Had to replace my stainless steel water tanks in 16 year old boat, due to corrosion.

[geem]

We have two stainless steel water tanks, and one of them started leaking, via a weld, within five years of its manufacture. The
tanks are made from recycled hospital water tanks, and these were labelled 316L. The rods, too, were 316L. However, the welding was not done by a pro, and the tanks were not pressure tested... Everything was done in a bit of a rush, as we were in a hurry to get the boat built and go sailing! The interesting thing is that before the job reached the top of a very long list of urgent chores, the leak healed itself...!

Our holding tank is another matter. That, too, is stainless. We don't know what grade, as it is actually a second-hand fuel tank. It doesn't just leak along the welds; it's porous in various other places, too!
Since we can't find a ready-made plastic tank which will fit the space, we're going to make a plywood one and sheathe it, inside and out, with epoxy GRP. We have a lot of misgivings about this idea... but the subject is probably not relevant to this thread.

So far as the original question is concerned - we reckon that rather than filling your water tanks when the boat is not in use, you might do better to drain them. As you say yourself, the oxygen content in water is sufficient to allow crevice corrosion. (Hence, stainless steel anchor chain is a disaster.) On the other hand, a wet stainless steel deck fitting tends not to corrode, provided there is sufficient exposure to the air to allow a chrome oxide coating to form.

our fuel and water tanks are GRP and built into the boat. They sit above the keel and have done so for 33 years. I was thinking I might paint the inside of the water tank with epoxy paint this winter

 

[vyv_cox]

our fuel and water tanks are GRP and built into the boat. They sit above the keel and have done so for 33 years. I was thinking I might paint the inside of the water tank with epoxy paint this winter

The water tank in my Sadler is built in, constructed of GRP. It looks to me that the whole thing has been coated internally with epoxy during manufacture, although I cannot be certain. Various other internal parts of the boat that often have fresh water sitting on them for extended periods, e.g. engine bilge, heads sink, beneath toilet, a cockpit locker, have suffered minor osmosis damage whereas the water tank is pristine.

 

[toolarts]

Thanks for the corrections, vy. What I know about welding stainless is the short story I got from Svendsens, so your information is much better.

It's is also good to know that crevice corrosion is not a big problem in tanks, sort of a relief. But it does sound like the real culprit in a failing SS tank would
probably be the quality of the welds. One comment would be that a well made tank should be made the way you describe it, with the right sheet and correct rods.

 

[ianabc]

Epoxy water tanks


from Gougeon Brothers:


"Considerations for potable water tanks

We have adopted the broad policy of not recommending epoxy for drinking water tanks because of regulatory and safety issues.

The potential problems outweigh the benefits.

To date, none of Gougeon Brothers' epoxies meet FDA regulations or any other drinking water certified approval.

The major long-term concern with any plastic water tank is extractives leaching out in the water. Off-ratio, poorly cured epoxy can release extractives, as noted above.

In the fabrication of water tanks and food handling equipment, the successful use of epoxy requires thorough mixing and adequate elevated temperature post-cure to assure the maximum cross-linking and cure of the polymer. These process controls are not always possible with the home-built tank. Unfortunately, neither Gougeon Brothers, Inc. nor any certification agency can verify the level of quality control exercised in the fabrication of the tank.

What about the builder who weighs the risks and decides to go ahead against our recommendation? For the homebuilder it is a personal choice. If you build a potable water tank, follow the general guidelines noted above; in addition, you may want to install an in-line filter to help remove any possible extracts and odd tastes. Professional builders should understand, again, that there are no formal approvals (awarded or pending) for Gougeon Brother's epoxy products for use in potable water tanks. Where certification is required, it is usually application specific.

We have looked at the 2000 Title 21 of the Code of Federal Regulations, Part 175.300 (Food and Drug Administration, HHS) and found that the testing is specific for end-use conditions, container sizes, and frequency of use. If you have a specific design/use in mind, it may be appropriate to contact the American or Canadian agency to identify their requirements. As with many things, the care used in the construction of the tank is the key to the overall quality.

ABYC also has a section dedicated to the general installation of potable water systems; the specific standard is H-23-Installation of Potable Water Systems for Use on Boats. "

 

[NormanS]

Was getting worried hear the tales of woe, so I'm pleased that there are those extolling the virtues. We have stainless water tanks (two) and a diesel tank, all of which seem fine after nearly 30 years.

Our diesel tank is a slight variation on the arrangements Vyv mentions - the engine supply is taken through the side of the tank, about an inch and a half from the bottom of the tank, while in the botom of the tank there's a drain plug which makes draining out water & debris relatively easy. In the fuel tank of my dreams the drain plug would sit in a small, steep sided sump to better collect/empty the water and debris.

My fuel tanks are the tanks of your dreams.

 

[Abraxas33]

I built a new holding tank using 10mm polyethylene sheet last winter and fitted it in May this year (no leaks so far). There is plenty of advice including utube video's explaining how to weld plastics and I found it fairly easy to master (as an ex electrical engineer), no doubt a skilled metalwork welder would find it easier. One piece of advice I would offer is that using the 'speed welding' nozzel is easier than conventional hand welding.

So... don't be afraid to build your own plastic tanks. From memory a 2metre X 1metre sheet of 10mm thick polyethylene costs about £110 - with internal baffles the tank is very strong. Lee Sanitation sell a range of pipe transitions to get though the tank walls.

 

[Norman_E]

My 15 year old Jeanneau had leaks from both stainless water tanks about a year ago. Unlike the OP's boat however it was possible to remove the tanks, so I had the leaks welded over. I consider that the leaks were probably caused by putting Milton into the taks and leaving them full when not in use. I now drain them before leaving the boat. How long the repairs will last I cannot tell, but if they fail again I will get replacements made in plastic. There is a firm in the Saniye in Marmaris that makes them, and prices them by weight. Curiously, on the Jeanneau, the water tanks are stainless, but the fuel tank is aluminium.