I'm trying to get a more complete picture of how the chemistry works within a typical marine setup including a sacrificial anode. The setup consists of an aluminum hull, another metal (say a stainless steel propellor) and then a zinc sacrificial anode. We can ignore the steel propellor. What are the oxidation and redox equations for the aluminum and zinc? Seems like marine blogs don't care about this level of detail and chemistry courses don't use saltwater as the solution...

I'm really curious about what is going on at the aluminum cathode. I think the oxidation equation for the zinc is something to the effect of:

$$ \ce{Zn + something -> Zn^2+ +H_2} $$

Therefore the zinc disappears. But what happens to the aluminum? There is no aluminum in solution to precipitate so where do the electrons go?

  • $\begingroup$ What you are showing is a half-cell reaction. In "theory" the reaction shouldn't take place at all. The boat would just have some potential relative to the water. In reality all sorts of things are going on. But rather than have some other metal oxidize into the water via corrosion, the zinc oxidation occurs. Hence its name - a sacrificial anode. $\endgroup$ – MaxW Nov 7 '15 at 19:01
  • $\begingroup$ Are you saying that no reaction should occur with the Aluminum? The aluminum cathode must still be shedding electrons somehow. $\endgroup$ – Vytas Nov 7 '15 at 19:22
  • $\begingroup$ The point with Zn being a sacrificial anode is that the zinc should oxidize before any other metal. So the Zn protects the aluminum, and the stainless steel propeller and so on... $\endgroup$ – MaxW Nov 7 '15 at 19:46
  • $\begingroup$ My understanding is that if the only metal was Aluminum there wouldn't be much of an issue. It would oxidize, but slowly since the negative charge would build up on the Aluminum. However, somehow the existence of a cathode helps in releasing the electrons back into the solution, thereby greatly speeding up the oxidation. Am I understanding that correctly? $\endgroup$ – Vytas Nov 7 '15 at 19:59
  • $\begingroup$ Aluminum is sort of a funny metal. Normally when a metal oxidizes (rusts) the rust has a greater volume than the metal so the rust flakes off exposing more fresh metal. Aluminum oxide should coat the aluminum metal and "seal" the metal surface. But you can scratch the aluminum and so on... $\endgroup$ – MaxW Nov 7 '15 at 20:09

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