Black art of anode chemistry … calling all metallurgists.

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Albert Ross

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In short .. my previous two Autoprop anodes (about £27+ each) lasted only a couple of months each. (No, no shore connection etc. and we were on a swing mooring). So I thought I’d add a hull anode (Yes, I checked for electrical continuity from anode to engine to shaft to prop .. all good).

Anyway, I fitted a new zinc prop anode and the afore mentioned hull electrode (which was aluminium) and was advised that the little zinc anode on the prop would inevitably fizz away as it tried to protect the aluminium hull anode that was only about a metre away.

Everyone I asked said “you mustn’t fit dissimilar metal anodes”. “ Zinc is less noble than aluminium and will be electrolysed first.”

Anyway, ten months later we lifted the boat. The prop anode (Zn) was not only still there, but was hardly spent at all. The aluminium anode on the hull was however, well corroded and about 50% used up.

Zinc is higher on the galvanic series than aluminium and should therefore have been used up first.

However, aluminium is higher (more reactive) than zinc on the electrochemical series (in seawater).

Why did my aluminium anode appear to protect the zinc one?

A nice man in the Chandlery at Troon Marina just said “anodes .. it’s a black art”.

What is going on?

P1020534.jpg
 
Sorry not a metallurgist

But the answer to your questions is a bit of simple electrochemistry... not black art

Aluminium anodes are not pure aluminium they are an alloy containing zinc and crucially a small amount of indium, usually to a US military specification.

The aluminium alloy is a bit higher in the galvanic series than zinc so a bit more active.

-1.09 Volts compared with - 1.05 Volts (relative to Ag/AgCI)

You should not mix aluminium and zinc anodes

BTW ... Chemist
 
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Electrochemical series is not an 'absolute'. Depends on so many factors... But then again I always thought aluminum is less noble than zinc (may be wrong at this). But aluminium which is in normal use is never just aluminium for this very reason, it's most often some alloy of bigger corrosion resistance - because plain aluminium would perish...
Then zinc anode also is not just zinc, it's an alloy that works better since normal zinc would form a protective layer in itself.
So unless you measure the actual potential of this very piece of metal, you never know.

Yup, checked base potential: Al -1,6; Zn -0,8
 
Thank you indeed VicS.
This now makes sense.

Except .. why shouldn't we mix aluminium and Zinc anodes?

Using Aluminium (with indium) to slow down the loss of the Zn on the prop was exactly what I was trying to achieve (even though I didnt understand the galvanic series properly and didnt know about the Indium).

The Al anode was £8 and the expensive Zn one has survived to last another season.
 
flat1 said:
Thank you indeed VicS.
This now makes sense.

Except .. why shouldn't we mix aluminium and Zinc anodes?

Using Aluminium (with indium) to slow down the loss of the Zn on the prop was exactly what I was trying to achieve (even though I didnt understand the galvanic series properly and didnt know about the Indium).

The Al anode was £8 and the expensive Zn one has survived to last another season.
Click to expand...

You can happily mix them, it just that the more reactive one will always waste first.
 
Thanks Davew,
That's what I was thinking till I had a look at an Aluminium Hulled Ovni at the boat show and found it to be fitted with Zinc Anodes. The salesman said the Zinc was to protect the Aluminium hull, but I suspect that the Aluminium hull will end up protecting the Zn anodes! It certainly will if the metals are the same alloys as in my different anodes.
 
flat1 said:
Thank you indeed VicS.
This now makes sense.

Except .. why shouldn't we mix aluminium and Zinc anodes?

Using Aluminium (with indium) to slow down the loss of the Zn on the prop was exactly what I was trying to achieve (even though I didnt understand the galvanic series properly and didnt know about the Indium).

The Al anode was £8 and the expensive Zn one has survived to last another season.
Click to expand...
Be careful as the Ali anode can make the zinc one sluggish so make sure you abrade the zinc one when the Ali has gone - but better still - don't mix them.
 
Rossynant said:
Electrochemical series is not an 'absolute'. Depends on so many factors... But then again I always thought aluminum is less noble than zinc (may be wrong at this). But aluminium which is in normal use is never just aluminium for this very reason, it's most often some alloy of bigger corrosion resistance - because plain aluminium would perish...
Then zinc anode also is not just zinc, it's an alloy that works better since normal zinc would form a protective layer in itself.
So unless you measure the actual potential of this very piece of metal, you never know.

Yup, checked base potential: Al -1,6; Zn -0,8
Click to expand...

The elecrochemical series is not what concerns us . It is a table of standard electode potentials, (measured relative to a hydrogen electrode) in which the electrode is immersed in a standard solution of one of its salts.

We are concerned with the potentials that exist when metals are immersed in seawater ... that gives rise to what we generaly call the "galvanic series"

The figures you quote are the standard electrode potentials for Al and Zn !
 
Yes, I know. Not mentioning what is seawater potential as not having at hand; just made some explanation on what I still remember. From chemistry ;) but I'm in biochemistry.
This was just an answer to OP's question: aluminium is less noble.
 
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My stern gear is almost identical to yours. Bruntons Autoprop, stainless steel shaft, P-bracket. The Autoprop anodes on their own last less than a season. I use a zinc shaft anode just ahead of the P-bracket, with this addition my Autoprop anode lasts two seasons and the shaft anode usually makes three.
 
Very interesting, thank you.:)

So the key information appears to me to be that the zinc (used in the autoprop anode) is in fact more noble (lower in the galvanic series) than the aluminium alloy (which may contain indium) used in the very nice and inexpensive hull anode that I bought on-line.

Are all aluminium anodes made out of the the same alloy (containing indium)?
 
flat1 said:
Thank you indeed VicS.
This now makes sense.

Except .. why shouldn't we mix aluminium and Zinc anodes?

Using Aluminium (with indium) to slow down the loss of the Zn on the prop was exactly what I was trying to achieve (even though I didnt understand the galvanic series properly and didnt know about the Indium).

The Al anode was £8 and the expensive Zn one has survived to last another season.
Click to expand...

I should word that differently perhaps.......... It is generally advised that zinc and aluminium anodes are not mixed.


Whether to use ZN or Al anodes is a balancing act between several facts.

Al has an RAM ( atomic weight ) of 27 and is trivalent. Zn has an RAM of 65 but is divalent. This means that 9g of aluminium will be consumed in a situation where 33 g of zinc might be consumed., except of course that the aluminium anodes are a bit more reactive than the zinc ones!

The relative prices of zinc and aluminium have to be figured in.. and I suppose the price of the Indium.

Aluminium is a lot less dense than zinc so much of the apparent gain in the relative weight loss of zinc and aluminium is offset ( if physical size is important) by this difference in density Aluminium is approx 2.7 Kg/m³ Zinc is about 7.1kg/m³
 
Vyv, thank you.

I tried using Zn shaft anodes the previous season but with much less success than you. The (Zn) prop anode still lasted less than the season and the shaft anode was nearly gone by the time we were lifted.

Aluminium is I understand considerably cheaper than Zn, and (used in the absence of other Zn anodes), longer lasting.
 
The advice on not mixing anodes is simply because when two different metals are mixed you never know. Be cautious. Aluminium anodes are not meant for seawater as they may vanish quickly. Well, in your situation the zincs are there as should be, so you're on safe side.
I understand even the anodes itself are made with different potentials - there is a standard, but others also are made and in use. Real potential depends not only on water, also on temperature and speed of the ship; there were, for instance, anodes designed for high speed craft like hydrofoil ships built here.
So effects are not so easy to predict.
 
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VicS said:
The elecrochemical series is not what concerns us . It is a table of standard electode potentials, (measured relative to a hydrogen electrode) in which the electrode is immersed in a standard solution of one of its salts. We are concerned with the potentials that exist when metals are immersed in seawater ... that gives rise to what we generaly call the "galvanic series"...
Click to expand...

Does this potential tally with the 0.47V I've reported in this thread? http://www.ybw.com/forums/showthread.php?347803-Saildrive-corrosion-galvanic-isolation-and-grounding
 
Well .. as I said .. it is a black art .. but .. in seawater if your drive leg has a chunk of Zinc on it (galvanic potential in seawater about -1.03v) and whatever you are grounding it to .. also in the same seawater (which sounds probable!) .. is made of Cast Iron .. your keel ?? (galvanic potential about -0.6v) .. then I think, all things being equal (which they wont be) you volt meter might show about -0.4v between them.

However I probably know rather less about Metallurgy than most of those posting above.
 
nigelmercier said:
Does this potential tally with the 0.47V I've reported in this thread? http://www.ybw.com/forums/showthread.php?347803-Saildrive-corrosion-galvanic-isolation-and-grounding
Click to expand...

The figure of .47v is significantly greater than any differences you would expect between zinc, aluminium and aluminium indium alloys in seawater.

It is even a little greater than one might expect between an aluminium alloy and cast iron, eg your keel. I'd not expect more than about 0.4v. When you measured it you said the battery was disconnected, hopefully fully disconnected and unable to be having any influence on the reading so unless you have any brass or bronze fittings linked to your "ground" Id be looking for some connection with the keel as a line of investigation after resolving the business of the saildrive isolation.

The 500 mA you measured would account for the largest part of a kg loss of zinc, or an equivalent loss of zinc and aluminium, over a 2 month period

However we are drifting the thread away from The OP's original question
 
Thank you.

I think you have now answered my original question:

In summary: Aluminium anodes may behave as being "less noble" than zinc anodes in seawater because they may contain Indium (which is added to prevent passivation of the surface of the aluminium by oxidation in seawater). In these circumstances, and when Zn and Al anodes are both connected to the same bronze propeller, the Al anode will be depleted the most rapidly and may itself provide some protection to the Zn anode (the depletion of which may be much reduced).
In general, anodes of different metals should not be used on the same vessel, except perhaps when trying to achieve what occurred in this case.

Al and Zn anodes are both good in seawater.
Al is cheaper than Zn but the same volume of Al may not last quite as long as you might expect because it is so much lighter than Zn.

Anodes are no longer "a black art".

Thank you all.
 
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