Would unpainted iron or steel rust in space?

Question

Iron and steel are made primarily of the same substance and they tend to rust. I believe other materials like non-ferrous metal are generally preferred for their lighter weight and strength, in most space applications.

Presumably it will be found to be as common in asteroids as it is on earth (4th most common). In the not to distant future, we should be harvesting minerals from asteroids and building things in space. So if we build say a frame to support solar panels out of steel, will we need to try and figure out how to apply paint in zero gravity and zero atmosphere, or would rust not be a concern?

Answer 1

"Rusting", or more chemically correct "oxidation", is a reaction with oxygen in the atmosphere. Iron reacts with oxygen and turns into iron oxide, the reddish-brown substance commonly referred to as "rust".

In space there is no atmosphere with any oxygen to react with, so any iron in space would not rust.

This, however, assumes that there is really no oxygen in space. When a spacecraft is in a low orbit around Earth, there are still small traces of atmosphere it flies through. These small traces could bond to any iron and cause it to rust, although far, far slower than on ground level. Another source of oxygen are any rocket engines. Most rocket engines run on some liquid fuel and liquid oxygen. A leak of the latter could cause corrosion to any iron which gets exposed to it.

By the way: The lack of oxidation in vacuum also has another interesting effect which could be either a blessing or a curse for space construction: It allows you to cold-weld. Pure, non-oxidized metals have an interesting property: When they touch, they stick and form a single piece. That means you could break a piece of metal into two, put the parts together again, and they would fuse without a trace. This effect is hard to reproduce on earth, because the moment you break a piece of metal, the exposed area comes into contact with oxygen and a nano-scale corrosion layer forms immediately which prevents cold-welding. But it works in an artificial pure vacuum environment.

For space construction, this could be a blessing because it makes it much easier to put large structures together. Just move two girders together and they fuse the moment they touch. No welding, screws or bolts required. But it could also be a curse, because it is easy for accidental welding to occur. Any surfaces which are designed to touch and separate again (like mechanical gears or joints) need to be coated to avoid accidental cold-welding from happening. Exposing such parts to an oxygen-atmosphere for a short duration could be enough, though.

Answer 2

Iron/steel in low earth orbit most likely would eventually rust, due to the presence of highly reactive atomic oxygen. I don't have data on steel specifically (most steel launched is stainless), but atomic oxygen does cause erosion of hydrocarbon polymers, silicone polymers, aluminum, and silver. I would direct your attention to this paper, which shows micrographs of surfaces exposed to AO.

Summary

Atomic oxygen is formed in the low Earth orbital environment (LEO) by photo dissociation of diatomic oxygen by short wavelength (< 243 nm) solar radiation which has sufficient energy to break the 5.12 eV O2 diatomic bond in an environment where the mean free path is sufficiently long (~ 108 meters) that the probability of reassociation or the formation of ozone (O3) is small. As a consequence, between the altitudes of 180 and 650 km, atomic oxygen is the most abundant species. Spacecraft impact the atomic oxygen resident in LEO with sufficient energy to break hydrocarbon polymer bonds, causing oxidation and thinning of the polymers due to loss of volatile oxidation products. Mitigation techniques, such as the development of materials with improved durability to atomic oxygen attack, as well as atomic oxygen protective coatings, have been employed with varying degrees of success to improve durability of polymers in the LEO environment. Atomic oxygen can also oxidize silicones and silicone contamination to produce non-volatile silica deposits. Such contaminants are present on most LEO missions and can be a threat to performance of optical surfaces. The LEO atomic oxygen environment, its interactions with materials, results of space testing, computational modeling, mitigation techniques, and ground laboratory simulation procedures and issues are presented.

Illustration

You can see it very clearly on this picture as discussed here:

   ^{Rick Mastracchio as EV1 (red stripes) and Mike Hopkins as EV2 (no stripes) leaving the airlock during US EVA-24 (Credit: NASA)}

Answer 3

It could be a concern, if it's anywhere near some oxidizer, say on the inside and having contact with atmospheric oxygen, propellant oxidizer, from your own propellants leaking forward of your path, freezing solid on the frame and slowly outgassing due to lack of pressure (baking when exposed to the Sun), splashing onto the frame from high oxidizer burn mixtures, or ionospheric atomic oxygen (e.g. see here).

But unless required oxygen is already present in the environment, or you somehow add it there (unwittingly or otherwise), there wouldn't be any oxidation, thus no rust. And with up to a few proton particles of matter per cubic centimeter in outer space, and only ~ 0.05% of that oxygen atoms, it would take a long, long time for any rust to form. One thing to add is that while your question is explicitly about iron, with some other metals like Aluminum, a few atoms thick oxidized layer can even be intentional and can protect material from further decay.