Mixing zinc and aluminium anodes

[BelleSerene]

This month’s PBO includes a fascinating article on the metals we can use for anodes. Apparently zinc is less effective than aluminium for the purpose, and we’re all so used to it simply because it has so far been a cheaper metal. No more: the price of zinc has doubled of late, so aluminium anodes are now both cheaper and more effective than zinc ones.

The article concludes, though, with Nigel Calder warning us that ‘you can’t mix zinc and aluminium in the same system’.

Does that mean, then, that if my folding prop manufacturer is only selling zinc prop anodes, I can’t replace my large hull anode with an aluminium one?

 

[lw395]

Aluminium outboards are protected by zinc anodes.
So connecting a big aluminium 'anode' to your little prop anode would have it trying to protect a large lump of bare metal.
The prop anode would have a very short life.

 

[VicS]

Aluminium outboards are protected by zinc anodes.
So connecting a big aluminium 'anode' to your little prop anode would have it trying to protect a large lump of bare metal.
The prop anode would have a very short life.

Sorry your logical thinking is floored by the fact that aluminium anodes are in fact a special aluminium-zinc- indium alloy which makes them slightly more anodic than zinc anodes (-1.09 volts compared with -1-05 volts) This means that an aluminium anode will in fact protect a zinc anode all other things being equal. In practice a zinc prop anode will probably have about the same life as it would without the hull anode, being in direct contact with the prop, its life will certainly not be shortened by an aluminium hull anode.

Aluminium alloys in general are less anodic than zinc anodes ( -1.0 to -0.75 volts ) hence the reason than a zinc anode will protect an aluminium alloy outboard or sail drive leg.

 

[Quandary]

I keep my boat in fresh water though it goes to sea for about 6-7 weeks of the 26 which it is afloat, it has a VP sail drive leg for which zinc anodes are the only type available, the new Flexofold folding prop is available with either metal so I asked Darglow which I should use, they said aluminium should be fine. When the boat came out yesterday the zinc on the leg seemed fairly clean but the aluminium cone on the bronze prop. seemed quite coated, the reverse of what I expected.
So with a prop that cost almost £2k. I am very interested in the answer to this question.

 

[noelex]

Zinc anodes do not work in fresh water. They develop a coating that effectively insulates the anode, preventing it from working.

If you mix zinc and aluminium anodes the aluminium anode will corrode reasonably rapidly, but this assumes the zinc anode is not coated.

So you are in a Quandary
Mix zinc and aluminium anodes, which is bad or
Use zinc anodes for fresh water, which is bad

I am not sure which is worse of the two evils. Are you sure no one makes an aluminium anode for your sail-drive? Can you perhaps modify an anode designed for another brand of sail-drive to fit?

 

[VicS]

I keep my boat in fresh water though it goes to sea for about 6-7 weeks of the 26 which it is afloat, it has a VP sail drive leg for which zinc anodes are the only type available, the new Flexofold folding prop is available with either metal so I asked Darglow which I should use, they said aluminium should be fine. When the boat came out yesterday the zinc on the leg seemed fairly clean but the aluminium cone on the bronze prop. seemed quite coated, the reverse of what I expected.
So with a prop that cost almost £2k. I am very interested in the answer to this question.

Zinc anodes do not work in fresh water. They develop a coating that effectively insulates the anode, preventing it from working.

If you mix zinc and aluminium anodes the aluminium anode will corrode reasonably rapidly, but this assumes the zinc anode is not coated.

So you are in a Quandary
Mix zinc and aluminium anodes, which is bad or
Use zinc anodes for fresh water, which is bad

I am not sure which is worse of the two evils. Are you sure no one makes an aluminium anode for your sail-drive? Can you perhaps modify an anode designed for another brand of sail-drive to fit?

Aluminium anodes are available:- https://www.solentanodes.co.uk/collections/sail-sterndrive-aluminium-anodes

 

[Tranona]

This is irrelevant for saildrives because the two anodes are independent and not connected electrically.

Calder was referring to bonding systems where more than one anode is connected to the thing you want to protect. For example on a shaft drive boat where there is a hull anode connected to the shaft and, say a shaft anode clamped to the exposed shaft or a prop anode.

 

[noelex]

This is irrelevant for saildrives because the two anodes are independent and not connected electrically.

Yes good point. I had forgotten about the isolation although I am always a little suspicious how effective this is. Has anyone checked on an older saildrive?

However, the saildrive anode would still be better in aluminium if you spend the majority of the time in fresh water and it looks like Vic has found some.

 

[Quandary]

Thanks Vic, I had searched in the past without finding that.
Previously I had attempted to resolve the problem by using a zinc hanging anode that I could pull up and clean regularly, wired back to the top of the leg, together with zinc anodes on leg and prop. That was for a VP folder with those little segmented anodes around the hub, which degraded rapidly but always seem to come out with a clean surface, the Flexofold cone anode at the back of the prop. is much bigger but does not seem to self clean in the same way, possibly because it is not as exposed to the washing machine effect of rotation?
When I bought the boat it had lived in seawater, had a bronze VP two blade without any prop anodes and no obvious degradation, so perhaps props. do not need anodes anyway?
The first boat I kept in the canal (Sigma 38) had a bronze folder on a conventional ss shaft I fitted with a pair of zinc shaft anodes ( they lasted two years, so one new and one used as backup in case of loss) and I never experienced any visible deterioration.

 

[john0740]

Yes good point. I had forgotten about the isolation although I am always a little suspicious how effective this is. Has anyone checked on an older saildrive?

My boat has a saildrive that's >20 years old and isolation is fine. There is a rubber grommet on the throttle lever on mine that was showing signs of wear, but easily replaced.

 

[noelex]

I think you are refering to isolation between the engine block and the sail-drive leg. This is is important because the engine is usually connected to battery negative.

However, for a different type of anode to be used on the prop and aluminium sail drive shell the prop and aluminium shell need to be electrically isolated from each other, which must be more difficult to achieve. Even if the sail-drive is isolated from the engine you cannot fit an aluminium anode on the prop and a zinc anode on the sail-drive itself if the prop is electrically connected to sail-drive leg.

It is important because the aluminium sail-drive leg is only a relativly small mass and is normally one of the six series aluminium’s, which are are not as corrosion resistant as some of the grades of aluminium that are available. Connect a bronze prop to an aluminium casing an immerse to two metals in seawater and there is the potential for a great deal of corrosion if the details are not correct.

 

[VicS]

My boat has a saildrive that's >20 years old and isolation is fine. There is a rubber grommet on the throttle lever on mine that was showing signs of wear, but easily replaced.

The insulation that is relevant to electrical separation of the saildrive leg anode from the prop anode is the insulating prop hub that insulates. the whole prop from the shaft.

I guess your installation is one in which the saildrive is insulated from the DC negative, either by insulation of the sail drive from the engine or by having an isolated DC system ( ie one that does not use the engine block as the common negative return) The grommet on the throttle lever presumably forms part of that isolation.

 

[Tranona]

I think you are refering to isolation between the engine block and the sail-drive leg. This is is important because the engine is usually connected to battery negative.

However, for a different type of anode to be used on the prop and aluminium sail drive shell the prop and aluminium shell need to be electrically isolated from each other, which must be much more difficult to achieve. Even if the sail-drive is isolated from the engine you cannot fit an aluminium anode on the prop and a zinc anode on the sail-drive itself if the prop is electrically connected to sail-drive leg.

The prop is isolated by the rubber bush in the hub and a Delrin washer behind the cone on the shaft that locks the prop on.

Important to remember that an "aluminium" is actually an alloy with a zinc content to ensure the different potential from the alloy used in the saildrive housing as VicS explained earlier.

 

[MM5AHO]

"... No more: the price of zinc has doubled of late, so aluminium anodes are now both cheaper and more effective than zinc ones. "

The price of zinc has not doubled of late.! It did rise during the previous 2 years, then started falling again.
In fact it has reduced markedly, to the extent the galvanizing got cheaper by 3-5% recently (depending which galvanizer you go to)