steel hull corrosion

[Conachair]

I have an Ebbtide 33' steel cutter which I had lifted out of the water last week. In Feb I had her lifted out in the Medway, lots large bubbles in the paint down one side down to the steel. A guy in the boatyard straight away asked if I had been tied up near an old wooden boat, which was the case. He said that the copper nails in the timber attack the steel. This all seemed to make sense as that was the side of the hull which was next the a decrepid old timber thing for a many months. When I got lifted out the other day I had the same problem in the same areas. Between then and now the boat has beed sailed from London to Portugal, very rarely on shore power and if so just into sterling charger which supposes to have no direct earth connection. In Feb I used an angle grinder to grind back to metal and gave it 2 or 3 coats of VC tar epoxy.
Any ideas whats happening? Could it be that paint was badly applied? LX wise I usually only have the lighting circuits on. Meter which goes down to 0.1a show zero draw when lights turned off.
All help much appreciated.

Ta

 

[savageseadog]

Perhaps fit another anode or increase the primer thickness, no substitute. You might also be getting electrolysis between the Antifouling and the hull. The situation of having this on one side might be due to stray currents from mains electricity.

 

[vyv_cox]

Sounds like an old seadog's tale. Steel is more electronegative than copper but the two would have to be in direct contact underwater for corrosion to occur. Copper and copper compounds are widely used for antifouling without any corrosion problems, even in direct contact.

This looks like pitting corrosion caused by 'holidays' in the paint system. Usual solution is the application of many coats. I suggest your paint system needs review.

 

[savageseadog]

[ QUOTE ]
Sounds like an old seadog's tale. Steel is more electronegative than copper but the two would have to be in direct contact underwater for corrosion to occur.

[/ QUOTE ]Sorry but they are in contact, enough to conduct, a thin layer of paint will allow electrolytic cells to form.

 

[KellysEye]

I agree it looks like a paint problem. If there was a trace of rust when the epoxy tar was applied it could account for the pitting. On the anode front (we also have a steel boat) apparently it is possible to over-anode a steel boat and that will 'blow' the paint off (we only use 4 of six anode points) - but I don't see how that would apply to only one side.

 

[Conachair]

alough it's from memory i think the bubbles have formed in the same place as they were before. Cold when vctar went on as well. There are quite a few anodes fitted, as the boat was a professional yard build of an alan pape design i never really though much about that. At the moment I'm thinking of getting the entire hull shot blasted and repainted but would like to know why the existint paint system was failing. Boat is 23 years old.

 

[Blueboatman]

Couple of questions really but no answers I'm afraid : When you hauled in February were the air temperature and humidity within the paint systems prescribed window for application ?
Did you thin the paint system before applying the coatings?(because solvents can evaporate leaving pin holes,thus requiring multiple coats)
You may have to ask for a different marina berth,as a scientific test...

 

[Conachair]

Air temp was just within the window. Think I thinned the first coat, pretty sure that was suggested in the data sheet. Just looked at website "do not use if there is a chance of condensation forming". Could have been some.

 

[KellysEye]

[ QUOTE ]
alough it's from memory i think the bubbles have formed in the same place as they were before...thinking of getting the entire hull shot blasted and repainted but would like to know why the existint paint system was failing. Boat is 23 years old.

[/ QUOTE ]

Well I guess if the paint has lasted 23 years that's pretty good going. If the bubbles have reappeared in the same place it does sound as though there was a problem when the paint was applied - maybe just re-do that patch?

On the anode front there is an electrical test for steel boats to establish the right level of anode protection but I'm not a techie and hopefully there might be someone around who knows what it involves.

 

[Ships_Cat]

I go along with Vyv_Cox. The talk of the antifouling causing it is extremely unlikely, and that of copper nails on a nearby boat causing it is complete rubbish.

A shortage of anodes will not cause the problem and most steel pleasure boats are over protected driven by irrational fears of the owners (our own has only one smallish anode).

The usual cause of blistering is poor preparation and reading other posts on these forums poor preparation is rampant. That you state that the problem, as you recall, is occuring at the same place also points to that.

Another cause can be the use of zinc rich primers which the major paint manufacturers no longer (for quite some time) recommend in the paint specifications for steel pleasure vessels (in fact the technical representative of one large paint manufacturer was talking to me about their specification for recoating the bottom of a large tug and that was no zinc rich epoxy primer too) - however, if this was the cause one would expect the symptoms to not be limited to repeated reoccurance at the same sites.

So, getting back to preparation, the only preparation suitable for the original coating and for any repairs is sand/grit blasting. Anything else will fail. Then recoat to the selected paint manufacturer's specification (which will include blasting requirement, recoat times, film thickness, etc) - if one is not competent then get it professionally sprayed so that can be sure that the required film thicknesses are reached. Wire brushing, sanding and even grinding are a waste of time, but if not possible to blast for any good reason then grinding is the best of the worst alternatives. Rust killers, etc only hide the problem for a while.

We have a similar problem on our own steel yacht (but not nearly so bad and bubbles are never broken) and occurs we have determined where when the bottom was being faired (the boat is fully faired) in places the fairing (sanding) board went through the fairing and epoxy primer to the steel taking the blast "key" off those places. Our boat only gets lifted every 2 years for antifouling and each time these minor areas needed touching up but not extensive enuff to make it worth getting a blaster in to spot blast. However, last lift there was blasting work going on next door to the boat yard and so they grabbed the boat in the travelift and delivered it to the blaster /forums/images/graemlins/smile.gif and got the troublesome areas spot blasted - will be interesting to see how it is next lift.

Trust that has given some ideas. A useful resort is the paint companies are usually very helpful in giving advice and also may have a specialist visit the boat if one is handy in order to prepare a specification (well they are here).

John

 

[cliff]

[ QUOTE ]
So, getting back to preparation, the only preparation suitable for the original coating and for any repairs is sand/grit blasting. Anything else will fail. Then recoat to the selected paint manufacturer's specification (which will include blasting requirement, recoat times, film thickness, etc) - if one is not competent then get it professionally sprayed so that can be sure that the required film thicknesses are reached. Wire brushing, sanding and even grinding are a waste of time, but if not possible to blast for any good reason then grinding is the best of the worst alternatives. Rust killers, etc only hide the problem for a while.

[/ QUOTE ]At last, the voice of experience/knowledge. The correct way to prepare the surface is to grit blast the surface to a minimum SA2½, preferably SA3, and apply a suitable paint system either by airless spray or roller ensuring atmospheric conditions are within allowances and the film thickness's are maintained - too thick is as bad as too thin.
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[nedmin]

A lot depends on the quality of the steel. if you have poor spec. steel it will have impurities in it and this can cause problems later. fortunately there is that much competition in the steel industry today that they have had to improve specs .Fiat had horrendous problems with Russian steel some years ago.Hope the above is not your prob.

 

[SlowlyButSurely]

"2 or 3 coats of VC tar epoxy"

That's your problem. Not enough paint. In my experience you need at least 5 coats. Personally I have never had very good results with epoxy tar and these days I only use International Interprotect as it seems to give consistently good results and lasts well.

The better the preparation the longer it will last. Grit blasting gives a surface that the epoxy sticks to fantastically well, but if that's not an option you can get a reasonable surface with a wire brush in an angle grinder and it will last 5 years or more. This is what I do for small patches. Eventually you will have more patches than original paint and grit blasting becomes a more economic proposition.

 

[Conachair]

Thanks for that. I can see poor preparation and application causing the problem this time but can't see why it happened before. Blistering was severe, many this size of tennis balls but not so round and only down one side. As far as i'm aware there have been no problems with this before I bought the boat. Pretty sure it happened in south dock, where the boat was for about 7 months, though it was in Milford haven before that for about 3 months. Stray current? My plan now is to get the whole hull grit blasted and repainted, maybe even hire a surveyor to mack sure it's done properly.

Thanks again
Padz - Concachair

 

[Ships_Cat]

If you are going to do the whole hull then my recommendation would be to get a specification from one of the reputable marine/industrial paint suppliers and use one of their approved applicators. If you are going to fair the boat and in any event for the sanding, etc preparation for the final coats you may wish to do that yourself as that is the high labour cost part of the job.

In the countries I have been involved with (which does not include the UK/France so may be different there) if you align yourself with a manufacturer and use an approved applicator you will likely get a guarantee on the coatings and if you are close to one of their technical representatives they will come and inspect the application (including taking thickness measurements or get evidence of them being taken). As I say though, UK/France may be different if that is where the work will be done.

Stray currents wandering around in marinas is largely a myth and will not in any event affect your paint. I have heard it said that significant overprotection of the boat with anodes can cause the paint to fail but have never seen it myself nor have I known of any confirmed case (I work mainly with aluminium boats and they are more heavily protected with anodes than any steel boat and and never a problem with them).

I have heard it said that a severe shore power electrical fault on board leading to high earth currents back through the sea (ie so a failed earth connection between the boat and shore in the shore cable plus a big AC fault to earth on board) can cause paint failure. A temporary installation possibility leading to this could also be a faulty welding setup/earthing if welding carried out on board. But again I have never seen or known of a validated case of this myself and suspect it would not occur.

A common myth, if you have an ssb radio on board and steel hull as the earth, is that the rf currents back through the water will damage the paint. This is rubbish as the paint just acts as a dielectric between the steel and the sea so damage is impossible.

The fact that the problem seems to be mainly on one side could be as simple as when originally painted (or repainted if that has occurred) it was the cold or damp side if done outside, the blaster operator got tired/changed, the spray painter with the primer was slow in following the blaster so the steel had cooled/rust flashed, etc, etc. Perhaps there was a paint repair on this side and preparation was not as it should have been. Lots of possible reasons, but who knows?

The raising of electrical currents, galvanic/anode issues, etc is mainly from the uninitiated who seem to think that steel vessels are fragile in that department and puts the problem into the mystical for them. In the end a properly painted steel vessel needs no anodes at all except perhaps to protect the shafts and propellor (if those are not of quaility materials/selection) as it is protected by the paint (as I have said, we have only one small anode on one side of the keel, which I suspect we could easily do without, and no corrosion issues after 10 years).

John

 

[Conachair]

Thanks very much for comprehensive replys, John. I feel much better about the whole thing now. The side which is bubbling was severly damaged in 1987 hurricane (apparently only boat which didn't sink in Portland harbour) so that may well have a part to play. Think I'll take your advice and get a paint manufacturer involved.
Thanks again, here's looking forward to a shiney rust streak free boat!

Padz - Conachair

 

[Ships_Cat]

No problem Paddy.

It is easy to feel a bit let down over some of these problems when they arise - that is until one sees some of the very expensive structural and other problems that boats built of FRP, etc turn up in the boatyards with and from which metal boats are mostly immune or at least much cheaper to remedy /forums/images/graemlins/smile.gif.

John