Anodes on bronze

[ripvan1]

VicS in case you didn't read query t'other day, any info gratefully received



Quote:
Originally Posted by VicS
It is not now considered correct or wise to bond everything underwater in this fashion. Only items that require the protection of anodes should be bonded to them.

Bronze and DZR through hulls do not need the protection of anodes and should not be bonded to anything, or even to each other.
Interconnecting underwater fittings can provide a low resistance path through the vessel for stray currents in the water.

Anything going pink is doing so due to dezincification. Bronze does not suffer from this because it does not contain zinc. It is something that only happens to brass.
Anything showing signs of zincification is therefore brass is best replaced with bronze or at least with dezincification resistant (DZR) brass if the effect is more than surface deep

Be aware however that there is now a trend for builders to fit brass through hulls with a life expectancy of 5 years!


Vic - that's very interesting - I have all bronze underwater fittings, prop and shaft, p/bracket, rudder and shaft and 1 through hull.

I'm about to launch soon and have been asking around about anoding and general consensus is that I need to employ some. (I bought the boat as resto project and out of the water - she was previously only in freshwater I believe, so no anodes present)

Can you point me in the direction of some literature that backs up your views please?

 

[vyv_cox]

Not too sure about the literature but what Vic says is perfectly correct. Bronze does not need anodes as it is extremely corrosion resistant in itself. Witness the thousands of Blakes seacocks that are all over the world, almost none bonded and some 35 years plus in service. Older ones such as those were bronze, a gunmetal type. For about 25 years they have been DZR.

 

[VicS]

Sorry I must have missed that.

Vyv has said pretty much what i would have said.

As for documentary support one of the recommendations from the MAIB in their report on the near sinking of the F.V Random Harvest, due to a failed through hull fitting, a few years ago was that the seacocks etc should not be bonded to the anodes. It was considered to be a contributing factor in this incident, although only one of several it has to be said.

If your prop is bronze and is on a bronze shaft then since there are no dissimilar metals involved, only a single corrosion resistant one, there should be no problem there. Your "P" bracket presumably is isolated from every thing so no problem with it.

The same applies to your rudder and shaft if both are bronze.

If you start fitting anodes you will find that they will waste away quickly... simply because they are connected to large surface areas of bronze
and the two are about 0.7 volt apart in the galvanic series.
People will tell you that the anodes must be working and "doing their" job if they are wasting away. Sadly not the case.

 

[ripvan1]

Thanks V & V, a comment made to me at the marina was that bronze was an alloy and that as such with dissimilar metals electrolysis would occur without an anode protecting it (this was after he'd lost a prop to e/sis).

I prefer your explanation tho'

Thanks again

 

[VicS]

Thanks V & V, a comment made to me at the marina was that bronze was an alloy and that as such with dissimilar metals electrolysis would occur without an anode protecting it (this was after he'd lost a prop to e/sis).

I prefer your explanation tho'

Thanks again

Theres no difference in potential on the galvanic series between copper and tin so even tin and copper in contact wont result in any galvanic activity.

Further more the bronze alloys with which we are concerned are a single phase as you will see from the copper tin phase diagram, so not even any potentaial differnce possible between different phases.

Electrolysis is a different kettle of fish. It implies that the electrochemistry is being driven by an externally applied emf rather than that resulting from dissimilar metals.

This leads to an interesting case on these forums not too long ago in which a forum member's boat sank on its moorings due to electrolysis of a skin fitting which was the result of the skin fitting having been bonded to the negative connection of a bilge pump ( presumably, but incorrectly, as a means of connecting it to the anodes). There must have been a bad connection in the negative supply to the pump, which was known not to work properly, resulting in a return path to battery negative via the water and the stern gear and anodes.

Another reason to steer clear of unnecessary bonding.

i

 

[mitiempo]

Proper bronze through hulls/seacocks are made of 85-5-5-5 bronze. The maximum zinc content is 5% and it requires closer to 10% before dezincification is an issue. Bonding these is not required and can invite issues even.

Propellers on the other hand are most often manganese bronze which is very prone to dezincification as the zinc content is about 39%. Even if the shaft is bronze I would install a zinc as the shaft is a very different alloy than the propeller.

 

[vyv_cox]

Thanks V & V, a comment made to me at the marina was that bronze was an alloy and that as such with dissimilar metals electrolysis would occur without an anode protecting it (this was after he'd lost a prop to e/sis).

I prefer your explanation tho'

Thanks again

Your 'commenter' is confused about the difference between an alloy and a mixture. An alloy consists of phases that are solid solutions. There may be one phase or more. In a single phase alloy there are no dissimilar metals to react together. In some fairly obscure alloys one phase can be galvanic by comparison with another but these are rare.

However, as has already been said, propellers are a different case. Although made from manganese bronze, this material is actually not a bronze at all, but a modified brass.

 

[ripvan1]

thanks all, so a shaft anode would be a wise precaution ? excuse my ignorance but does this go inside or outside the boat and do i need to do any wiring or other stuff

 

[vyv_cox]

thanks all, so a shaft anode would be a wise precaution ? excuse my ignorance but does this go inside or outside the boat and do i need to do any wiring or other stuff

Yes, you need an anode to protect dissimilar metal reactions in seawater. In your case it sounds like everything is pretty high quality so it is possible that the prop is a real bronze. Even so, a shaft anode will do no harm and may well do good. Put it fairly close to the P-bracket, leaving a gap of about 12mm to allow cooling of the cutless bearing. Putting it further away can cause imbalance in the shaft, which will vibrate badly.

When fitting the anode do the two bolts up as tightly as possible, then bang the two halves together with a hammer in each hand and tighten again. Keep doing it until the bolts tighten no more. This will avoid the likely problem of the anode loosening at a later date.