Sewage Tank Connection concerns...

[Sailor Simon]

Under the v berth in mySwiftsure33, there is a custom made ss sewage tank. The connection nipples appear to be mild steel, as they are badly corroded, the lower, suction clean-out is the worst. How is a mild steel nipple attaced to a ss tank? It doesn't appear to be welded, it's just there... I want to: use a Dremel tool to hone out the inside of the nipple, to ensure the mild steel has not been pressfitted or something. If it has been, I'll clean it all out anyway, thus leaving a slightly larger hole. At any rate, eventually all of the inside of that fitting will be clear metal.

It is my intention to replace both nipples with a plastic flange assembly ( both nipple are located on the horizontal plane, facing aft).and sufficient sealant/adhesive that this concern never need be addressed again. The lower flange on the inside of the tank, would have to be shaved down somewhat, (like adapting a washer to fit in a tight spot), as the suction is fairly close to the bottom of the tank.

Adhesive suggestions?
Tool suggestions?
Am I going in the right direction suggestions?
Other ideas suggestions?

Sailor Simon

[Paulus]

Was this perhaps a custom made potable water tank, then replumbed as a holding tank? The contents of any holding tank is extremely corrosive and conductive (galvanic action) and therefor not best suited for that application. Whatever you decide to do, you will likely have to re-visit in a few years and make additional repairs/corrections.

If you have good access to the area/tank at this moment, you may wish to remove it and invest in a plastic, polyethylene or ABS tank instead...

That said, what you propose to do, i.e. remove the disimilar metal and leave the stainless by itself with rfp/polyethylene connections, will certainly remove most of the galvanic corrosion eating away at the tank (but still leave you with some oxydation). If you are thinking of sandwiching the stainless between polyethylene flanges, I would add some good reinforced rubber packing material to help with the seal....

No matter what you do, it will be a patch job and chances are there will be some leaks... not a pleasant thought considering the contents... ;-o

[Hirilondë]

Was this perhaps a custom made potable water tank, then replumbed as a holding tank?

This would be my guess. I strongly suggest pulling the tank and going plastic. Not only are the repairs tedious and in the end futile, but a leaky holding tank is more than annoying, it is just plain unacceptable.

[mitiempo]

I agree replacing it with plastic is the solution. Nothing on a boat is more corrosive. Sheet stainless survives fairly well but welds become porous after not too long and fittings as you've found.