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Apart from the noticeable cost difference and weight has anyone any experience of using these on keels in salt water ?
Aluminium vs Zinc Anodes
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mortehoe
Well-Known Member
Aluminium is already surfaced oxidized --- So I don't think that aluminium can do anything at all, not even in salt water. Stick with zinc.
FAITIRA
Well-Known Member
No Pete, but I will be interested in responces, as far as I know? they are for f.w. use only. Bill.
BrendanS
Well-Known Member
I can't comment on keels, but companies like Mercruiser now provide their outdrives with aluminium anodes as standard (for use in salt water) as the zinc anodes were not providing enough protection
[10753]
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I have read that Aluminium Anodes are supposed to work in salt water but I need convincing as the cost saving here could turn out to be an horrendous expense !
Apparently Magnesium Anodes are for Fresh Water only.
Apparently Magnesium Anodes are for Fresh Water only.
VicS
Well-Known Member
In salt water aluminium or zinc anodes can be used. They are both ineffective in fresh water due to the formation of an oxide crust and magnesium is used. Zinc is also ineffective in brackish water but aluminium contiues to be effective.
Aluminium anodes are not pure aluminium
See http://www.aquafax.co.uk/images/aceimages/Corrosion%20Card%202005.pdf
Aluminium anodes are not pure aluminium
See http://www.aquafax.co.uk/images/aceimages/Corrosion%20Card%202005.pdf
Trundlebug
Well-Known Member
Aluminium is supposed to be used as an allrounder between Magnesium and Zinc.
For boats that spend their time in brackish waters, or who go between salt and fresh, neither zinc nor Mag are any good. Zinc oxidises and crusts over in fresh water, rendering itself useless, and Magnesium dissolves like aspirin when in salt water.
We fitted aluminium anodes to our boat last year, but I can't give you a performance report because we haven't lifted it out of the water yet.
As for the oxidised layer, to be strictly accurate I believe the so-called aluminium isn't pure aluminium but some kind of alloy that has the requisite properties for the job it has to do.
It's a relatively new material, or seems to be, as it's not widely available. Seems to have originated in the US.
I fitted it because the "brochure spoke highly"...
For boats that spend their time in brackish waters, or who go between salt and fresh, neither zinc nor Mag are any good. Zinc oxidises and crusts over in fresh water, rendering itself useless, and Magnesium dissolves like aspirin when in salt water.
We fitted aluminium anodes to our boat last year, but I can't give you a performance report because we haven't lifted it out of the water yet.
As for the oxidised layer, to be strictly accurate I believe the so-called aluminium isn't pure aluminium but some kind of alloy that has the requisite properties for the job it has to do.
It's a relatively new material, or seems to be, as it's not widely available. Seems to have originated in the US.
I fitted it because the "brochure spoke highly"...
mortehoe
Well-Known Member
So how does reoxidisation on aluminium work?
Post a website to show that Al is as good as Zn under seawater
Post a website to show that Al is as good as Zn under seawater
VicS
Well-Known Member
Re: So how does reoxidisation on aluminium work?
[ QUOTE ]
Post a website to show that Al is as good as Zn under seawater
[/ QUOTE ]
Please?
Pure aluminium is not. Aluminium anodes are an alloy, to a US military spec, with zinc and indium. They are marginally more effective in theory than zinc but there is only 0.04 of a volt in it.
Google will find masses of info for you to read.
[ QUOTE ]
Post a website to show that Al is as good as Zn under seawater
[/ QUOTE ]
Please?
Pure aluminium is not. Aluminium anodes are an alloy, to a US military spec, with zinc and indium. They are marginally more effective in theory than zinc but there is only 0.04 of a volt in it.
Google will find masses of info for you to read.
joha
Well-Known Member
Re: So how does reoxidisation on aluminium work?
Hi
Used on North sea oil rigs, I am involved in marine construction and we use them on steel piles, steel pontoons etc and they work!
joha
Hi
Used on North sea oil rigs, I am involved in marine construction and we use them on steel piles, steel pontoons etc and they work!
joha
geoffatstanpit
Well-Known Member
Re: So how does reoxidisation on aluminium work?
I have an outboard in a well that dangles its bottom in the water. Brackish if it rains a lot round Salisbury but mostly salty. The standard Mercury anodes get eaten in less than a season (April - end of Oct) so I use 2 on a longer bolt. They are rather expensive (about a pound a gram!!!) so I've turned up some copies from "aluminium" bar (probably some kind of alloy, a bit heavier than they should be in theory!) to try for this year.
The bit of the motor with the anode on it goes above the water-line at low tide so I have a pair of those round zinc ones bolted through the skeg. They get eaten a bit but seem to last a couple of years.
I have an outboard in a well that dangles its bottom in the water. Brackish if it rains a lot round Salisbury but mostly salty. The standard Mercury anodes get eaten in less than a season (April - end of Oct) so I use 2 on a longer bolt. They are rather expensive (about a pound a gram!!!) so I've turned up some copies from "aluminium" bar (probably some kind of alloy, a bit heavier than they should be in theory!) to try for this year.
The bit of the motor with the anode on it goes above the water-line at low tide so I have a pair of those round zinc ones bolted through the skeg. They get eaten a bit but seem to last a couple of years.
BrendanS
Well-Known Member
Re: So how does reoxidisation on aluminium work?
The alloy in aluminium anodes is very carefully concocted to make it work as an anode. The specifications are very exact. If you use any old aluminium alloy, you may find it won't work well at all.
The alloy in aluminium anodes is very carefully concocted to make it work as an anode. The specifications are very exact. If you use any old aluminium alloy, you may find it won't work well at all.
neale
Well-Known Member
Re: So how does reoxidisation on aluminium work?
Exactly, The aluminium alloy must be lower on the galvanic series table than the Alloy that your outboard is made from. If it's the other way around, your outboard will become the anode to protect you lump of aliminium bar /forums/images/graemlins/shocked.gif
Exactly, The aluminium alloy must be lower on the galvanic series table than the Alloy that your outboard is made from. If it's the other way around, your outboard will become the anode to protect you lump of aliminium bar /forums/images/graemlins/shocked.gif
