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

[mocruising]

I have a reasonably well know GRP cruising boat build in Sweden with a big blue stripe round the topsides. Two years ago we sprung a leak in the water tank. (Tank capacity 1000 L) Actually its two tanks adjacent in the floor with a common pipe and only one filler. The tanks are installed under the floor prior to putting the deck on the boat so each individual tank could not be removed by conventional means. Both tanks sit above the bilge and do not come in to contact with accumulated bilge water. To cut a very long and expensive story short we had to remove the tanks from the floor, build a habitat down below, cut the tanks in half with an angle grinder and remove them. We built new tanks out of food grade SS in three sections so they could be installed down the hatch and joined them all together. For the past ten years we have never chlorinated the water.

The two leaks were from the inside out and were at the weld points. (Perforations) We could have patched them but considering all the work and thoughts of more perforations we replaced them. The boat has spent about two thirds of her life in the Med and completed one trip to the Caribbean and was shipped back. We were in Greece the three years before the tanks started leaking. I always load water through a 10 micron filter. The water in parts of Greece can be very hard. Although we always top up our fuel tanks for winter lay up in the past we never bothered to either top up or drain our water tanks. We have a water maker but its the one bit of equipment that we have never used. It was used by the previous owner. For the past ten years the boat is parked on the hard from Oct-Nov. through to April- May.

We now load all our water through a water softener plus ten micron filter and when ever we leave the boat for any length of time fill the water tanks to the top. The theory here is to exclude as much oxygen as possible, I do realise water contains dissolved oxygen.

Five years ago we had to replace both the SS holding tanks due to perforations but we replaced them with plastic which was the recommendation from this forum.

I can understand the holding tank issue but have we just been un-lucky or have others had similar experiences.

 

[vyv_cox]

It is a well known problem referred to as weld decay, almost certainly the result of welding 'ordinary' 316 stainless steel sheet. Using 316L sheet should overcome the problem. See http://events.nace.org/library/corrosion/MatSelect/corrstainsteel.asp for a full explanation of the phenomenon. I guess that if you had only ever put deionised water in your tanks it might not have happened but any realistic fresh water will contain sufficient ions to cause it.

Not the only example of poor materials selection from that company. I hope you have checked you seacocks?

 

[charles_reed]

I suspect Vyv that "food-grade stainless" is synonymous with 304 rather than 316 - in which case I'm pleasantly surprised that they've lasted as long as they have.

 

[Tranona]

Yes. I had the plastic OE tank in my Bavaria replaced with stainless and foamed into the forward locker. Leaked after 3 years at a failed weld. Took a week to get it out of the boat. Decided in the end to have a plate welded on, partly because a new tank from Tek Tanks was £800. Built new mountings in the boat so it is easy to remove next time it leaks, but if I was keeping the boat long term would have bit the bullet and had a plastic one made. Welded stainless is not a good choice for tanks.

 

[RobbieW]

I replaced the water tanks in an '83 Rival 41 about 18 months ago. Similar problems, though these had been sitting in about an inch of bilge water for extended periods and the bottom inch of the tanks had extensive crevice corrosion - replaced with plastic. What surprised and pleased me was that the floor was obviously designed to allow removal of the tanks - they sit under the saloon on top of the ballast in the keel, 2xc.200 ltrs. All the 'joists' had joints in the places they were needed, and all the screws came out!, to remove the woodwork leaving a large coffin shaped hole.

Given the debates in other threads about British vs German quality, this British built boat had had some thought put into that area (though the headlining installation is less impressive). Also delivering a similar era HR W-E across the Atlantic took the shine off that marque for me, though it was in need of a lot of tlc having been in the carib for 10 years +.

 

[macd]

if I was keeping the boat long term would have bit the bullet and had a plastic one made. Welded stainless is not a good choice for tanks.

+1 redoubled. If only Calder designed boats...
It's not such a big problem in the US: they seem to like aluminum for tanks, gawd help 'em.

 

[vyv_cox]

Welded stainless is not a good choice for tanks.

It's an interesting argument, which I believe throws accepted best practice in tank design on its head. My stainless steel fuel tank is now coming up to 30 years old and appears to be faultless. Fuel is drawn from the bottom of the tank. I used to work with a man who was an expert at oil fired central heating systems, who maintained that the way to keep the mild steel tanks corrosion free was to tilt them slightly towards the discharge port, so that any entrained water in the fuel was drawn off and burnt.

Best practice in fuel tank design has been that the delivery dip tube came from a point an inch or so above the bottom of the tank. Provided that any water in the bottom was drawn off from time to time this was perfectly adequate. Now that we additionally have diesel bug to contend with the picture changes a little, as this has an excellent and undisturbed part of the tank to breed in, until the time when it reaches the dip tube, when it becomes a major exercise to remove. Again, regular draining of any water should take care of it but it is all too easy to forget. Taking the delivery from the base of the tank removes the water naturally, especially when monitored with a primary filter equipped with a glass bowl, and will also remove diesel bug at its early stages.

 

[Tranona]

I guess stainless (and aluminium) were seen as a step up from mild steel and it is several years before the problems occur. Each time I have replaced tanks I have got quotes for both stainless and plastic and there has been little difference in price. However if you are a production builder the difference must be massive if you have the volume to justify rotational moulding and baffles can be incorporated in the shape. In the Bavaria tanks there are vertical tunnels which create the baffle.

As well as the effects of the contents, metal tanks, particularly mild steel can corrode from thye outside, for example where they sit on bearers, probably insulated with old bits of (wet) carpet!

 

[Tradewinds]

An alternative to Tek-Tanks for waste & water tanks.

Stansa Plastic Fabrication - nr Ipswich Suffolk.

Very good pricing.

No connection - just a satisfied customer.

http://www.stansa.co.uk/products.htm

 

[catmandoo]

I

Best practice in fuel tank design has been that the delivery dip tube came from a point an inch or so above the bottom of the tank. Provided that any water in the bottom was drawn off from time to time this was perfectly adequate. Now that we additionally have diesel bug to contend with the picture changes a little, as this has an excellent and undisturbed part of the tank to breed in, until the time when it reaches the dip tube, when it becomes a major exercise to remove. Again, regular draining of any water should take care of it but it is all too easy to forget. Taking the delivery from the base of the tank removes the water naturally, especially when monitored with a primary filter equipped with a glass bowl, and will also remove diesel bug at its early stages.

What about the outside of the tank with bilge water in direct contact with the welds - probably worse that a little water in the tank ? In that case best to keep water level below tank and let air get at it .

 

[vyv_cox]

What about the outside of the tank with bilge water in direct contact with the welds - probably worse that a little water in the tank ? In that case best to keep water level below tank and let air get at it .

Can't argue with that. One thing is for certain, if there is no water there is no corrosion, no matter what the materials or weld quality.

 

[Yacht Mollymawk]

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.

 

[macd]

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.

I've made several tanks, both holding and fuel, 'flat-packed' from GRP (without the ply). Please feel free to PM me if you want any tips.

 

[Chris_Robb]

Can't argue with that. One thing is for certain, if there is no water there is no corrosion, no matter what the materials or weld quality.

Just hope the new diesel tank I am having made in Leros will be good - Vyv are you there at the moment to cast a beady eye?- apparently Manelos is starting work any day now on making it.....

 

[vyv_cox]

No Chris, came home three days ago.

 

[Chris_Robb]

No Chris, came home three days ago.

Ah well, so no quality control available!!!! Its forecast to get pretty hot in the next week out there so your better off back here. We won't get out there until around the 16th September. Hopefully all will be mended. I feel quite relaxed about the engineer now I have seen him at work.

 

[chinita]

I had a similar problem with the same boat as the OP; except that the boat was only seven years old.

Detected a freshwater leak in the bilge and identified (eventually)the source from the corner weld on the bottom of the upper tank.

Removed it and repaired with coldweld. Seemed to do the trick - at least for the next three years before I sold her!

 

[toolarts]

Please note that oxygen in the water does NOT cause crevice corrosion in stainless steel. Look it up if you don't believe me.

Stainless steel is corrosion resistant because it is oxidized on the surface layer. Exposure to circulating, oxygenated water is good for stainless steel.

What causes crevice corrosion is when the water is not circulating. When it becomes stagnant, the oxygen content drops, and then the water absorbs oxygen from the oxidization layer on the stainless steel. The steel then begins to corrode. It's possible that if water sits in a stainless tank and does not circulate, areas around the welds will lose oxygen and crevice corrosion begins. Most likely, it is worse if a boat is sitting for long periods on the hard.

I'm having a new 80 gallon drinking water tank built out of 316 Stainless. It is one of the materials approved by ABYC. I have reputable company building it (Svendsen's Metal Shop, Alameda CA) and they will leak test and guarantee the tank. It is not cheap, but they do excellent work.

I have no problem installing a properly constructed SS tank. I need a rigid tank because it is suspended over the bilge. Plastic will not work in this application. I am also having them put inspection ports in each chamber of the tank so I can get in and clean it thoroughly. Since I use my boat on a regular basis, I fill and empty the tank about once per month. I am absolutely unworried about crevice corrosion in this tank.

When I got my boat, it had been sitting unsed for a long time. There were numerous problems with it due to neglect and disuse. I have had it since 2004,
and use it regularly, and the boat has been improving all that time. The engine always starts because I start it frequently and service it regularly. I am replacing the aluminum water tank because I have the cabin sole out for other reasons, and I am installing pressure, hot water, a shower, and so I want the heart of all these new systems to be clean and reliable.

In talking with Svendsen's I asked them about the welds, and failures. They told me that many tank builders "wire-weld" the seams. This tends to create a
dirty seam. Svendsen's uses tig welding, which creates a clean seam that if polished is indistinguishable from the surrounding metal. I think there are a lot
of reasons why someone's stainless tank starts leaking: Construction methods, materials, installation, and of course, usage. But I wouldn't jump to the conclusion that stainless is a poor choice for a water tank. Stainless is used throughout the commercial food industry for everything from vats to piping to dishwasers and food grinders. It is a good material for a water tank, if the construction is quality and it is maintained properly.

Plastic is also a good material, but plastic tanks must be properly supported or they will destroy themselves in a pounding head sea.

Just my two cent's worth.

toolarts

 

[LittleSister]

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.

 

[vyv_cox]

In talking with Svendsen's I asked them about the welds, and failures. They told me that many tank builders "wire-weld" the seams. This tends to create a
dirty seam. Svendsen's uses tig welding, which creates a clean seam that if polished is indistinguishable from the surrounding metal. I think there are a lot
of reasons why someone's stainless tank starts leaking: Construction methods, materials, installation, and of course, usage. But I wouldn't jump to the conclusion that stainless is a poor choice for a water tank. Stainless is used throughout the commercial food industry for everything from vats to piping to dishwasers and food grinders. It is a good material for a water tank, if the construction is quality and it is maintained properly.

There are a few misconceptions in this post. Firstly, crevice corrosion is not normally a problem in tanks, provided they have been made well. TIG welding may use filler rods or it may not, it depends on a number of factors. TIG is somewhat similar to gas welding in that the arc provides the heat source, filler may be needed, maybe not, just as in gas welding.

Tanks fail due to sensitisation, which is the formation of chromium carbides adjacent to the weld. The effect of this is to reduce the local chromium content to well under 18%, upon which corrosion occurs. It is avoided by using low-carbon sheet, 304L or 316L, and by using rods of the same metal or better. There are some even better rods that contain small amounts of preferential carbide formers, e.g. titanium and niobium.

There is also a form of tank corrosion caused by using rods that are lower in chromium and nickel than the sheet material. In this case a different problem, known as preferential weld corrosion, can occur. The answer is to use rods that are higher in both elements than the parent metal.