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Old 17-03-2014, 13:28   #16
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Re: Casting anodes

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The result will not be 99.99 pur zinc or whatever millspec is, but the nobility of the zinc itself does not change, you'll just has a little less per volume.
Yes and no.

Alloys often have properties which are not a straight sum of properties of ingredients. In this particular case I don't know, but I would love to hear from a metallurgist about a Zinc-Copper alloy structure at a microscopic level.

Mil spec is 99.314% by the way.
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Old 17-03-2014, 13:37   #17
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Re: Casting anodes

The article cited below probably has as good an answer as anyone's:

Mil-Spec Zinc Alloy A-18001K
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Old 17-03-2014, 13:50   #18
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Re: Casting anodes

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No offense, but on what do you base that statement?
Oh gee, that 97.5% zinc will be practically as reactive galvanic wise as 99.3% as the difference is just 1.01%. It would provide the same basic protection, but with a slightly shorter lifetime. Plus I base it on 30+ years in engineering.

A 1% difference in zinc concentration really is not going to effect galvanic voltage potential difference at all in an anode. So the homemade penny zinc would provide about the same protection as one in the marine store, which also is not made to mil spec, for the most part.

BTW the core of the 1983 onward penny is 99.2% zinc with a 0.8% copper electroplate. I'd say pretty darn close.
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Old 17-03-2014, 14:05   #19
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Re: Casting anodes

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...the homemade penny zinc would provide about the same protection as one in the marine store, which also is not made to mil spec, for the most part.
Well, not to be argumentative, but almost all anodes available to sailors in chandleries are mil-spec. Reliance, Sea Shield, Martyr and Camp all use military specifications when producing anodes. You'd be hard pressed to walk into a marine store and find a zinc that wasn't.

But my reluctance to accept your position was based on this:

"(Studies) determined that the maximum allowable iron content for reliable sacrificial zinc Corrosion Control with Sacrificial Zinc Anodes anode performance was 0.0014 percent. (Mil-spec) also limited several other contaminating elements such as copper and silicon, but excessive iron was the main cause of anode passivation. To produce anodes that met the specification, manufactures had to start with the purest grade of zinc available from the smelters (Special High Grade) and not contaminate it while producing the anodes. This was, and is, very difficult to do.

Installing zinc anodes that are not certified to meet the current U.S. Military Specification runs the risk that the anodes will be contaminated and will fail to protect the metals to which they are attached."


Mil-Spec Zinc Alloy A-18001K
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Old 17-03-2014, 14:35   #20
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Re: Casting anodes

Yes, read that manufacturers writeup too before my previous post. Then they show a Non-milspec anode. Wonder where that came from...

You know I've seen lots of materials that indicated that they met a certain specification, not actually meet it. Many made in the far east for example. Lots O pipe and fittings come from china with data sheets indicating it meets a ASTM or mil-spec. Thing is in more then some cases it doesn't.

But the home brewed anode, could be made with pennys. Zinc melts at about 717 degrees F, copper at roughly 1980 degrees F, Iron at 2800 degrees. So the home alchemist could produce a zinc that was pretty darn close to mil spec, just by controlling temperature roughly. I mean we're starting with 99.2% zinc to begin with. How do you think they refine the zinc. I'm betting the iron content in the zinc in penny's is really low. I've yet to see a penny rust.

So I still say it would be easy to make a zinc from penny's. I'll have to start saving up a pound or two and give it a try.
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Old 17-03-2014, 14:59   #21
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Re: Casting anodes

I agree that it is risky to assume that the properties of an alloy are the sum of the properties of the constituents.

Aluminium bronze is a case in point: the addition of aluminium confers properties which are diametrically opposite to what the above assumption (and intuition) would lead you to expect.

And the presence of iron does not necessarily cause an alloy to rust.

It's an oversimplification to say that this is because intermetallic compounds behave differently (sometimes very differently) from the individual elemental metals, but it's a helpful oversimplification.
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Old 17-03-2014, 15:25   #22
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Re: Casting anodes

Not trying to change the subject or anything but -
zinc for salt, magnesium for fresh, aluminium for... brackish is it?

These three metals are pretty close on the galvanic scale IIRC - can it really be that only one of the three will be effective in each water condition?

Or is it rather that zinc is just better than the other two in salt - etc.?

I really am asking because I don't know - not trying to make a point.
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Old 17-03-2014, 15:35   #23
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Re: Casting anodes

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And the presence of iron does not necessarily cause an alloy to rust.
You know I knew that, but now everyone knows that...

Another thought I had was just use aluminum cans to fabricate Aluminum anodes. Take 12 aluminum can lids (roughly 90% of aluminum can metal is in the lid, this from engineers at Budweiser BTW). Run a long bolt with a washer, through the holes in the tops, spacing the lids equally around in a circle about the bolt. Should be a cheap low cost prop end anode. Just call it the twelve beer anode... Or, you can use a microwave to melt the aluminum (or penny's) for sand cast anodes.
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Old 17-03-2014, 15:37   #24
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Re: Casting anodes

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Not trying to change the subject or anything but -
zinc for salt, magnesium for fresh, aluminium for... brackish is it?

These three metals are pretty close on the galvanic scale IIRC - can it really be that only one of the three will be effective in each water condition?

Or is it rather that zinc is just better than the other two in salt - etc.?

I really am asking because I don't know - not trying to make a point.
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Magnesium is over reactive in salt water. So it should only be used in fresh water. Aluminum would work in salt water too.
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Old 17-03-2014, 15:53   #25
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Re: Casting anodes

In any slipway that deals with larger power boats (that I have ever been in) one will find discarded anodes that still contain considerable unconsumed Zn. These are often in the form of rectangular "bricks", and are discarded into the dumpsters in many yards.

If one removes all the biota and the other crap on the outside, one finds a lump of fairly clean Zn which is unchanged in its alloy's constituents. Using that as feed stock and rough temperature control, I don't see why the galvanic properties of home made anodes would not be usable.

IIRC, one of the British mags - Practical Boat Owner most likely - ran an article a few years ago on this very subject. They seemed to think that it was a viable practice.

I've never made anodes, but sure did cast a lot of dive weights using a Coleman stove and coffee cans as my smelter. Done outside and with some care the health hazards seem negligible.

Cheers,

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Old 17-03-2014, 16:02   #26
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Re: Casting anodes

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Originally Posted by bornyesterday View Post
Not trying to change the subject or anything but -
zinc for salt, magnesium for fresh, aluminium for... brackish is it?

These three metals are pretty close on the galvanic scale IIRC - can it really be that only one of the three will be effective in each water condition?

Or is it rather that zinc is just better than the other two in salt - etc.?

I really am asking because I don't know - not trying to make a point.
For once
Another regrettable complication (of the real world vs the textbooks) is that the galvanic series is not the whole story.

It's not quite as useless as buying digital cameras solely on the number of pixels, but the comparison is not completely barmy.

Some combinations should fizz like an Alka-Seltzer, but in practice do not.

And the reason zinc is useless in fresh water is not galvanic, it's because it quickly forms a surface layer which prevents it interacting as the galvanic series predicts.

Pure aluminium is also useless as an anode, in any sort of water, because it forms an impervious protective oxide layer. Traces of zinc and indium in the aluminium prevent this happening; until this discovery, aluminium was not able to be used for anodes.

And trace elements can similarly affect both the surface reactivity and (I think) current density, neither of which the galvanic series captures.
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Old 17-03-2014, 16:31   #27
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Re: Casting anodes

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Anodes (whether they be zinc, aluminum or magnesium) are made of an alloy of metals and the best ones are made to miltary specifications. Will smelting spent anodes to be recast as new provide the proper metal ratios? Who knows? Unless you can duplicate these alloys, you are doing your boat a big disservice by putting homemade anodes on it, IMHO. Zincs are cheap but the result of improperly protecting your boat's metals can be expensive.
I totally agree. Many off-brand anodes contain impurities that cause a galvanic reaction within the anode (they consume themselves). Unless you have a way to measure the position of your homemade anode relative to its position on the Noble scale, you could increase, rather than decrease corrosion.

If you smelt from 40% depleted anodes then 60% of the active ingredient (so to speak) is already gone. Can you replace those missing components at your beach fire ?
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Old 17-03-2014, 16:49   #28
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Re: Casting anodes

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I totally agree. Many off-brand anodes contain impurities that cause a galvanic reaction within the anode (they consume themselves). Unless you have a way to measure the position of your homemade anode relative to its position on the Noble scale, you could increase, rather than decrease corrosion.

If you smelt from 40% depleted anodes then 60% of the active ingredient (so to speak) is already gone. Can you replace those missing components at your beach fire ?
Thats why you would use multiple old anodes and remelt them into newer larger ones.

Actually the Milspec includes trace elements so that the zinc oxide that forms on the surface of the zinc breaks off exposing fresh surface area. Like aluminum a zinc anode needs some impurities to make it more effective.

I guess I just need to try my hand with melting old zincs and then pennys and compare the results over time.
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Old 17-03-2014, 16:50   #29
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Re: Casting anodes

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Thats why you would use multiple old anodes and remelt them into newer larger ones.
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You miss the point. You don't know if all components of the alloy have depleted equally (highly unlikely).
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Old 17-03-2014, 16:58   #30
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Re: Casting anodes

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Originally Posted by Andrew Troup View Post
Another regrettable complication (of the real world vs the textbooks) is that the galvanic series is not the whole story.

It's not quite as useless as buying digital cameras solely on the number of pixels, but the comparison is not completely barmy.

Some combinations should fizz like an Alka-Seltzer, but in practice do not.

And the reason zinc is useless in fresh water is not galvanic, it's because it quickly forms a surface layer which prevents it interacting as the galvanic series predicts.

Pure aluminium is also useless as an anode, in any sort of water, because it forms an impervious protective oxide layer. Traces of zinc and indium in the aluminium prevent this happening; until this discovery, aluminium was not able to be used for anodes.

And trace elements can similarly affect both the surface reactivity and (I think) current density, neither of which the galvanic series captures.
Aluminum cans are of course also an alloy, mostly 3004, 3105 and a few others. Mostly aluminum with some manganese and magnesium, plus other trace elements. I'll have to do a few experiments with the ubiquitous aluminum can.
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