Why not mix anodes types?

[zAr]

Why should we not mix anodes, for example putting both a zinc and a magnesium on the prop shaft of a fiberglass boat in freshwater?

Follow up question: How can you tell which type is being used? As for example when the previous owner doesn't know these sorts of things and the old anode is difficult to get off and the yard says to let it dissolve on its own and you want to add another anode.

[RichH]

If you have a magnesium anode, there is no need to remove the zinc anode.

The magnesium anode will be 'less noble' in comparison to the zinc anode .... the magnesium anode will be sacrificed (protecting the zinc anode), until the magnesium is fully 'consumed' .... then the zinc will 'take over' as the least noble anode.

[sailingdog]

Yes, but in fresh water, a zinc anode might not be reactive enough to prevent galvanic corrosion, especially if he has any underwater bits that are aluminum.

[zAr]

Yeah, I wouldn't let it get to that point. I'm kinda paranoid about corrosion.

I've been to several sites that say things like "WARNING: Do not mix anode types."

My understanding is along the same lines as what RichH is saying, the least noble metal will just take over. I don't understand why it would be a problem to mix.

By some wild leap of the imagination does mixing metals somehow cause each anode to cancel each other out, effectively rendering you without the protection of either?

[mitiempo]

No, the least noble metal is the first to be sacrificed in every case.

[Gene T]

The least noble metal will be sacrificed first, but adding a more noble metal (an aditional zinc) will increase the decay of the least noble. It that makes sense. So if you use both you will just decay the magnesium anode faster, but then the zinc would start protecting when the magnesium is gone.

At least that is the way I see it.

Gene

[hellosailor]

Rob, I'll bet that if you hit each one with a wire wheel or cutting disc, the sparks that they throw are different in size, color, and shape. Spark tests are a pretty simple way to tell metals apart, take a look on the web & I'll bet you can find pictures of what each look like. (Or compare /vs/ a new one of your choice.)

There are all sorts of ways to identify metals, but this should be one of the simplest--if you're hauled or have them in hand.

[zAr]

Originally Posted by hellosailor

Rob, I'll bet that if you hit each one with a wire wheel or cutting disc, the sparks that they throw are different in size, color, and shape. Spark tests are a pretty simple way to tell metals apart, take a look on the web & I'll bet you can find pictures of what each look like. (Or compare /vs/ a new one of your choice.)

Well, I did do a search and apparently the sparks are too similar, but magnesium does catch fire, so that's something. One source said to heat the metal, then add water, if it burns down the shop then it was magnesium.
I could also take some shavings and try to burn them...zinc won't burn. Another test is electrical, but you need to immerse the part in water. Most tests seem to be chemical, using an acid or alkali..

All of which is far too complicated, so when I'm hauled out I'll just instruct the yard to drill out the anode of unknown composition.

Gene: I'm not sure how adding a more noble anode would increase the rate of decay of the least noble anode?

[Fstbttms]

Originally Posted by zAr

Follow up question: How can you tell which type is being used?

If you were to hold both anodes in either hand, comparing weight, it would be obvious which is which. Magnesium anodes are extremely light while zinc anodes are exactly the opposite.

[Gene T]

Originally Posted by zAr

Gene: I'm not sure how adding a more noble anode would increase the rate of decay of the least noble anode?

Two different metals in an electrolyte create a battery. Even though the potential between the two metals might be close electrolysis is still created between them.

Gene