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galvanic action

50ft semi trad built 5 years ago. we recently took our narrowboat out of the water for Interzone 954 blacking. We discovered water blisters on both swims and on the side which is normally the side which we moor. these are the normal metal bank pilons. we contacted International paints concerning the problem and sent off a sample which they requested. in their opinion it was a galvanic action. the boat is not connected to shore power and no other boat in this area has a mains connection as this is not available on this environment mooring. What can we do to correct this? and also what can we do to prevent it from happening in the future.

Asked by: MUNNS  | 8.08pm, Monday 3 August

WW says:

A quick first question would be: have you anodes fitted (and are they working- so when the boat was docked, do they show obvious wear).
If they are fitted but don't show any degradation, then they are not makng cgood electrical contact with the hull. Also, check that they are magnesium anodes, and not zinc (zinc being used on salt water, as magnesium is too reactive).
The metal pilling is usually zinc-galvanised (or hot-dipped in zinc) so that other, less reactive metals will not corrode (such as steel). If you have magnesium anodes, these should corrode in prefence to your steelwork- however, if they are used up, or insufficient, then the steelwork may corrode near contact with the bank.
It is also worth ensuring that no 12V connections (apart from the starter motor, if at all) have an earth return- ensure that all 12V connections are good inside.
It is also possible that variances in the composition of different steel plates on your own boat can set up electrolytic cells, leading to local pitting. More likely around the waterline, where there might be abrasion.
Hopefully that helps- if in doubt, let us know more information, particularly regards the sacrifical anodes fitted to your boat.

Mark Langley  | 4.35PM, Tuesday 4 August

Interzone 954 is a two pack epoxy paint which should only be used to cover a similar and compatible paint system. If the boat has been painted with a more basic finish like Bitumastic or even Comastic, this should be completely removed by grit blasting before applying the new two pack paint. If you have one of these finishes and are planning to grit blast, that may solve the problem. If you already have a two pack paint on the boat, the problem may be the anodes.
I have a similar two pack paint system on my own boat and, when it is dry docked for maintenance, there is always blistering around the rear anodes and, to a lesser extent, the front ones. I have sought advice on this and some specialists say that this is not unusual. They reckon that the anodes may be to blame and some have suggested dispensing with them. Since the paint is so good at protecting the metal and since the anodes only affect a relatively small area around themselves, I am tempted, but I have not gone as far as removing them yet so I cannot speak from experience.

Graham Booth  | 6.08PM, Tuesday 4 August

Readers say:

Mark Langley
1st question We do have 4 magnesium anodes fitted and they were half eaten away. We have fitted another 4 magnesium anodes near to them.
The metal piling is of mild steel, not galvanised.
We have a Beta 38 engine and the negative is bolted to the engine frame at the point provided by Beta.
The blisters are below water line.
We do not know any of the history of the steel.
To Graham Booth
Our boat was shot blasted by the boat builder 5 years ago, and then Interzone 954 was applied.
We do have 4 new magnesium anodes fitted near the 4 old ones which were half eaten away.

MUNNS  | 11.17AM, Friday 7 August

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