Coppercoat - galvanic reaction around anodes?!

[vyv_cox]

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.

Bruntons have been supplying nylon screws with theirs for many years, more than 10. Overcomes all the galvanic problems that lead to anodes falling off.

 

[Ammonite]

Bruntons have been supplying nylon screws with theirs for many years, more than 10. Overcomes all the galvanic problems that lead to anodes falling off.

These look suspiciously like stainless steel to me, as are the ones they sold me
APS-H5 Saildrive Anode Kit Saltwater (Zinc)
I just put a dab of promocon and trilux on the area in the immediate vicinity of the screw head. Works fine.

 

[vyv_cox]

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.

I'm having real problems with this explanation. In the first place there is no free copper, AMC state that each grain is isolated by epoxy, so effectively that it can be used on aluminium saildrives.

But far more than that, the explanation completely ignores the galvanic series. If it was true we would not have dezincification, it would be decopperification, which as far as I know does not exist. Assuming the two metals are in contact and immersed in the same solution, not two different ones as in a galvanic cell, the zinc will corrode to protect the copper.

I feel certain there is a chemical explanation for the deposition of calcite around the zinc but I cannot believe this is it. A term called 'burnback' is described on USA literature as a condition associated with excessive numbers of anodes, which does not seem to be the case here. The high level of protection inhibits the release of copper ions, and thus fouling occurs. I am not quite sure about this one either but apparently some paint companies subscribe to it. How Much Sacrificial Zinc Does Your Boat Need? | Yacht Management South Florida

 

[vyv_cox]

These look suspiciously like stainless steel to me, as are the ones they sold me
APS-H5 Saildrive Anode Kit Saltwater (Zinc)
I just put a dab of trilux on the area in the immediate vicinity of the screw head. Works fine.

I agree, they look like stainless steel but all the ones I have bought since the very first one in 1998 have come with nylon screws. Like these. Aluminium Anode Kit for H5 Autoprop - Anodes Direct - Bruntons Anodes

 

[RobbieW]

These look suspiciously like stainless steel to me, as are the ones they sold me
APS-H5 Saildrive Anode Kit Saltwater (Zinc)
I just put a dab of promocon and trilux on the area in the immediate vicinity of the screw head. Works fine.

Looks like they are different screws for a saildrive (APS), tthe H5 screws are shown as nylon

 

[Ammonite]

Looks like they are different screws for a saildrive (APS), tthe H5 screws are shown as nylon

Yes. I just spotted that as well.
H5 Anode Kit Saltwater (Zinc)
The APS version is a lot more substantial than the hollow standard H5 version. Maybe that's why? The link in Vyv's post was the freshwater variety but standard saltwater H5 has plastic screws as you say.

 

[vyv_cox]

Yes. I just spotted that as well.
H5 Anode Kit Saltwater (Zinc)
The APS version is a lot more substantial than the hollow standard H5 version. Maybe that's why? The link in Vyv's post was the freshwater variety but standard saltwater H5 has plastic screws as you say.

Bruntons seem not to have caught up with anode developments. These days we use aluminium anodes for all waters. I have written several articles on the subject. Anodes

 

[Ammonite]

Bruntons seem not to have caught up with anode developments. These days we use aluminium anodes for all waters. I have written several articles on the subject. Anodes

Yes, I've seen that one. It was very helpful and I was just using their terminology. However, I think I read somewhere that aluminium anodes should be replaced when 30% worn rather than 50% (was that one of yours and is that specific to saltwater use?) in which case maybe Bruntons are being a little cautious as the APS autoprop consumes anodes at a prestigious rate which is presumably why there's more meat to them.

 

[Babylon]

[...] 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).

I've got the answer: there is bronze down there, hence the need for an anode.

 

[vyv_cox]

I've got the answer: there is bronze down there, hence the need for an anode.

Bronze resists corrosion very well, e.g cannon 400 years old pulled from shipwrecks. These probably had 10% tin whereas modern alloys may only have 2 or 4 %. Even these should not need an anode.

 

[Babylon]

Contrary to what I said two post above, it appears that there might in fact be no bronze at all below the waterline on my boat. Earlier versions were all bronze, but my later-production one seems to be all stainless, certainly above the waterline, and this photo of the heel-plate (which I've not quite yet finished scraping in preparation for fresh epoxy primer and CC) shows that it is clearly stainless.

To skew things further, an owner with an earlier version of my boat, i.e. all-bronze gudgeons and pintles etc, says that his boat only has an anode on the prop-shaft, no-where else... and the boat has done two circumnavigations!

So, one could probably just get away with just the prop anode (?) but, as that would eat away too fast for comfort, I'm going to stick with the hull and skeg anodes as in the past. Overkill maybe, but that's how its always been and I only had the calcification problem last winter because (I think) I left the battery banks switched on when leaving the mooring and that meant stray electric currents for a few months. I also need, when the new CC is patched on, to lightly abrade the whole of the underwater parts to reveal fresh copper, as its already 8yrs old.

Re removing the calcification, brick cleaner certainly did the job fast! (Note that I didn't use brick-cleaner on the prop, as I didn't want acid all over the mechanism - just old-fashioned hand scraping and rotary-brush.)

 

[Tranona]

The hull anode is not connected to the skeg heel fitting so it will not be affected. An anode has to be connected directly to the metal it is protecting and the item being protected has to be connected to another metal of higher potential. So your hull anode is connected to the shaft/propeller assembly which is a mixture of stainless and bronze. It is protecting the bronze, as is the anode on the prop itself. The latter goes first because it is closer and the hull anode a little bit. If the prop anode goes the hull anode will go more, but because of the distance away it is not as effective as the direct prop anode.

Your heel fitting is not connected to anything but GRP and as it is all stainless, including the fastenings does not need an anode. Likewise all bronze assemblies do not need anodes. On my Eventide I made a bronze heel fitting with bronze bolts and a Delrin bush which the stainless pintle on the end of the rudder stock fitted in so isolating it from the bronze. No anode. Yours is all stainless with a Delrin bush - no need for anode.

 

[vyv_cox]

The bronze rudder shoe on my Sadler is 40 years old, no anode, no corrosion. Photo of it on my website. Sadler 34 and 32 rudder bearings

It is attached with stainless bolts. They are same age, no corrosion.

 

[Petronella]

We have a very similar issue. 50cm diameter of eroded(?) Coppercoat around the original, bonded starboard (only) bronze through hulls. The through hulls appear in perfect condition.

 

[Fr J Hackett]

To reply to vic it does not require the copper coat and the zinc to be in physical contact or even the zinc to be connected electrically to the boats systems. The copper particles will go into solution once in salt water and form a weak electrolytic solution the zinc will sense this and displace copper ions which will then be deposited as copper. This will fall away as there will be no strong physical bond to the hull. It is a simple redox reaction and will continue as long as the zinc can chemically sense the copper ions in solution which will be highly localised. Once the copper is exhausted the small area will be subject to normal fouling.

 

[Whaup367]

To reply to vic it does not require the copper coat and the zinc to be in physical contact or even the zinc to be connected electrically to the boats systems. The copper particles will go into solution once in salt water and form a weak electrolytic solution the zinc will sense this and displace copper ions which will then be deposited as copper. This will fall away as there will be no strong physical bond to the hull. It is a simple redox reaction and will continue as long as the zinc can chemically sense the copper ions in solution which will be highly localised. Once the copper is exhausted the small area will be subject to normal fouling.

This sounds like exactly what I'm seeing (post 8 above) and I'm wondering what to do about it... I'm currently using an aluminium anode and wondering if this may be exacerbating the problem, due to the slightly higher galvanic activity. Maybe Zinc would be more suitable?

Cheers,
W.

 

[vyv_cox]

This sounds like exactly what I'm seeing (post 8 above) and I'm wondering what to do about it... I'm currently using an aluminium anode and wondering if this may be exacerbating the problem, due to the slightly higher galvanic activity. Maybe Zinc would be more suitable?

Cheers,
W.

Aluminium and zinc anodes have just about exactly the same galvanic potential. Article here Anodes

I cannot comment about its influence on the theory given by Fr J Hackett as I have no knowledge about it.