Coppercoat - galvanic reaction around anodes?!

[srm]

This is my boat's area of lost Coppercoat around the sintered bronze cooling plate for the fridge. Width of loss is approx 0.5 metres. Black, grey, and white layers are the epoxy coatings applied over the clear gel coat before the CC. This was at lift out in 2019, 10 years after application. The cooling plate was fitted a couple of years after the CC so mounted on top of it with sealant.

At earlier cleanings I noticed that barnacles had settled on the cooling plate then spread outwards on to the Coppercoat.

Note measurement given in previous post was from memory and too high, have now checked another pic with tape measure across area.

 

[Fr J Hackett]

There is a significant potential difference between the copper in the copper coat and zinc so I would expect the copper to be gradually used up as it forms copper sulphate or carbonate the zinc will then take part in a redox reaction and reddish brown copper will be dropped out of solution some may adhere to the surface but I expect it to be lost as there will be little mechanical adhesion and no chemical adhesion. The reaction will go on as long as the sink can "see" the copper in solution once the copper surrounding the zinc is used there will be nothing to stop marine growth hence the barnacle patches and the circular erosion patch of the copper coat which will eventually reach a point that it stops growing. There should be an accompanying loss of zinc which will match the loss of copper and be relatively small.
That's what I think is happening.

 

[VicS]

There is a significant potential difference between the copper in the copper coat and zinc so I would expect the copper to be gradually used up as it forms copper sulphate or carbonate the zinc will then take part in a redox reaction and reddish brown copper will be dropped out of solution some may adhere to the surface but I expect it to be lost as there will be little mechanical adhesion and no chemical adhesion. The reaction will go on as long as the sink can "see" the copper in solution once the copper surrounding the zinc is used there will be nothing to stop marine growth hence the barnacle patches and the circular erosion patch of the copper coat which will eventually reach a point that it stops growing. There should be an accompanying loss of zinc which will match the loss of copper and be relatively small.
That's what I think is happening.

Surely it will be the zinc, being the less noble metal, which is dissolved

.

 

[Fr J Hackett]

Surely it will be the zinc, being the less noble metal, which is dissolved

.

Yes the copper forms an electrolyte immediately around the zinc then there is a redox reaction with a soluble zinc salt going into solution and copper being deposited out of solution, ie an exchange of copper and zinc taking place but first the copper needs to form an electrolyte hence the gradual loss of copper coat. There will come a point at which this stops governed by the concentration of copper based electrolyte around the zinc.
I think this is the basic chemistry that is occurring, if there was no zinc anode then the copper coat would last much longer.

 

[VicS]

Yes the copper forms an electrolyte immediately around the zinc then there is a redox reaction with a soluble zinc salt going into solution and copper being deposited out of solution, ie an exchange of copper and zinc taking place but first the copper needs to form an electrolyte hence the gradual loss of copper coat. There will come a point at which this stops governed by the concentration of copper based electrolyte around the zinc.
I think this is the basic chemistry that is occurring, if there was no zinc anode then the copper coat would last much longer.

But why is the copper going into solution in the first place or rather why is it going into solution at a greater rale in the vicinity of a zinc anode

Is the normal activity of the zinc anode creating a local environment ( loweed pH ???? ) which is causing the local dissolution of copper.

 

[Fr J Hackett]

But why is the copper going into solution in the first place or rather why is it going into solution at a greater rale in the vicinity of a zinc anode

That's the way that coppercoat will work it will go into solution slowly but in the presence of the zinc its concentration will drop as copper is deposited and zinc replaces it and then more copper will go into solution.

If you take a solution of copper sulphate or chloride then put a zinc rod in it you will see brown black copper deposited on the zinc.Eventually the solution will go colourless and there will be a deposit of brown black copper in the container if the ph is suitable the copper will go into solution and more zinc will react with it. It will stop once no more copper for whatever reason can go into solution. In a dynamic situation like a moving boat the copper will be washed away. If you look at the picture in #21 you can see the dark brown copper that has been deposited left long enough this will either fall away or go into solution as a copper salt and then the merry go round will start with sink replacing it in solution.

 

[Babylon]

Looking at your photos raises the question as to why you need anodes at all. You don't say how the hull anode is connected and what it is protecting, nor how long it has been on. The skeg one is one the heel fitting but that may all be the same material, probably all 316. Presumably you have a feathering propeller with its own anode. How long does that one last?

The hull-anode is wired to the engine earth (nothing else is bonded, i.e. all skin-fittings are isolated). These big pill anodes seem to last years, so the current one is probably something like 4-5yrs old. The skeg-anode lasts for 3yrs or so before needing replacement.

The prop is a Darglow Featherstream, the cone-anode seen below was fitted just over a year ago; I replace this annually, not because that much is eaten away but because enough is eaten away around the mounting bolts that there's a risk it could fall off.

What exactly would happen to the heel-fitting and various skin-fittings (two bronze Blakes forward for the heads, five others in total) if I only retained the prop-anode?

 

[srm]

If you look at the picture in #21 you can see the dark brown copper that has been deposited left long enough this will either fall away or go into solution as a copper salt and then the merry go round will start with sink replacing it in solution.

The picture is of a sintered bronze cooling plate for the fridge. However, the two discs on the plate are small zinc anodes originally about 2.5cm dia x 0.5 cm thick. These lasted 12 years (replaced 2 weeks ago as the spares arrived the day after launching in 2019). Zinc and Coppercoat are separated by the bronze and are not on the same plane as the cooling plate is about 2 cm thick.

The loss of Coppercoat here was significantly more than around the large zinc anode connected to engine and shaft.

 

[Tranona]

You have almost the same as on my boat. I fitted the new engine and Featherstream in September last. Lifted last Thursday and the anode looked just like yours except one of the screws had gone. No hull anode, but planning to fit one. What prompted my question is that I don't think your hull anode is doing anything, It is probably connected to the housing between the engine and the gearbox to provide a path to the shaft and then the prop. Do you have a flexible coupling? and is it bridged? How far is the hull anode away from the prop? It should be close and there should be continuity between the anode and the prop for it to be effective. The slow depletion suggests it is not effective. Suggest you get the surveyor to check continuity from the anode to the prop as in theory it should take some of the load off the prop anode. I was with Darglow on Friday discussing exactly this issue because of the short life of the prop anode and clear recommendation to try a hull anode. Slight complication because I have a flexible Bullflex coupling, but the engineer who installed the engine says he can bridge it to maintain continuity to the shaft.

Your skeg fitting is similar to mine except mine is on the rudder. When I bought the boat it had an anode on it which did not seem to do anything. We stripped the rudder for modifications and decided that as it was all 316 no anode was needed. As you can see from the photo it looks fine after 8 months in the water. Anodes are only needed if there are mixed metals (as in the prop) but if your fitting is stainless with stainless fastenings should be no need for the anode. The pintle on the rudder is also stainless but looks like it is in a Delrin bush so not in contact with the skeg fitting. worth cleaning the fitting to determine the material.

None of this has any effect on your skin fittings which are not connected electrically to anything else - or at least should not be. There was a time when enthusiastic (and ignorant) builders bonded skin fittings into a circuit with an anode, but this is totally unnecessary.

Hope this helps

 

[Babylon]

Hull anode is located as per picture below. There is an electrical bridge between engine and shaft (I'm almost certain the gearbox has a flexible coupling). I suspect the size of the hull anode was due to the fact that all the skin-fittings were in fact originally electrically bonded (boat was built in 1982, I isolated them after re-engining in 2011).

That is a Delrin bush, but I don't recall if there is any bronze in the rudder pintle/gudgeon assembly, or why there is an anode at the base of the skeg (I've asked the owners association, will see what comes back).

Re your prop anode screw: did the screw itself come out? surely it would have had waterproof thread-locker on it?

 

[Fr J Hackett]

The picture is of a sintered bronze cooling plate for the fridge. However, the two discs on the plate are small zinc anodes originally about 2.5cm dia x 0.5 cm thick. These lasted 12 years (replaced 2 weeks ago as the spares arrived the day after launching in 2019). Zinc and Coppercoat are separated by the bronze and are not on the same plane as the cooling plate is about 2 cm thick.

The loss of Coppercoat here was significantly more than around the large zinc anode connected to engine and shaft.

Yes entirely understandable.

 

[Tranona]

Hull anode is located as per picture below. There is an electrical bridge between engine and shaft (I'm almost certain the gearbox has a flexible coupling). I suspect the size of the hull anode was due to the fact that all the skin-fittings were in fact originally electrically bonded (boat was built in 1982, I isolated them after re-engining in 2011).

That is a Delrin bush, but I don't recall if there is any bronze in the rudder pintle/gudgeon assembly, or why there is an anode at the base of the skeg (I've asked the owners association, will see what comes back).

Re your prop anode screw: did the screw itself come out? surely it would have had waterproof thread-locker on it?

View attachment 158050

The hull anode is as close as you could reasonably get. There is some debate about whether a second anode some distance away is of any value because the prop anode is in direct contact with the propeller so goes first. This seems to be borne out in practice as your anode was almost gone but the hull anode barely touched. My view (not really supported by any evidence) is that should the prop anode go completely or fall off, the hull anode will come into play. On my old Eventide which had an earlier version of the Featherstream with bronze blades, the anode was a bit smaller but used to last at least a year. I had removed the hull anode as it did not seem to be doing anything except for causing problems with the wood hull around the mounting holes.

I did not attach the prop or anode so not sure if thread locker was used. The area around the hole where the screw would have ben had gone completely

 

[Fr J Hackett]

The hull anode is as close as you could reasonably get. There is some debate about whether a second anode some distance away is of any value because the prop anode is in direct contact with the propeller so goes first. This seems to be borne out in practice as your anode was almost gone but the hull anode barely touched. My view (not really supported by any evidence) is that should the prop anode go completely or fall off, the hull anode will come into play. On my old Eventide which had an earlier version of the Featherstream with bronze blades, the anode was a bit smaller but used to last at least a year. I had removed the hull anode as it did not seem to be doing anything except for causing problems with the wood hull around the mounting holes.

I did not attach the prop or anode so not sure if thread locker was used. The area around the hole where the screw would have ben had gone completely

Given the distance between prop and the 2nd anode unless it was directly connected to the shaft and prop I doubt that it would be efficient and ther would be some corrosion of the prop.

 

[Tranona]

Given the distance between prop and the 2nd anode unless it was directly connected to the shaft and prop I doubt that it would be efficient and ther would be some corrosion of the prop.

Babylon said clearly that it is. If it was not then it would not do anything at all. The requirement is that the prop should be able to "see" - that is direct line through the water, the closer the better and connected electrically. Not easy to achieve with a hull shaped like this compared, say with a modern flat bottom and prop/shaft on a P bracket.

 

[RobbieW]

The prop is a Darglow Featherstream, the cone-anode seen below was fitted just over a year ago; I replace this annually, not because that much is eaten away but because enough is eaten away around the mounting bolts that there's a risk it could fall off...

How to make the anode last a bit longer without falling off...

Available from your wife/mistress/lovers make up box or any good Boots (Mercadona for this Hot Pink)

 

[VicS]

How to make the anode last a bit longer without falling off.

Available from your wife/mistress/lovers make up box or any good Boots (Mercadona for this Hot Pink)

Lipstick ?

.

 

[Babylon]

Now had another chat with the GRP guy at the yard. He says CC also told him directly that it was 'calcification' but didn't elaborate as to why. His view is that it could only have happened if there were stray electrical currents in the vicinity. Normally there never are (I'm on a mid-river mooring) but the only difference between this winter and all the previous ones is that from sometime last November until this February I'd mistakenly left the isolator switches for both battery banks on (which resulted in my old but otherwise good condition batteries from circa 2010 draining to death despite wind-gen charging). Notwithstanding that it is only the hull anode and not the skeg one that is bonded to the engine earth, the fact is that there were for the first time ever 'stray currents' in the vicinity - and I'm sure that it is just this that triggered and then exacerbated the spread of the calcium around both anodes.

 

[RobbieW]

Lipstick ?

.

Close, nail varnish

 

[Babylon]

How to make the anode last a bit longer without falling off...

Available from your wife/mistress/lovers make up box or any good Boots (Mercadona for this Hot Pink)

How did you know I had a mistress?!

Interesting that you're using a nylon screw instead of a s/s one - but what exactly is the hot pink bit?

I use a standard s/s one, with waterproof Loctite. I used to line the hole in the anode with sikaflex or similar, but didn't really notice any difference.

 

[RobbieW]

Interesting that you're using a nylon screw instead of a s/s one - but what exactly is the hot pink bit?

Nylon screws are what Bruntons and OEMs supply in the baggy with the anode, they feel slightly oversize when screwed in and provide a good bite on the thread. The pink stuff is nail varnish