. It is however a stainless tank. However the salt level in the Baltic is pretty low so the chloride part of the mechanism is going to be less important. Any comments?

<hr width=100% size=1>Roger

 

[Talbot]

A holding tank should not be stainless steel whatever protection methods are advocated, any boat builder who is recommending tjhis needs to go back to school. If they can fit the stainless tank, then they can also fit a plastic one. Why go to the expense of creating a major problem for yourself in a very few years time.

If you dont believe me, ask the Headmistress who is an acknowledged authority on marine sanitation and posts sometimes on S'butt.

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[HeadMistress]

Urine is the culprit, not salt. A metal holding tank on a boat in fresh water won't last any longer than a boat in salt water. It has nothing to do with alkalinity or acidity...urine is corrosive. The ONLY material recommended for waste holding is thick-walled (at least 3/8") polyethylene.

<hr width=100% size=1>Peggie Hall
Specializing in marine sanitation since 1987

 

[richardandtracy]

I can concur about the corrosive strength of urine.

I've seen the result of someone foolishly putting aluminium cored 0.5" thick honeycomb material in an aircraft lavatory structure. After 6 months of gentlemen missing the bowl, the aluminium core had turned to powder, so the structure was down to 1/4260th of its original bending strength. Not very satisfactory as it was then at the point of collapse at 1g, let alone the loads seen in a crash.

Regards

Richard.


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[ArthurWood]

But it's probably the chloride ion in the urine that's the culprit. Not all acids will attack stainless, but chloride ion will.
Merry Christmas (Happy Hoildays), Peggie

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[Talbot]

Thought this post might draw you out into the open again /forums/images/icons/smile.gif

have a happy christmas Peggy

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[davel]

Timely post as I've just noticed some corrosion around the welds of my S/S holding tank.

Can anyone recommend a source of thick walled plastic holding tanks?


<hr width=100% size=1>Dave L.

 

[jfm]

Peggie, your book

Peggie:
Sorry to thread drift but is your book out of print? I just bought a copy on Amazon but they said they could only supply a secondhand copy. I dont mind a secondhand copy (so long as it has not been too close to any holding tank disasters....), but was just wondering if it's out of print.

Pretty tight supply over here means it cost me a hundred bucks. It may be my least glamourous christmas present but look forward to reading it, I think!

Thanks

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[BrendanS]

Re: Peggie, your book

It's certainly in print in US. Amazon.com has it for a shade under $17new, though not published in UK, hence amazon.co.uk has it available only through other vendors at hugely greater cost. Might actually have been cheaper to buy from Amazon.com, and pay the shipping costs from the US

<hr width=100% size=1>Me transmitte sursum, caledoni

 

[HeadMistress]

Re: Peggie, your book

As long as y'all are asking...my book is most certainly NOT out of print...and the least expensive source MAY just be direct from my publisher in the US--$19.95 USD, plus air mail postage, which isn't that much. The direct link is: http://www.seaworthy.com/store/customer/product.php?productid=40&cat=6&page=1

Tek-Tanks is the best source I know of for a top quality poly tank in the UK: http://www.tek-tanks.com

And a very Merry Christmas/Happy Holidays to all of you too!

Peggie


<hr width=100% size=1>Peggie Hall
Specializing in marine sanitation since 1987