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Comment by fryguy | Hacker News Reader

For a, you can put a sacrificial anode on the frame. Given that the rate of corrosion should be known, the size of it can be calculated to however long the pod is expected to be in service.

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I will say that my only experience with sacrificial anodes is that they help but don't prevent corrosion entirely. We had screws starting to turn to dust from a couple hundred hours of pool water exposure even with a sizeable anode. Maybe it wasn't in sufficient contact or whatever. They're unlikely to be curealls anyway.

One thing about sacrificial anodes is that multiple seem to be required. E.g. even though the hull of a ship may be a good conductor, you don't just have one sacrificial anode.

Wikipedia ["Galvanic anode"]: The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.2 mi) long needs 10 anodes, then approximately one anode per kilometere would be more effective than putting all 10 anodes at one end or in the centre.

I came here to say the exact same thing. The resistance of the metal surface will mean you want to cover the metal needing protection all around. I think dependant on the salinity of the water different metals should be tested to best suit you environment.

Hmm, my experience is with pretty small objects: light enough to be carried by two people.

This sounds really interesting, can you explain it further for a layman? (Not being snarky, I genuinely know very little about chemistry)

See this article: https://en.wikipedia.org/wiki/Cathodic_protection

This is probably what OP meant:

"They are made from a metal alloy with a more "active" voltage (more negative reduction potential / more positive electrochemical potential) than the metal of the structure. The difference in potential between the two metals means that the galvanic anode corrodes, so that the anode material is consumed in preference to the structure." https://en.wikipedia.org/wiki/Galvanic_anode

Basically, the reason that things corrode is that there are dissimilar metals, one with more electrons than the other creating a voltage between them. So your part is a battery and the ocean is the "wire". The electrons move from your part (corroding them), to other parts with less electrons. If instead you have a sacrificial piece of zinc, the zinc has more electrons than your part so the electrons will come from the sacrificial piece of zinc instead of your part. Think of it like a lightning rod, but for corrosion instead of lightning. It's more complicated than this, but that's the general idea.

It's fairly common on boats to have exposed zinc whose only purpose is to be sacrificially electrolized (IIRC) to protect your steel or aluminum.

I worked on a 120m old passenger ship for a while. During dry-dock each year we'd put 8 or so 2kg (ish) zinc anodes around the hull, a welded bracket around each one.

The trouble was that they're pretty expensive, and zinc has a great resale value - so we had to do extra watches around the lower decks with rigged firehoses to try and stop local divers stealing them who knew we'd just come out of drydock. (This was in the Philippines). We lost several that way.

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