Anodes for fresh and salt water???

SHUG

SHUG

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I am planning to use the French canal system in a steel boat which is kitted out for seawater use.
I am aware that different anodes are used for fresh and salt water but is there a compromise solution in case I decide to overwinter in the canals.
 
SHUG said:
I am planning to use the French canal system in a steel boat which is kitted out for seawater use.
I am aware that different anodes are used for fresh and salt water but is there a compromise solution in case I decide to overwinter in the canals.
Click to expand...

I would leave your existing anode in situ in case you go back to sea water. Attach a magnesium anode to a piece of stainless cable attach it to the boat and hang it over the side
 
SHUG said:
I am planning to use the French canal system in a steel boat which is kitted out for seawater use.
I am aware that different anodes are used for fresh and salt water but is there a compromise solution in case I decide to overwinter in the canals.
Click to expand...

Aluminium is the best compromise.

Except perhaps in unpolluted fresh water where magnesium would be preferable if cathodic protection really is needed although care would be needed to avoid over-protection

Zinc anodes will require cleaning off the oxide coating before returning to salt water or they will not be effective
 
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We are based in a fresh water marina but sail at sea.

I use the following:

Permanent Aluminium shaft and hull anode.

+ Hanging Mg anode when in fresh water.

+ Hanging Zn anode when at anchor/in marina when at sea.

No problems after 6 seasons.

The largest depletion is of the hanging Mg anode which is only used in fresh water. The Zn anode shows almost no depletion and and Al anodes very little. I bought a large Mg anode from a narrowboat chandlers which has a lug at each end. I then cut this in half and made up a wire to hang it and it is connected through the hull to the internal electrical system. The first half did about 3 seasons and then I replaced it with the second half. Although the surface of the used Mg anode was very pitted, there was still plenty left so I used an angle grinder to clean it up back to the smooth metal. That went back in at the end of last season and I am about to do the same cleaning up job with the second half.

If you leave a Zn anode on the hull for any length of time, it will get a coating on it that will stop ity working. You will need to lift out and clean back to sare Zn before going back to sea.
 
davewarburton said:
We are based in a fresh water marina but sail at sea.

I use the following:

Permanent Aluminium shaft and hull anode.

+ Hanging Mg anode when in fresh water.

+ Hanging Zn anode when at anchor/in marina when at sea.

No problems after 6 seasons.
Click to expand...

I dont think I'd bother with the hanging zinc anode. I cannot see that it serves any useful purpose if you have fixed aluminium anodes.
 
VicS said:
I dont think I'd bother with the hanging zinc anode. I cannot see that it serves any useful purpose if you have fixed aluminium anodes.
Click to expand...

Judging by the complete lack of erosion of the hanging Zn anode, I think you are correct. In fact, half the time I forget to deploy it anyway.
 
davewarburton said:
We are based in a fresh water marina but sail at sea.

I use the following:

Permanent Aluminium shaft and hull anode.

+ Hanging Mg anode when in fresh water.

+ Hanging Zn anode when at anchor/in marina when at sea.

No problems after 6 seasons.

The largest depletion is of the hanging Mg anode which is only used in fresh water. The Zn anode shows almost no depletion and and Al anodes very little. I bought a large Mg anode from a narrowboat chandlers which has a lug at each end. I then cut this in half and made up a wire to hang it and it is connected through the hull to the internal electrical system. The first half did about 3 seasons and then I replaced it with the second half. Although the surface of the used Mg anode was very pitted, there was still plenty left so I used an angle grinder to clean it up back to the smooth metal. That went back in at the end of last season and I am about to do the same cleaning up job with the second half.

If you leave a Zn anode on the hull for any length of time, it will get a coating on it that will stop ity working. You will need to lift out and clean back to sare Zn before going back to sea.
Click to expand...
This looks like the most complete answer...thanks.
So, two things (1) use a Mg anode over the side in fresh water
(2) clean up the Zn anodes before returning to the sea.
 
You should'nt mix anodes materials. According to corrosion basics, magnesium is worn quickly because it protects zinc, which, in turn, can't do its job correctly... It's a waste of money. Best material for sea/fresh waters remains aluminium. Anyway, only potential control will tell if protecting devices work properly and if protection is effective.
 
Has someone got an electropotential table to hand to help here.
Is the new answer that Aluminium, is the all-purpose electrode material for a steel hull? Salt and Fresh?
 
SHUG said:
Has someone got an electropotential table to hand to help here.
Is the new answer that Aluminium, is the all-purpose electrode material for a steel hull? Salt and Fresh?
Click to expand...

My usual link is not working ("down for maintenance") but Goggle for " galvanic series" and you'll get a selection.

Remember though that aluminium anodes are a special aluminium-zinc-indium alloy and that they have an an electrode potential of -1.09 volts relative to a silver/silver chloride electrode ( zinc anodes are -1.05 volts )
 
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SHUG said:
Has someone got an electropotential table to hand to help here.
Is the new answer that Aluminium, is the all-purpose electrode material for a steel hull? Salt and Fresh?
Click to expand...

Large offshore steel structures such as jetties, oil platforms, ships, choose aluminium for corrosion protection. The alloy may be the one Vics suggests, I don't know specific details. The reason may well be partly financial, as aluminium is cheaper than zinc, but it undoubtedly is effective.
 
vyv_cox said:
Large offshore steel structures such as jetties, oil platforms, ships, choose aluminium for corrosion protection. The alloy may be the one Vics suggests, I don't know specific details. The reason may well be partly financial, as aluminium is cheaper than zinc, but it undoubtedly is effective.
Click to expand...

The reason is quite simple. Aluminium provides identical protective values (current, duration) for only a third of the weight of zinc. Of course, aluminium used for anodes is a special alloy every anodes maker knows (keep to mil. specs. though). However, when you alternate soft and sea water (aluminium is quite efficient in both), you might have to brush your anodes for maximum efficiency.
 
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I eventually contacted MGDuff , which is perhaps what I should have done at the outset, and got this excellent reply



The correct anode material for fresh water is magnesium and for salt water it is zinc. The only anode material that works in more than one water type is Aluminium, which works in both salt and brackish water. Aluminium will not work in truly fresh water. Usually you would choose the anode material to suit where the vessel is moored, as this is where the vessel will spend the majority of its time during the year.

The problem with vessels changing from salt to fresh water regularly is that zinc and magnesium are not suitable in fresh and salt water respectively. In salt water magnesium is very reactive and will corrode at a high rate, and may damage some paint systems. The official recommendation for magnesium anodes being fitted and a vessel traveling in salt water is no longer than 2-3 days at a time and no more than 2 weeks total in a year. By the same token zinc in fresh water will passivize, develop a crust like layer of calcareous deposits and cease to offer any protection.

Essentially what we are saying is that ideally you would have magnesium anodes fitted the whole time you are in fresh water and then have the boat lifted and zincs fitted when you change to salt. Realistically this is expensive and time consuming. There is a solution however, in the form of hanging anodes. By using hanging anodes instead of hull mounted anodes, you can simply lift the anode out of the water and change the anode on the end of the cable to suit the water type. The number and location of hanging anodes is the same as hull mounted anodes. As your vessel is steel, you will need to make several connections to the inside of the hull to achieve cathodic protection of the hull.

Below is a link to one of our online distributor's sites where we have information about the leisure hanging anode. These can be purchased from any of our distributors, of which there is a full list on our website.

http://www.anodeoutlet.co.uk/fitting-instructions/57l-hanging-anode-range/

I hope this is useful information. If you have any further questions or I can be of further help please let me know.

Kind Regards,

Ollie Robinson
Sales & Marketing Technician
Email: orobinson@mgduff.co.uk

MGDUFF International Ltd.
1 Timberlaine Estate, Tel: 00 44 [0] 1243 533336
Gravel Lane, Fax: 00 44 [0] 1243 533422
Quarry Lane, www.mgduff.co.uk
Chichester, sales@mgduff.co.uk
West Sussex, PO19 8PP
 
Pretty much what I suggested in my earlier response. Not surprising really as I had consulted the MG Duff website for info when we moved from berthing in salt to fresh water.

SHUG said:
I eventually contacted MGDuff , which is perhaps what I should have done at the outset, and got this excellent reply



The correct anode material for fresh water is magnesium and for salt water it is zinc. The only anode material that works in more than one water type is Aluminium, which works in both salt and brackish water. Aluminium will not work in truly fresh water. Usually you would choose the anode material to suit where the vessel is moored, as this is where the vessel will spend the majority of its time during the year.

The problem with vessels changing from salt to fresh water regularly is that zinc and magnesium are not suitable in fresh and salt water respectively. In salt water magnesium is very reactive and will corrode at a high rate, and may damage some paint systems. The official recommendation for magnesium anodes being fitted and a vessel traveling in salt water is no longer than 2-3 days at a time and no more than 2 weeks total in a year. By the same token zinc in fresh water will passivize, develop a crust like layer of calcareous deposits and cease to offer any protection.

Essentially what we are saying is that ideally you would have magnesium anodes fitted the whole time you are in fresh water and then have the boat lifted and zincs fitted when you change to salt. Realistically this is expensive and time consuming. There is a solution however, in the form of hanging anodes. By using hanging anodes instead of hull mounted anodes, you can simply lift the anode out of the water and change the anode on the end of the cable to suit the water type. The number and location of hanging anodes is the same as hull mounted anodes. As your vessel is steel, you will need to make several connections to the inside of the hull to achieve cathodic protection of the hull.

Below is a link to one of our online distributor's sites where we have information about the leisure hanging anode. These can be purchased from any of our distributors, of which there is a full list on our website.

http://www.anodeoutlet.co.uk/fitting-instructions/57l-hanging-anode-range/

I hope this is useful information. If you have any further questions or I can be of further help please let me know.

Kind Regards,

Ollie Robinson
Sales & Marketing Technician
Email: orobinson@mgduff.co.uk

MGDUFF International Ltd.
1 Timberlaine Estate, Tel: 00 44 [0] 1243 533336
Gravel Lane, Fax: 00 44 [0] 1243 533422
Quarry Lane, www.mgduff.co.uk
Chichester, sales@mgduff.co.uk
West Sussex, PO19 8PP
Click to expand...
 
How long would you need to be in fresh water before the zinc anodes wouldn't work in salt? I am thinking about a canal passage.
 
Irrealistic

May I suggest it is irrealistic to rely on just hanging anodes for a steel boat in any waters (not aluminium one, though, and what about moving on...)? Irrealistic too switching from magnesium to zinc at every change of waters. Aluminium remains the best solution: suitable for both waters for a third of the zinc's weight (and small maintenance). No guess...
 
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