Water Tank Corrosion

[pevil]

Got an integral water tank on my narrowboat, nothing strange in that it's been about 6 years since I last painted it so decided I'd better have a look and probably do it again, when I got in there (nice big hatch) and cleared away the rust deposits I found quiet a bit of corrosion and about 6 x 2mm to 3mm deep pits on the port side plate along with shallower pitting to the bottom plate, particularly in the corners, the metal looked like it had been cleaned with acid! I'm not sure what's caused this, no AC just 12volt, has anyone seen corrosion like this before or got an idea as to the cause, I want to stop it getting any worse.

As a side note the boat has just been out for blacking and new anodes and there was no sign of pitting to the hull, in fact it looked very good for it's 13 years.

 

Thanks.

 

 

[dmr]

Im not an expert but have read that there can be some unusual pitting mechanisms in watertanks, especially on the bottom plate.

As its only a few pits you could get them welded. My pits were 1.5 to 2mm in the 10mm bottom so I was not too concerned. Whatever you decide the best thing to do is to take the entire water tank back to bare metal (or very close to bare metal) and paint it in a suitable epoxy.

 

..............Dave

[MtB]

Im not an expert but have read that there can be some unusual pitting mechanisms in watertanks, especially on the bottom plate.

As its only a few pits you could get them welded. My pits were 1.5 to 2mm in the 10mm bottom so I was not too concerned. Whatever you decide the best thing to do is to take the entire water tank back to bare metal (or very close to bare metal) and paint it in a suitable epoxy.

 

..............Dave

 

 

Or better in my opinion, fit a tank liner.

[dmr]

 

 

Or better in my opinion, fit a tank liner.

 

I would still worry about condensation and corrosion so would still recommend the two pack before fitting the liner, but once you've done the two pack why bother with the liner????. I have pondered getting one of those little camera things so that I can have a look inside my tank to see how the epoxy is doing without the fag of lifting the cover plate, not sure how effective it would be.

 

.............Dave

[MtB]

 

I would still worry about condensation and corrosion so would still recommend the two pack before fitting the liner, but once you've done the two pack why bother with the liner????. I have pondered getting one of those little camera things so that I can have a look inside my tank to see how the epoxy is doing without the fag of lifting the cover plate, not sure how effective it would be.

 

.............Dave

 

 

You wouldn't worry for years that there might not be even just one little pinhole that you missed when doing the two pack then?

 

[Murflynn]

what we really need is the equivalent of the Victorian chimney sweep boy, to get inside the tank and give it a good inspection once a year.

 

perhaps a new service that could be offered by vertically challenged folk, willing to travel all over the system.

[dmr]

 

 

You wouldn't worry for years that there might not be even just one little pinhole that you missed when doing the two pack then?

 

 

Thats why you put several coats on init? More realistic failures are rust breaking through where preparation was less than perfect (rust left at bottom of small pits) or epoxy failing due to impacts, the front of the boat does take a fair bit of stick. The Armourguard epoxy is supposed to be slightly flexible to cope with impacts but who knows? I worry more 'bout whats going on under the floor than what goes on in the water tank. If the tank fails we don't sink, just get a bit of flavour in the water.

 

Fancy a pint or two mid November????

 

.............Dave

[Chewbacka]

You haven't got any copper pipe in the tank have you? As steel will corrode preferentially to copper.

[Bee]

Personally I would do the epoxy in the tank and some sort of liner/something else I have yet to discover/bladder route, I'm not happy with integral tanks (my diesel tank is integral and just about acceptable)

[MtB]

You haven't got any copper pipe in the tank have you? As steel will corrode preferentially to copper.

 

I notice there are patches of bright shiny corrosion in the OP photos, suggesting to me there is some very active electrolysis still happening somehow. A more noble metal in the system might explain it.

 

Fancy a pint or two mid November????

 

.............Dave

 

 

That sounds a Good Idea.

 

Will take you bell ringing too. I think you'll be more than intrigued!

[pevil]

Im not an expert but have read that there can be some unusual pitting mechanisms in watertanks, especially on the bottom plate.

As its only a few pits you could get them welded. My pits were 1.5 to 2mm in the 10mm bottom so I was not too concerned. Whatever you decide the best thing to do is to take the entire water tank back to bare metal (or very close to bare metal) and paint it in a suitable epoxy.

 

..............Dave

 

I do plan to get the pitting welded but having just had the boat blacked I'd prefer to leave it until she comes out again, for now I'd just like to find out what's causing the corrosion, stop it and get it painted.

 

 

 

Or better in my opinion, fit a tank liner.

 

Thought about a tank liner, even got a quote but at £672 it's a bit beyond my budget, probably go down the two pack route once I get the welding done.

 

You haven't got any copper pipe in the tank have you? As steel will corrode preferentially to copper.

 

There's no copper in the tank, only other metal is the brass filler cap but that's not actually in the water.

 

 

I notice there are patches of bright shiny corrosion in the OP photos, suggesting to me there is some very active electrolysis still happening somehow. A more noble metal in the system might explain it.

 

 

 

 

My first thoughts as well but there's nothing in there, I did wonder about a current flow through the water pipes, the outlet is on the worst side, then I'm thinking should I bond the negative or not, don't think it is at the moment but there's loads of different views on that!! I'll have to do some checking with a multi meter.

 

Anyway thanks guys for your input, it's all welcome.

[Chewbacka]

I still have a suspicion that something was dropped into the tank, such as a bit of copper pipe (used as a dip stick or hose connector), copper olives or even a brass hose fitting.

When you cleaned out the tank where there any bits (even fairly small) of metal (not steel) lying on the bottom?

Edited October 15, 2016 by Chewbacka

[MtB]

 

There's no copper in the tank, only other metal is the brass filler cap but that's not actually in the water.

 

 

I'm no expert in this field but I would have thought a more noble metal would only need to be somewhere in the system in contact with the electrolyte (the water). ANY copper or brass pipework also electrically connected to the steel of the boat has the potential to cause electrolysis.

 

But the thing is, is copper/brass/zinc more or less noble than iron? I don't actually know.

[Chewbacka]

 

 

I'm no expert in this field but I would have thought a more noble metal would only need to be somewhere in the system in contact with the electrolyte (the water). ANY copper or brass pipework also electrically connected to the steel of the boat has the potential to cause electrolysis.

 

But the thing is, is copper/brass/zinc more or less noble than iron? I don't actually know.

For corrosion inside the tank the circuit needs to be via the water in the tank, so is most probably inside.

Copper is more noble than iron (steel) and so the steel will corrode if in contact with copper in the presence of an electrolyte.

 

As an aside this was actually a problem with the statue of liberty which is an iron frame with copper cladding, the copper being insulated from the iron frame by a shellac film. This film aged and failed and corrosion started.

 

Compared to Iron the more positive metals in the electrochemical series include

Nickel

Tin

Lead

Copper

Silver

Platinum

Gold

 

So any of these lying in the water tank would encourage Iron/steel corrosion. So if I dropped a lump of gold in I would make a big effort to get it back, though not for corrosion reasons

Edited October 15, 2016 by Chewbacka

[Neil2]

 

 

I'm no expert in this field but I would have thought a more noble metal would only need to be somewhere in the system in contact with the electrolyte (the water). ANY copper or brass pipework also electrically connected to the steel of the boat has the potential to cause electrolysis.

 

But the thing is, is copper/brass/zinc more or less noble than iron? I don't actually know.

 

Isn't this the very reason why we put corrosion inhibitors in central heating systems? Copper pipes and steel radiators set up the conditions for electrolysis. Same reason why galvanised water storage tanks rust, the zinc acts as the anode to the copper cathode. Probably another good reason for plastic piping.

 

Once the protective coating on your integral tank wears off there's nothing to stop electrolysis causing corrosion, I shudder to think what's going on inside my tank, the lid looks like it hasn't been touched since the boat was built...

 

I suppose you could drop a zinc or magnesium anode in there - would that work?

 

Anyhow, I'm having a duratank liner made just as soon as I can pluck up the courage to look inside my tank.

[Chewbacka]

If you want to make a galvanised steel tank leak - though it will take a year or three depending on thickness - drop a mild steel nail in. It will rest on the Zinc layer and the zinc will corrode away to protect the steel nail. After a while there will be a big bald area of zinc on the tank base around the nail, whereupon the steel base will start to corrode and eventually perforate.

[pevil]

 

 

I'm no expert in this field but I would have thought a more noble metal would only need to be somewhere in the system in contact with the electrolyte (the water). ANY copper or brass pipework also electrically connected to the steel of the boat has the potential to cause electrolysis.

 

But the thing is, is copper/brass/zinc more or less noble than iron? I don't actually know.

 

We can probably over think these things because lets face it, if you've got a calorifier in the system, which is copper, that would be connected to the water tank by the water in the pipes, the water in the calorifier is connected indirectly to the engine via the coolant flowing through the coil inside, the engine is connected to the negative side of the battery, the same thing applies as to whether to bond the hull, the engine 'which already is' will be connected to the hull via water in the engine and skin tank, via the exhaust pipe and as said already via the calorifier, this being the case why aren't all our boats corroding away? do we need to introduce some stray positive current to this mix in order to trigger corrosion? I don't know and now my head hurts it's all very technical, oh the joys of boating

Edited October 16, 2016 by pevil

[Chewbacka]

 

We can probably over think these things because lets face it, if you've got a calorifier in the system, which is copper, that would be connected to the water tank by the water in the pipes, the water in the calorifier is connected indirectly to the engine via the coolant flowing through the coil inside, the engine is connected to the negative side of the battery, the same thing applies as to whether to bond the hull, the engine 'which already is' will be connected to the hull via water in the engine and skin tank, via the exhaust pipe and as said already via the calorifier, this being the case why aren't all our boats corroding away? do we need to introduce some stray positive current to this mix in order to trigger corrosion? I don't know and now my head hurts it's all very technical, oh the joys of boating

Ignoring stray currents because of bad wiring and just talking about galvanic corrosion.

In this case the dissimilar metals must be touching each other with a conductive fluid between them. The corrosion being within the fluid.

So a copper pipe to an iron radiator would cause some corrosion inside the radiator and close to where the copper pipe enters the rad, hence corrosion inhibitors are used.

Alde boilers now have aluminium parts within the water system inside the boiler hence the instruction not to use copper pipes as the aluminium will corrode away inside the boiler.

[Neil2]

The truth is boats with copper piping and integral steel tanks probably are "corroding away" unless you are very disciplined in keeping the inside of your tank protected with some sort of coating. It could take thirty years or more for pitting to work through to the outside of the hull but it does happen. When we had our boat galvanised at Debdale Wharf one chap on the marina was telling me how he discovered perforations in his integral tank when the boat was last docked, though the rest of the hull was sound. IIRC the boat was some forty years old and he wasn't that concerned about it as the internal bulkhead was still watertight. I suppose it would save him having to fill the tank up as well...

 

Central heating systems and engine cooling systems can have additives to restrict/prevent galvanic action, obviously we can't do this with domestic/drinkng water systems.

[Sea Dog]

Ignoring stray currents because of bad wiring and just talking about galvanic corrosion.

In this case the dissimilar metals must be touching each other with a conductive fluid between them.

 

Are we not just talking dissimilar metal corrosion here as opposed to galvanic action?

[Chewbacka]

Are we not just talking dissimilar metal corrosion here as opposed to galvanic action?

I think dissimilar metal corrosion and galvanic corrosion is the same thing.

 

The Wikipedia definition is "Galvanic corrosion (also called bimetallic corrosion or contact corrosion) is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte."

[WotEver]

I think dissimilar metal corrosion and galvanic corrosion is the same thing.

 

The Wikipedia definition is "Galvanic corrosion (also called bimetallic corrosion or contact corrosion) is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte."

As opposed to electrolytic corrosion which is caused by an applied voltage.

[Sea Dog]

I think dissimilar metal corrosion and galvanic corrosion is the same thing.

 

The Wikipedia definition is "Galvanic corrosion (also called bimetallic corrosion or contact corrosion) is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte."

As opposed to electrolytic corrosion which is caused by an applied voltage.

Yeah, all agreed. I was more referring to the 'in contact' bit really which I read as physically and, whilst clearly bimetallic corrosion, would be resulting in a local reaction and sit nicely under the 'dissimilar metal corrosion' description. The more distant issues, now being in electrical contact but not physical, perhaps sitting better under 'galvanic corrosion'. A rotting steel screw in a fitting being an example of the former with a brass underwater fitting in contact only with grp but losing its zinc being an example of the latter. My use of 'as opposed to' wasn't a good choice, making them appear to be different processes rather than selective terms for the same process in different situations. A poor bit of clarification seeking on my part I think!

 

 

Edited to change an 'is' back to an 'in'.

Edited October 16, 2016 by Sea Dog