I think I understand it as some metals are more prone to rust and wear then others? These are used as sacrificial pieces that dissolve(?) to prevent the item you’d like to preserve from wearing or rusting. Help me understand how for instance in a boat the Zink weight bolted to the propeller assembly slows the other metal parts from rusting, as in why do both the sacrificial puck of zinc and the propeller not rust away together when they’re both submerged in the ocean?
In: Chemistry
Sacrificial anodes aren’t for regular corrosion. Your steel hull will still rust and will still need to be scraped and painted to preserve it. Sacrificial anodes help with galvanic corrosion, which occurs when you have dissimilar metals in the presence of an electrolyte.
The more anodic metals will be eaten away, while the more cathodic metals will gain deposits on them. The idea for a sacrificial anode is that you intentionally add a more anodic material to protect what would otherwise be the anode in the system. For example, if your boat has an aluminum hull, steel screws will cause galvanic corrosion and cause the aluminum to be eaten away in spots. Adding zinc causes the zinc to be eaten away instead, and you simply replace it when you need to.
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Rusting (corrosion/oxidation) is all about taking a metal made of atoms, removing some of their electrons and making them into ions that can then chemically react. It’s called the anodic reaction because that’s what happens on the anode in a galvanic cell.
So the most important point is that different metals are resistant to this to different degrees (their oxidation potential).
The other fact is that one metal that is more resistant will just steal some of those electrons from another metal of they have electrical contact.
Gold just never rusts except it some truly unreasonable conditions. Copper will oxidise on its own, but will steal electrons from most other metals, like iron. People found that out when the first iron hulled ships were covered in copper sheets like their wooden predecessors and the iron got just eaten away rapidly.
If you want to protect the iron (or steel), you have to find a metal that’s to iron as iron is to copper. Zinc is that metal. The iron will keep stealing electrons from the zinc until the zinc is corroded away and can’t give any more.
Another way to protect the hull of the ship from rusting is to apply a voltage to the hull directly, to stop electrons from flowing the way they’d prefer and stop chemistry from happening to begin with.
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