Copper based antifouling on bronze?

[Plum]

I have been using copper based antifouling on the Vetus bow-thruster bronze gear housing for 10 years without a problem. I am now fitting a new Vetus thruster, also with a bronze housing, and the instructions say "do not use copper based antifouling" on the housing. Given that copper and bronzes are very close in the galvanic series, why?

 

[MikeBz]

Good question. I know that when Fox's coppercoat a hull they mask off skin fittings and hard-antifoul them separately - but like you I’m curious as to why.

 

[vyv_cox]

Copper metal is anodic to bronze, if galvanic corrosion was to take place the copper would be wasted. Not sure what happens when the copper becomes oxy-chloride but I guess it may no longer be conducting. I suspect they do not really know what they are talking about.

I have tried coppercoating bronze and brass fittings. It falls off almost immediately. The answer is use an etch primer first but this is probably not economically viable. Better to hard antifoul.

 

[Plum]

Copper metal is anodic to bronze, if galvanic corrosion was to take place the copper would be wasted. Not sure what happens when the copper becomes oxy-chloride but I guess it may no longer be conducting. I suspect they do not really know what they are talking about.

I have tried coppercoating bronze and brass fittings. It falls off almost immediately. The answer is use an etch primer first but this is probably not economically viable. Better to hard antifoul.

Thanks Vyv, appreciate your reply but need to check I understand. Are you saying the bronze housing will not suffer corrosion as a result of using a copper based antifouling but the copper based antifouling may detach? Given that the copper based erroding antifouling, recoated every 2 years, on my old/previous bronze housing did not detach during 10 years, sounds like I should continue with that. Maybe Hammrite special metals primer first.

Although a hard non-copper antifouling, in theory, would be better, trying to prepare that and recoating each time in the confines of a 125mm bore bowthruster tunnel with a different antifouling on the rest of the surfaces will not be fun! I am getting a 2 year life from Micron 350, only lifting out every other year, and not sure a hard antifouling will last that long.

 

[jdc]

I have only anecdotal experience of this, but it was serious. I have a bronze foot for the skeg in which the rudder lower bearing is housed. I had copper-coat applied in 2015, prior to which the bronze skeg foot had been stable for 10 years. Only 2 years later (we had applied copper-coat because we were going away for 2 years or longer) on haul-out I was horrified to see that the bolts holding the two halves of the skeg were almost completely eroded away. We removed the copper-coat from the skeg, renewed the bolts and attached zinc anodes to it and the issue hasn't returned.

But in that simple last sentence there was much head scratching by the builder, the yard doing the copper-coat and me, and the effect was dramatic and very real. 10 years later, we renew the zinc anodes every year as they are really much eroded in the months from April - October that we say afloat. Yet we needed no such anodes in the first 10 years. We changed nothing else, and live on a mooring without mains power. So I tentatively conclude that copper-coat can affect bronze.

 

[Tranona]

I have only anecdotal experience of this, but it was serious. I have a bronze foot for the skeg in which the rudder lower bearing is housed. I had copper-coat applied in 2015, prior to which the bronze skeg foot had been stable for 10 years. Only 2 years later (we had applied copper-coat because we were going away for 2 years or longer) on haul-out I was horrified to see that the bolts holding the two halves of the skeg were almost completely eroded away. We removed the copper-coat from the skeg, renewed the bolts and attached zinc anodes to it and the issue hasn't returned.

But in that simple last sentence there was much head scratching by the builder, the yard doing the copper-coat and me, and the effect was dramatic and very real. 10 years later, we renew the zinc anodes every year as they are really much eroded in the months from April - October that we say afloat. Yet we needed no such anodes in the first 10 years. We changed nothing else, and live on a mooring without mains power. So I tentatively conclude that copper-coat can affect bronze.

Not sure why your anodes are wasting. There are different bronze alloys and maybe your new bolts are different from the casting. If they are the same you should not need anodes. It maybe there is still some contact between the copper and the bronze. On my Bavaria which was Coppercoated from new we left a 15cm circle around the aluminium lower bearing housing and the Coopercoat and painted that with Trilux

 

[Neeves]

Paint formulations change, due to advances in the effectiveness and availability of potential ingredients and as result of environmental issues introducing restrictions, but the paint name, sometimes, remains the same or similar.

Dont assume that the paint you used for 10 years, upto 10 years ago, is the same formulation as the similarly named product today.

Trilux contains Cuprous Thiocyanate which is said to be safe on aluminium hulls but other AF use cuprous oxide which can cause galvanic corrosion (according to International). My experience with Trilux as a AF has never been remarkable.

It might be interesting to know if the copper powders in Copper Coat are exactly the same as have been supplied in the past. In the grand scheme of things copper powder usage in Copper Coat is a minor application for copper powder (used in pigments, electronics, powder metallurgy etc) , and other pressures may have impacted the chemistry of some copper powder.

Basically - what experience anyone had 10 years ago may not be a relevant base for comparison, it might be totally accurate but there are too many variables.

Jonathan

 

[vyv_cox]

According to the makers of Coppercoat, each grain of copper is surrounded by epoxy resin, making the coating as a whole non-conducting. Assuming this to be true, and I suppose they have tested it, there can be no galvanic reaction.

Coppercoat cannot be applied over a solvent based primer, although I was told recently that leaving the primer for two weeks after application should be enough. Hammerite and other etch primers are water based, which suggests that Coppercoat should be ok over it.

To jdc: what was the composition of the bolts? The galvanic potential between all of the copper alloys is very small and even between them and copper it is not large. My bronze shoe has been attached to the skeg by stainless steel bolts for 40 years without corrosion. It seems there is something unusual in the case of yours

 

[geem]

According to the makers of Coppercoat, each grain of copper is surrounded by epoxy resin, making the coating as a whole non-conducting. Assuming this to be true, and I suppose they have tested it, there can be no galvanic reaction.

Coppercoat cannot be applied over a solvent based primer, although I was told recently that leaving the primer for two weeks after application should be enough. Hammerite and other etch primers are water based, which suggests that Coppercoat should be ok over it.

To jdc: what was the composition of the bolts? The galvanic potential between all of the copper alloys is very small and even between them and copper it is not large. My bronze shoe has been attached to the skeg by stainless steel bolts for 40 years without corrosion. It seems there is something unusual in the case of yours

Coppercoat advise 5 days for primer VOCs to offgas.
We have just primed our bronze through hulls and bearing housing with two coats of etching epoxy primer, ready for Coppercoat on Tuesday. I am hoping that the primer will assist with adhesion. When I applied Coppercoat direct to the bearing housing last time, it fell of pretty quickly. Interestingly, it stayed on the through hulls far better.

 

[Plum]

We seem to have drifted off from my original posting, which was not about Coppercoat..........

 

[vyv_cox]

We seem to have drifted off from my original posting, which was not about Coppercoat..........

I think the same applies. Copper is anodic to bronze and brass but I don't know how this is applicable to whatever copper compounds may be in antifouling paint. I would guess that they can be considered the same as the metal, as this seems to be the case where application to aluminium is concerned.

 

[jdc]

The bolts holding the bronze skeg were and are made of bronze. So it's a mystery.

In view of the various expert's comments here, I suspect the most likely explanation is coincidence on timing, and something else is causing the galvanic issue. After all, we had various things done at the same time as the first copprcoat.

No corrosion is apparent to the bronze seacocks, nor to the bronze prop (which hardly eats its anodes) so why the skeg? I don't think it's even bonded to 'earth' internally.

 

[Neeves]

According to the makers of Coppercoat, each grain of copper is surrounded by epoxy resin, making the coating as a whole non-conducting. Assuming this to be true, and I suppose they have tested it, there can be no galvanic reaction.

I've seen this quoted previously.

In theory I can see CC's logic.

On a larger scale - imagine a glass jar full of ball bearings or glass marbles, all covered in grease with enough grease to coat all the interstices - would the ball bearings or marbles touch each other? Or even ball bearings in resin.

I tried to think. of a more appropriate example (with a more viscous product than grease) - but did not come up with one, fill in tarmac, coarse fill in concrete....?

But it must depend on how well the copper is mixed into the resin, how the resin/powder is applied and the form, shape, of the powder. The mix is a liquid and if you apply with vigour you. are going to squeeze the liquid such that some particles will touch other particles. The way to minimise the particles touching is to increase the resin content but apply more mix - and don't roll the mix hard. I would assume that, along with testing for conductivity, CC have tuned the mix carefully - are applicators as careful with their mixing.

If you apply conventional AF all the copper compounds sink to the bottom of the tin. Initially the mix is not viscous, at the top of the tin, but becomes more so if you do not constantly mix. Applicators might focus on simple application and don't worry about poorly mixed CC - they think a high concentration of power is good - ignoring the fact a high concentration might cause other issues.

It would be easy to check, simply make up a sample, allow to set off and look at cut sections with a microscope. Easier... try passing an electric current through the applied and hardened coating.

Though the current flow might be tortuous I can imagine current flow to be quite possible, especially over short distances.

It would be easy for owners with CC to test. Test over a range of distances. How much flow would be an issue?

Jonathan

 

[Plum]

As part of my research regarding my original posting, I sent an email to International Paint (akzonobel ). Here is their reply "I would steer toward following Vetus’s advice about not using copper based antifouling on the bronze housing as it can lead to galvanic corrosion and may affect the bearings. Usually, I’d antifoul as far up the tunnel as close to the propeller only. You may end up with some fouling on the housing but this shouldn’t affect the performance of the bow-thruster."