Anodes

[PerlMac]

As first time boat buyers in September, we allowed the marina to fit new anodes to our stern drives whilst the boat was out of the water having a survey. We were told that they had fitted zinc ones which were really for use in salt water (we are river based) but they would be okay. Are these going to do their job properly? I understand that magnesium is normal for fresh water??

 

[ccscott49]

If your boat is to be used exclusively or 90% of its time in freshwater, use magnesium, if it's just berthed in fresh and used mainly at sea, zinc. there are, I'm sure a lot more opinions on this.

 

[heerenleed]

Zinc will cover itself in a thin protective layer of oxide when in fresh water. After that, it does not work properly. if you change between fresh and seawater, there is a third alternative; aluminium. We use them in the netherlands, were many boats are kept in freshwater and go out to sea more or less frequently. An aluminium anode is not 'eaten' with the same speed as a magnesium one when in seawater. i usually have to change mine (aluminium) every second year.

Vetus has got them here, may be not in the standard collection for the UK, but they should be able to order them here.

happy sailing



Peter a/b SV Heerenleed, Steenbergen, Netherlands

 

[ccscott49]

Trouble with aluminium on an aluminium sterndrive, is which is going to erode first?? Magnesium for aluminium sterndrives. IMHO

 

[Trevor_swfyc]

Re: Anodes - Electrochemical Series

The electrochemical series which gives an indication of the relative reactivity is Sodium -2.712 v, Calcium -2.500 v, Magnesium -1.550 v, Aluminium -1.240 v, Zinc -0.758 v, Iron -0.441 v, Tin -0.136 v, Lead -0.122 v. Hydrogen 0.0 v, Copper + 0.3448 v, Mercury +0.7986 v, Silver + 0.7995 v, Platinum +1.359 v and Gold +1.500 v. The higher the electronegativity the more reactive also the ions Na & Cl in salt water will increase the reactivity. As has been pointed out Al is an alternative to protect Fe but will react faster than Zn but should give better protection. In theory Zn will protect Fe being more electronegative but you are right it is usual to use Magnesium anodes in fresh water to protect iron. If you are trying to protect Aluminium then you must use Magnesium and not Zinc.
All the best, good luck in getting the anode changed.