Anode wear again

Daydream believer said:
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 :rolleyes:)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.:oops:

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.
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Sorry, that is metallurgical gibberish. Now that I have looked it up I see that a Zn-Al alloy does exist but it would probably not be anodic to a saildrive leg.
 
Daydream believer said:
I said to a bronze prop.
WHY is it gibberish, or is it just your opinion?
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I think you do not appreciate the difference between an alloy and a mixture. Anode composition is extremely closely controlled, having additions of less than 1% of critical elements. Mixing two closely controlled but totally different alloys together to make an unrecognisable third cannot possibly produce a useful anode.

Not sure what the prop composition has to do with it.
 
vyv_cox said:
I think you do not appreciate the difference between an alloy and a mixture. Anode composition is extremely closely controlled, having additions of less than 1% of critical elements. Mixing two closely controlled but totally different alloys together to make an unrecognisable third cannot possibly produce a useful anode.

Not sure what the prop composition has to do with it.
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An alloy is a mixture of 2 combined elements. Provided, of course they combine. In my example they need not actually combine. As in coppercoat where copper is held in suspension. That is the difference.

But first the 2 elements of zinc & aluminium could well be the correct mix of each. ie the zinc could be old anodes. Then as for the prop I pointed that out because I was not expecting aluminium to protect aluminium to any extent. But aluminium to bronze would be different. So, with due respect, you have not actually given me any hard evidence. Only an opinion. Which so far is unsbstantiated. Which is fair enough. We are all entitled to an opinion. As long as one is prepared to qualify it as such
 
Daydream believer said:
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 :rolleyes:)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.:oops:

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.
Click to expand...
Why make life so complicated? Zinc is zinc, and is easy to work with. "Aluminium" comes in all sorts of alloys, some presumably better as anodes than others. My understanding is that anode manufacturers are turning to "aluminium" because it's cheaper than zinc. If you can use discarded zinc anodes to cast new ones, the cost of materials is zilch.
 
NormanS said:
Why make life so complicated? Zinc is zinc, and is easy to work with. "Aluminium" comes in all sorts of alloys, some presumably better as anodes than others. My understanding is that anode manufacturers are turning to "aluminium" because it's cheaper than zinc. If you can use discarded zinc anodes to cast new ones, the cost of materials is zilch.
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Actually a kilo of zinc is only about £26-00 plus delivery. Lab grade is the same price for half a kilo in small ingots which should melt down easier.
But you are right. Trouble is that when touting for old anodes, one might pick up an aluminium one. How does one know it is not zinc?
 
Daydream believer said:
An alloy is a mixture of 2 combined elements. Provided, of course they combine. In my example they need not actually combine. As in coppercoat where copper is held in suspension. That is the difference.

But first the 2 elements of zinc & aluminium could well be the correct mix of each. ie the zinc could be old anodes. Then as for the prop I pointed that out because I was not expecting aluminium to protect aluminium to any extent. But aluminium to bronze would be different. So, with due respect, you have not actually given me any hard evidence. Only an opinion. Which so far is unsbstantiated. Which is fair enough. We are all entitled to an opinion. As long as one is prepared to qualify it as such
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No it isn't. An alloy and a mixture are just about as different as they could be. And in this case there will be considerably more than two elements, more like eight.

My opinion is backed up by a degree in metallurgy. You do whatever you think best.
 
Daydream believer said:
Actually a kilo of zinc is only about £26-00 plus delivery. Lab grade is the same price for half a kilo in small ingots which should melt down easier.
But you are right. Trouble is that when touting for old anodes, one might pick up an aluminium one. How does one know it is not zinc?
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Daydream believer said:
Actually a kilo of zinc is only about £26-00 plus delivery. Lab grade is the same price for half a kilo in small ingots which should melt down easier.
But you are right. Trouble is that when touting for old anodes, one might pick up an aluminium one. How does one know it is not zinc?
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I don't think you would have any difficulty. Zinc is roughly three times as heavy as aluminium.
 
The minor constituents of alloys are often surprisingly important for the properties. I would not say that the carbon content of steel is irrelevant because it is so minor. I read somewhere that the pretext to the MIL-DTL-18001L was that the US navy experienced that some anodes worked and others did nothing. The investigation of why, resulted in the MIL-DTL-18001L.
I could not found any text explain what the listed impurities do, but I'm sure it is out there somewhere. It is possible that the effect of some impurities is not related to the galvanic reaction, but how hard or adhering the crust of oxide product becomes. An incapsulated zink is useless. Also, if an impurity form a more active compound in the grain boundaries of the zinc, then the anode may crumble to dust in short time.
That said, I think what NormanS suggests, remelting old anodes sound ok. I were considering it myself some years ago, but came to the conclusion that despite the stiff price per kg, it was not worth the hassle. Zink is after all a minor expense in my boat economy, and I have no problem finding other boat related things to fill available time.
 
NormanS said:
Why make life so complicated? Zinc is zinc, and is easy to work with. "Aluminium" comes in all sorts of alloys, some presumably better as anodes than others. My understanding is that anode manufacturers are turning to "aluminium" because it's cheaper than zinc. If you can use discarded zinc anodes to cast new ones, the cost of materials is zilch.
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I always wonder how far I should let my hull anode wear away before replacement and whether there is anything I should do mid life to make them more effective.
 
Never Grumble said:
I always wonder how far I should let my hull anode wear away before replacement and whether there is anything I should do mid life to make them more effective.
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I work on the assumption that if it wears away, say 40% in a year, it'll probably do something similar the next, and act accordingly. If you're getting a white crust on them, scrape it off to present clean metal to the sea.
 
fontmell said:
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
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Some bedtime reading for you for what is a very complex subject ...

Boatkeeper - Original full publication by Terry Johnson.pdf

Corrosion - Stray Current.pdf

Corrosion Zincs & Bonding.pdf

Galvanic Isolators.pdf

Underwater Corrosion Article.pdf

Nigel Calder also provides an excellent explanation of electrolytic and galvanic corrosion in his excellent Boatowner's Mechanical & Electrical Manual (~£45 very well spent IMHO).
 
superheat6k said:
Some bedtime reading for you for what is a very complex subject ...

Boatkeeper - Original full publication by Terry Johnson.pdf

Corrosion - Stray Current.pdf

Corrosion Zincs & Bonding.pdf

Galvanic Isolators.pdf

Underwater Corrosion Article.pdf

Nigel Calder also provides an excellent explanation of electrolytic and galvanic corrosion in his excellent Boatowner's Mechanical & Electrical Manual (~£45 very well spent IMHO).
Click to expand...
I recommend the third article, Corrosion Zincs and Bonding. Thoroughly comprehensive and accurate. My only criticism is use of the words 'cavitation corrosion'. Cavitation has nothing to do with corrosion, it is entirely mechanical.
 
As an aside this press release might be of interest - and merit comment

Propspeed partners with CMP Group Ltd for international distribution

Part of the release mentions that Prop Speed reduces anode consumption by isolating the metals being protected. I wonder thus why the partnership as Prop Speed would appear to reduce sales of anodes ?? :)

I've never used a CMP anode, a Martyr Anode, and am ambivalent of Prop Speed - its difficult to determine if it offers any lifespan advantage over Velox (both are expensive and both require care in application). I note that in Europe Prop Speed can only be applied by a qualified applicator (this restriction seems to have been removed in other markets).

Jonathan.
 
vyv_cox said:
I recommend the third article, Corrosion Zincs and Bonding. Thoroughly comprehensive and accurate. My only criticism is use of the words 'cavitation corrosion'. Cavitation has nothing to do with corrosion, it is entirely mechanical.
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My understanding of the term is that it refers to chemical corrosion at a site where the protective oxide film has ben removed by the effects of cavitation.

We some times used the term "erosion corrosion " where corrosion ( of say a condenser tube) occurred at he site of a partial obstruction ( often a shell) as a result of the increased water velocity having eroded the surface film.
 
VicS said:
My understanding of the term is that it refers to chemical corrosion at a site where the protective oxide film has ben removed by the effects of cavitation.

We some times used the term "erosion corrosion " where corrosion ( of say a condenser tube) occurred at he site of a partial obstruction ( often a shell) as a result of the increased water velocity having eroded the surface film.
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Yes, that is a known combination but the article does not use it in that way.
 
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