Convert existing steel freshwater tank to blackwater holding tank?
 

I have no blackwater holding tank on my boat (an old English boat with just a sea head) and have a large steel freshwater tank under the vberth exactly where I'd install a blackwater tank if the other tank wasn't already there.

I'm thinking about just using that steel tank for the blackwater holding tank, and installing two new collapsible freshwater tanks in the bilge (there are two perfect spots on each side of the for them).

The main benefit of having the blackwater holding tank in the vberth is that it would be trivial to configure a gravity drain to the current sea head thruhull. If I put a holding tank in the bilge, I'd need to always pump it out when offshore since it would be below the water line.

My only concern is whether the salt in the seawater will cause the steel holding tank to rust out (since the head flushes with seawater, not fresh). Would that be a problem?

I would think if its steel and still alive it must have some type of internal coating ?


I would think that seawater would find the weak links in the tank and you would have a pretty crappy event at some point :)

Something in the back of my mind told me this wasn't a good idea. I googled something like "steel waste holding tank" and found the following at www.tec-tanks.com. Caveat: this is a commercial site and they're selling polyprop tanks, but the following is consistent with my recollections.

I'm not sure he's correct about "pee" being the most corrosive material -- hydrochloric acid is probably just a tad worse.

I'd second billyruffn. I doubt the welds would last a year. And if you think a leaking water tank can be messy....

I don't see why stainless steel wouldn't work. High Cr stainless holds up to salt water. It should work for urine too.

The issue is how long will it be before it fails and when it does will it be a catastrophic failure or a small leak in some inaccessible location.

I'm with BillyR and TomM....get a new tank. An old 'steel' (does this mean 'black iron' ?) is a recipe for whiffy.

Steel
It's not the stainless but the welds. Ever notice most corrosive materials are shipped in plastic barrels.