fighting anodes?

[Bill Gibbs]

I just finished a repower of my catamaran with new Yanmars engines, Yanmar SD-25 saildrives, and Variprofile props. The prop zincs are burning very fast, and the saildrive aluminum anodes are coated in a thick white "frosting" that I guess is zinc oxide from the prop Zincs. Both sides are doing the same thing, one side a bit faster.

Are my prop zincs trying to save my aluminum saildrive anodes?
Are saildrives and props supposed to be electrically isolated?

No shore power involved.

Any guesses?

[noelex 77]

You should not mix aluminium and zinc anodes, but the presence of zinc anodes should cause some extra erosion in the aluminium anodes, rather than vice versa, so there may be some other problems.

Stray current (usually DC current) should always be ruled out whenever there is rapid erosion of anodes.

[Pauls]

If they're being used up very fast I'd suspect that you have an electrical source supplying voltage. Check if there is any electrical continuity between the saildrive housings or prop with your electrical system.

[Bill Gibbs]

Originally Posted by noelex 77

the presence of zinc anodes should cause some extra erosion in the aluminium anodes, rather than vice versa.

Are you sure about this? On the Galvanic series Zinc is less noble than Aluminum.
https://www.corrosionpedia.com/an-in...rrosion/2/1403

[chrisr]

OMG ! how do you think this happened ? the anode on the destroyed leg looks brand new - yet the leg has disintegrated !

was copper based antifoul used ?

cheers

ps : this post was in response to a dive video posted here showing a badly corroded sail drive leg. however that post has now disappeared

don't know why ???

[fstbttms]

https://www.youtube.com/watch?v=88ax4YKe9H0

[fstbttms]

Originally Posted by chrisr

OMG ! how do you think this happened ? the anode on the destroyed leg looks brand new - yet the leg has disintegrated !

was copper based antifoul used ?

This was caused by a 12-volt stray current issue aboard the boat in question.

[HeywoodJ]

Originally Posted by Bill Gibbs

Are you sure about this? On the Galvanic series Zinc is less noble than Aluminum.
https://www.corrosionpedia.com/an-in...rrosion/2/1403

In a general sense that is true, but most anodes are alloys, for the commonly specified anode alloys:

Zinc: MIL-A-18001K - 1040mV

Aluminum: MIL-A-24779(SH) - 1100 mV

Magnesium: MIL-A-21412 - 1600mV

In theory at least, using genuine MilSpec anodes, an aluminum anode should marginally/barely protect a zinc anode. In practice they are so close that it probably makes little difference.

Using none-spec anodes all bets are off.

[noelex 77]

Originally Posted by Bill Gibbs

Are you sure about this? On the Galvanic series Zinc is less noble than Aluminum.
https://www.corrosionpedia.com/an-in...rrosion/2/1403

Aluminium has very different properties depending primarily on its composition (it is an alloy or mixture of metals) and heat treatment. Aluminium anodes are specifically made to be less noble and thus very different to the aluminium that you might use in everyday construction, let alone the specific marine alloys.

Aluminium anodes are often used on an aluminium boat in preference to zinc.

Aluminium anodes are slightly more noble than zinc anodes. Thus the aluminium will tend to protect the zinc from corrosion.

While mixing anode materials is not advisable I think the forum posters are in a rare unanimous agreement in suspecting stray DC current as the probable cause. In some cases this can cause very rapid corrosion particularly on the small mass of a saildrive. Therefore it would be wise to at least investigate this possibility ASAP.

[sailorboy1]

Originally Posted by Bill Gibbs

Any guesses?

bad engine ground connection

[Bill Gibbs]

Thanks for the comments. Had the “experts in”.
We passed the stay currents in water test. Marina is ok.
We installed an engine to engine ground wire.
Contemplating a transom zinc.
Fingers crossed.