Anode query

[Joe_Cole]

I was looking for an anode for my outboard at Beaulieu yesterday and was surprised when one stall holder insisted that outboard anodes are made from alluminium these days. He was only selling anodes so, presumably knows what he was talking about, but how can an alluminium anode protect an alluminium outboard?

I know that you can have different alloys of any metal, but surely one alluminium will not be that much different from another?

Just curious, but I must admit I was surprised.

 

[BrendanS]

Mercruiser provide aluminium anodes as standard for sea going boats on their stern drives these days, as Zincs were too problematic for many of their drives. The drives are covered in paint, so the differential in current is still there - just don't damage the covering paint layer! You still need magnesium if you are in fresh water

 

[nedmin]

dont use magnesium anodes on steel boats as they are too reactive and will possibly take your paint off.Use aluminium.

 

[BrendanS]

I'd love to hear your reasoning behind this

 

[Ships_Cat]

So would I.

In fact aluminium is lower on the galvanic scale so the risk of coating damage from overprotection on a steel vessel, even if attached directly to the hull instead of to the drives, is less than that with zinc.

I any event, while people talk of coating damage from overprotection on steel vessels I have never seen or heard of a validated case (the few actual real life claims I have heard of have subsequently been shown to be a problem of the coating itself or the preparation for it).

John

 

[nedmin]

Had a steel cruiser for 10yrs fitted with magnesium anodes, took it out every year, noticed paint had "disappeared" in various places on hull but was covered in magnesium oxide,didnt make much of it,just touched it up. When I bought my first dutch steel boat in 1990 I noticed that the anodes were aluminium. asked why not magnesium ? Told they had found that magnesium were too reactive, It answered the above prob. Another interesting one, With the first boat moored alongside steel piling,the side with the most paint off was the side away from the steel piling!