Saturday, July 4, 2015

More Active Anode = More Growth?

I read this in a guidance document by maker of lower potential anode material, and I admit it didn't make obvious sense. Then I started soaking a few trial anode/pipe combinations to help develop a test method for an up-coming anode review.


By December it became clear that unprotected copper fouled less, no doubt the result of more free copper near the surface.

Clearly the more active anode has more growth (aluminum on left, zinc on right, same brand).

Why?

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3 comments:

  1. Robert SalnickJuly 5, 2015 at 3:19 PM

    Are those copper pipes? If so then I assume that the more active anode is doing a better job of preventing copper dissolution. Assuming that it is copper ions rather than copper that repels marine growth, that is...

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  2. Drew FryeJuly 5, 2015 at 9:40 PM

    That is one obvious interpretation. It certainly explains why the anodes remain clean (lots of Zn and Al). On the other hand, the Cu visually seemed well protected. I will be weighing the pipes to the nearest mg before and after a 6-12 month exposure period; that should give some measure of Cu ion release. As a practical matter, if a bronze prop lost a few mg of Cu per year, and that resulted in less growth, what does that mean? How much Zn loss is concurrent (since Cu pipe is not bronze, I cannot easily measure that)? Not having bronze running gear, this is something I've never considered.

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    Replies
    1. Robert SalnickJuly 7, 2015 at 10:29 AM

      Sadly, with unprotected bronze, it is the alloying Sn and Zn that are preferentially lost, leaving behind an atomic lace of Cu. The metal *looks* OK, but is fatally weakened. I have a 20" prop in our living room that has suffered this fate...

      (The anodes stay clean because the surface is constantly sluffing)

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