Storing bulk metals for centuries

Question

If you are building a megastructure like a Banks Orbital, it will be necessary/useful to provide it with reserves of various useful metals for the use of the civilization inhabiting the structure.

However, it seems unlikely to be considered necessary or desirable for those reserves to be buried so that the way to get at them is to dig up the ground; even if the interior environment is designed to look natural in most ways, there is such a thing as going too far. It seems more likely that metals would just be stashed in stockpiles, in a conveniently accessible form.

But what form, exactly? There is plenty of discussion on the Internet about how to store objects made of iron, copper etc. on individual scales of space and time. However, if you are putting down a stockpile of a billion tons of metal, that might stay where it is for centuries, that's a very different proposition.

Would it make sense to store a large stack of ingots? Or would it make more sense to say that on a timescale of centuries, everything oxidizes anyway, so you might as well store the oxide and let whoever gets around to using it, run it through a smelter?

What sort of granularity would make sense? Ingots of what sort of size? Or would powder be more efficient for handling by machine? Or would it make sense to store very large ingots to minimize surface area therefore oxidation loss? It sounds plausible to say 'a very large ingot only loses a thin surface layer to oxidation, that's nearly as good as no oxidation at all'; is that actually right? What's the largest granularity that wouldn't give headaches to people trying to move the stuff to where it's needed?

Of course, oxidation is not an issue for metals like gold or platinum. (For those, the big problem is securing the stockpile so it doesn't get stolen.) Are there differences among other metals that would affect the best way to store them?

Answer 1

Two keywords here: pigs and cosmoline.

First, pigs are large ingots, typically close to the size of the smelter crucible that originally produced the pure metal. This (as suggesed by L.Dutch) minimizes area available for corrosion, as well as minimizing the handling necessary to put the metal in storable form. Different metals will have different pig sizes, because they're smelted by different methods, but they're all made into pigs first before they get any other treatment -- and if you're storing them for raw material, pigs are the easiest, quickest, cheapest form to store.

Second, cosmoline is a term for an antioxidant bearing grease coating used for more than a century to protect finished metal goods (firearms, originally) from corrosion in storage. A few years ago, I bought a rifle that had seen service in the Second World War; after the war, it was refurbished and put into storage (before 1950, almost certainly) and coated with the Soviet version of cosmoline; fifty years later, after a process of removing the cosmoline (hot water and a brass brush worked well) the rifle was corrosion free and ready to load and fire. Obviously this would work as well or better for pigs -- with the additional bonus that there would generally be no need for the labor of cleaning off the cosmoline if the pig is to be remelted for further processing.

The composition of cosmoline is a mix of oils and waxes, so that as the volatiles evaporate off, the wax forms an impervious coating. An alternative to this that has been used for tooling is a wax dip -- simply immersing the tool in molten wax, like dipping a candle.

Answer 2

Airless Chambers

The atmosphere is artificially generated. Some pockets of the structure have no atmosphere. Or perhaps they have a different oxygen-free atmosphere. For example pure nitrogen like inside bags of potato crisps.

The metal is stored in these pockets and does not rust, since there is no oxygen, and rust is caused by oxidation.

For example a rotating ringworld might have the metal stored on the outer surface, while people live on the inner surface.

Answer 3

If you want to store it for very long time, first of all you want to minimize the available surface: it both reduces oxidation and depredation.

While a stash of 1 billion ingots 1 kg each can be taken away 1 ingot at a time, a single block of 1 billion kg will be much more cumbersome to move. And it will have also less exposed surface for oxidation.

Then, if you can minimize exposure to at least one between oxygen and water, you will greatly reduce the oxidation rate. Don't forget that in the dry desert iron scraps last much longer than in more humid places, despite the same presence of oxygen. And, as an anecdote, I have recently found a 5 cm long slab of raw unpainted iron that I cut back in high school from a longer bar for a science experiment: stored in my study desk, it has gone a quarter of a century without getting any rust, while the gate made with it has needed more than one painting and sanding.

Answer 4

I believe ingots would be sufficient. Over the timescale of centuries or single digit millennia, ingots will be resistant to oxidation anyway. Certainly those at the surface of these large stacks will become oxidized, but for a stack that is 40 meters deep, those at the bottom will remain virtually pristine.

The ingots could be sized arbitrarily... whether they are 25g ingots or 1kg makes little difference, I think. There is probably an upper limit beyond which they are simple inconvenient to use, and another limit beyond that where it becomes a real technical challenge (how would a neolithic person carve off a piece of a gigantic copper ingot, after all?). This will be dependent on the nature/size of the beings the ingots are intended for, so you'll have to work on that detail yourself. For humans or human-likes, I would say that you shouldn't go much past 500kg though, because it becomes difficult to lever anything much bigger out onto rollers with primitive tech. Hell, how much heat would the first one suck up before you could melt it down? Probably go smaller, some that can fit in a more practical crucible.

As for how common these stockpiles need to be, ancient peoples apparently sailed far to get tin necessary for bronze. The people of Greece and its surrounding areas may have gone as far the British Isles for theirs. So hundreds or low thousands of kilometers might not be out of the question if they can sail the distance.

This should work for most of the important metals: copper, tin, iron, gold, etc. My chemistry's bad, but I know some of the non-manufacturing ones (sodium) can be quite violent... those can probably be ignored. And if they need fissiles, then eeeeesh, maybe you'll just have to have little Star Trek replicators everywhere that they can dial it in and watch it materialize out of nothing. Those don't really keep anyway.

Answer 5

Passivating layers will help. Avoid water and especially chlorides. Ignore oxidation when choosing ingot size.

A combination of a thick passivating layer and a dry atmosphere should slow corrosion to negligible rates for most metals. For iron, passivate with phosphate. For aluminium, put on an extra thick anodised layer.

Many others just need to be kept dry and chloride free. A hot, dry climate will do more than almost anything else to help.

Chlorides are the devil and will attack most metals (including many but not all that we don't consider especially vulnerable to them) over time, so keep them the hell away.

As another commenter has pointed out, you can apply wax (I'd add: or a good paint).

The only factor in the decision re: size of ingots for most metals is the human one; do you want it convenient or inconvenient to move?

Oxidation's going to be negligible even over centuries for passivated ingots in any good storage facility buried under a desert.

Answer 6

Seal the Ingots in the Gold

As the various metal ingots are made take them into a nitrogen gas chamber and seal them in gold foil. The gold does not react and acts as a barrier against the air thus greatly slowing down the oxidation of the metal.

This has a side benefit that it will be hard to steal the gold since it distributed as foil around some rather heavy cheaper metal ingots.

Answer 7

Moonlets

If you're building a mega-structure like a Banks Orbital, then your civilization has mastered spaceflight to the point that it's a non-event. Your people can hop out to the Kuiper Belt the way most people go to the corner store.

So once the Orbital is complete, they take all the left over material, press it up into a couple of decent sized dwarf planets, and park them in whatever orbit is most aesthetically pleasing. If they need more material in the future, they process it out of the planetesimals as required.