One for the chemists: Zinc or Magnesium?

nigelmercier

nigelmercier

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Live in Kent, boat in Canary Islands
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I'm planning on having a go at casting a new Zinc anode for my sail drive. I've collected a bunch of old anodes over the years, some from boat yards.

The problem is, some of them may be Mg rather than Zn [Later] or even Al.

I know the melting point of Mg is 650° C, wheras Zn is 419.5° C. However, I don't have any temperature control, I just blast it until it all melts.

I did consider scraping a sliver off each sample and trying to ignite it, but are there any better methods?

By the way, I'd quite like to do a batch. Does anyone know where I can get a few more kg of Zinc?
 
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I think that if you put a drop of vinegar on magnesium it will fizz actively; zinc does not react so quickly.
 
I got some zinc from a local galvaniser. They buy it in small round ingots .
Please let us know how you get on with the casting and PM me if you get really stuck for zinc material.
Martin
 
Magnesium is less than a quarter of the density of zinc.
Weigh each anode then measure its volume in jug of water, calculate density ... ... et voila!
 
You could try burning some fillings in a blow lamp flame to see if there is a difference?

The magnesium should be bright white, I don't know what zinc would be but if there is a difference between the two it doesn't matter what colour it is, they are different.
 
I think:

produce a small quantity of swarf from each piece with a drill.

Dissolve the swarf in some dilute acid...... the relative rates of reaction will, as Sarabande suugests, give a pretty good indication of which is which.

Confirm by adding some dilute sodium hydroxide (caustic soda) solution. Both will produce a white precipitate but the ppt from the zinc will dissolve in excess sodium hydroxide.

Be very wary of heating and setting fire to magnesium. Do not attempt to extinguish burning magnesium with water. It reacts with steam to produce hydrogen!
 
Zinc Anodes

I recently purchased two of these from UK supplier zincsmart, and at around £8 each it hardly seems worth the hassle of trying to make some!
As I have an ambassador rope cutter the anodes need to be modified, which is not difficult with a hacksaw, file and a drill (even with my minimal skills with tools):(

Cheers,

Michael.
 
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Zinc Anodes

Yes Nigel, that is the firm.

I got fed up with paying a high price from the rope cutter manufacturers, who supply an original Volvo anode machined to fit the rope cutter lugs- even comes in the volvo packaging.
The work involved to machine it, as I said earlier, is well within the capabilities of anyone with a few tools and a vice.
The anode in the picture was made in Canada.

Cheers,

Michael.
 
I would have reservations about using mixed all sorts for an anode, contamination could cause the anode to dissolve rapidly.
 
savageseadog said:
I would have reservations about using mixed all sorts for an anode, contamination could cause the anode to dissolve rapidly.
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Absolutely

If the two ( or more) metals do not form a uniform solid solution, resulting in two separate phases being present, then an anode could have a very short life . I could see Mg or a Mg rich phase being dissolved in a similar manner to dezincification of brasses.

Even if the two (or three) are completely soluble in each other I'd not like to predict what the electrode potential of the resulting alloy would be.
 
VicS said:
Absolutely

If the two ( or more) metals do not form a uniform solid solution, resulting in two separate phases being present, then an anode could have a very short life . I could see Mg or a Mg rich phase being dissolved in a similar manner to dezincification of brasses.

Even if the two (or three) are completely soluble in each other I'd not like to predict what the electrode potential of the resulting alloy would be.
Click to expand...

That's confirmed what I always thought. Despite all the economies I've made to my yachting I've never used a cheap third party anodes for my Yanmar saildrive, or been tempted to cast one from the bits I have, it's just too important. I notice that Yanmar are now selling aluminium anodes rather than zinc for seawater use.
 
aquaplane said:
What is the Saildrive made of? If it's aluminium an aluminium anode won't give any protection at all.
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"Aluminium" anodes are an alloy to a US military spec ( Youll find the details on the anode manufacturers websites) crucially containing a small amount of Indium

If you check the electrode potential you will find that it is in fact slightly more negative than that of zinc anodes
 
VicS said:
Absolutely

If the two ( or more) metals do not form a uniform solid solution, resulting in two separate phases being present, then an anode could have a very short life . I could see Mg or a Mg rich phase being dissolved in a similar manner to dezincification of brasses.

Even if the two (or three) are completely soluble in each other I'd not like to predict what the electrode potential of the resulting alloy would be.
Click to expand...

It's a very complex phase diagram with around 7 phases present. There is a diagram, table 9, on page 14 here.
 
VicS said:
"Aluminium" anodes are an alloy to a US military spec ( Youll find the details on the anode manufacturers websites) crucially containing a small amount of Indium

If you check the electrode potential you will find that it is in fact slightly more negative than that of zinc anodes
Click to expand...

Ahhh, so it's not an aluminium anode, it's an anode for aluminium, TFT.
 
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