Anode wear again

[fontmell]

Please can someone explain to a simpleton why the anode on my sail drive and additional one over the side connnected to saildrive are both pitted /worn
The boat was re launched lat march motored to berth (10 mins) in marina the batteries all taken out no shore power connected and left till this week when towed for hhaul out again
Thanks for educating me

 

[vyv_cox]

Your saildrive is made of an aluminium alloy that is corroded in seawater. Corrosion can be slowed or arrested by connecting it to a metal that is more anodic on the galvanic series in seawater, most usually zinc. The zinc anode corrodes preferentially, protecting the saildrive.

When you see loss of metal on the anodes you are being assured that the anode is doing its job. The time to worry is if they are not corroded at all.

 

[Spirit (of Glenans)]

+1. And if it is a Volvo Penta installation, (I don't know about other brands), it will be electrically isolated from any other metal on the boat, and the introduction of a cable attached to an "over-the-side" anode is unnecessary and possibly introducing the risk of compromising that isolation.

 

[vyv_cox]

An over the side anode, if connected directly to the saildrive and nothing else, can be a useful means of prolonging the life of the anode on the saildrive.

 

[yotter]

I have an over the side anode connected to a bolt on the saildrive, and as Vyv says, hopefully it will prolong the life of the very expensive saildrive anode.

 

[fontmell]

An over the side anode, if connected directly to the saildrive and nothing else, can be a useful means of prolonging the life of the anode on the saildrive.

Many thanks all the above. I understand the anodes doing its job but please explain simply why the erosion. Is there current in water from somewhere ? Is the pitting just corrosion? I’m just interested and curios. As to why eroding with no batteries or shore power when I hear comment of anodes lasting x years whilst mine last a lot shorter period
Yes overboard anode connected to leg not block thanks your patience

 

[IanCC]

Please can someone explain to a simpleton why the anode on my sail drive and additional one over the side connnected to saildrive are both pitted /worn
The boat was re launched lat march motored to berth (10 mins) in marina the batteries all taken out no shore power connected and left till this week when towed for hhaul out again
Thanks for educating me

My understanding is two dissimilar metals, in proximity, in an electrolyte (sea water) create an electric current. It is the principal on which the wet cell battery is based. Or something like that.

 

[Tranona]

Yes, exactly like that. The aluminium housing is a cathode and the zinc, lower down the scale is the anode and seawater a very weak electrolyte so current flows between them and erodes the anode rather than the housing.

 

[emmalina]

Make a mold buy a blowtorch and make your own thats what I do and plenty or dumped anodes in most boatyards !

 

[VicS]

Make a mold buy a blowtorch and make your own thats what I do and plenty or dumped anodes in most boatyards !

But be aware that zinc anodes are made of a special high purity zinc, usually to a US military specification:

 

[NormanS]

But be aware that zinc anodes are made of a special high purity zinc, usually to a US military specification:
View attachment 146879

In the real world, why do they have to be of "a special high purity zinc"? Bearing in mind that all that's required is for the anode to be less "noble" than the metal being protected. I've been making my own for years, and they perform as expected.
I can understand the US military demanding a particular spec, it makes them feel important, but I very much doubt if it means that their anodes work any better.

 

[VicS]

In the real world, why do they have to be of "a special high purity zinc"? Bearing in mind that all that's required is for the anode to be less "noble" than the metal being protected. I've been making my own for years, and they perform as expected.
I can understand the US military demanding a particular spec, it makes them feel important, but I very much doubt if it means that their anodes work any better.

As I understand it higher levels of some of the impurities listed in the spec , in particular iron, can make the anodes ineffective.

See Use Mil-Spec Zinc Alloy A-18001K

I am not in a position to doubt the validity of the US research , but perhaps you know better

 

[NormanS]

As I understand it higher levels of some of the impurities listed in the spec , in particular iron, can make the anodes ineffective.

See Use Mil-Spec Zinc Alloy A-18001K

I am not in a position to doubt the validity of the US research , but perhaps you know better

I only know better, in that in the real world in which I live, my home made anodes work fine. As they are only required to be less noble than the metals which they are protecting, it's difficult to see why any tiny imperfections in content, matter.
Perhaps you have personal experience to the contrary?

 

[Daydream believer]

Make a mold buy a blowtorch and make your own thats what I do and plenty or dumped anodes in most boatyards !

But be aware that zinc anodes are made of a special high purity zinc, usually to a US military specification:
View attachment 146879

But if emmalina is using old anodes, then one might expect that issue to have been solved already.

I intend to do the same having just had a shock when buying 6 Brunton autoprop 2 hole anodes at nearly £40 each . Less the, "I know the MD", discount of course, . But still stupidly expensive. I use 2 per season plus one saildrive anode. I never intend to buy another new one. He tells me there will be a price increase in the new year.
I was actually quoted £77.7 each plus delivery, by one anode supply company- That was after they applied 15% discount, as I am an RYA gold member

What I would like to know is whether or not re melting affects the material structure? I cannot image that it should.

 

[vyv_cox]

But if emmalina is using old anodes, then one might expect that issue to have been solved already.

I intend to do the same having just had a shock when buying 6 Brunton autoprop 2 hole anodes at nearly £40 each . Less the, "I know the MD", discount of course, . But still stupidly expensive. I use 2 per season plus one saildrive anode. I never intend to buy another new one. He tells me there will be a price increase in the new year.
I was actually quoted £77.7 each plus delivery, by one anode supply company- That was after they applied 15% discount, as I am an RYA gold member

What I would like to know is whether or not re melting affects the material structure? I cannot image that it should.

Remelting a zinc anode should not compromise its effectiveness in the slightest but note the rider shown in VicS post. Very small traces of iron will reduce or even cancel the action of the anode, so melting should not be carried out in a steel vessel.

These days aluminium anodes are becoming more common and of course should not be mixed with zinc ones if remelting.

 

[GHA]

I only know better, in that in the real world in which I live, my home made anodes work fine. As they are only required to be less noble than the metals which they are protecting, it's difficult to see why any tiny imperfections in content, matter.
Perhaps you have personal experience to the contrary?

What voltage does it pull steel down to? I've tried a few anodes against a silver half cell in seawater, one was way less than all the others, probably would still help but it was without doubt different to the others. thrown out.
Been many centuries since humans worked out the looking beyond unreliable human biased senses works far better..

 

[Daydream believer]

These days aluminium anodes are becoming more common and of course should not be mixed with zinc ones if remelting.

Why not? Genuine question.

 

[vyv_cox]

Why not? Genuine question.

Because they are totally different alloys. I am not even certain that zinc and aluminium would form an alloy if melted together but they would certainly not form an alloy with the accurate compositional content required.

 

[NormanS]

I'll just keep on making mine from discarded anodes, and not worry overmuch about possible microscopic impurities. As long as they work, that'll do me.

 

[Daydream believer]

Because they are totally different alloys. I am not even certain that zinc and aluminium would form an alloy if melted together but they would certainly not form an alloy with the accurate compositional content required.

I hope that I am using the right analogy here. I do not want to get into a coppercoat arguement. Only use the principle as I understand it
Lets' work out how they MIGHT interact. Assuming ( & we all know what assumption is )they do actually mix Ok; rather than like oil floating on water, but more like ( using something many of us could realte to) coppercoat, ie where the copper is in suspension with the epoxy.

The zinc in suspension with the aluminium. It should do this in the melting, stirring & pouring process if a single pour for a single anode is done.

Now we can expect the zinc to erode first- In my case I am trying to protect a bronze prop not an aluminium sail drive. Then, having eroded first, the zinc is surrounded in aluminium & stops working. Just like my coppercoat. ie the copper stops working until the epoxy erodes, or I burnish it.
But we have already decided that aluminium works as an anode because it has been stated that suppliers are supplying aluminum anodes. One assumes it does of course work.

Now when the aluminium begins to erode more zinc is exposed ( As is the copper in coppercoat when the epoxy erodes) so the cycle continues.

Now what is wrong with that as a possibility?
I know someone will produce a paper to the contrary- But can we see it please.