If there is a steel plate floating in space, for ever, within the solar system, how is it going to degrade? Of course without oxygen it is not going to oxidize (rust), but how are cosmic rays, UV light, alpha particles and the general space environment going to affect the structure of steel on the long term?
EDIT: Let's imagine a plane of steel on orbit around the earth.
Just curious.
As previously mentioned, micrometeorite pitting will occur over sufficiently long time scales.
Steel is composed of microcrystalline grains, and as such it is likely that cosmic rays will cause the formation of crystallographic defects, leading to embrittlement of at least the periphery of the steel in the long term. Inelastic collision between the steel and ions or molecules floating in space may result in interesting chemistry on the surface of the steel, however this absolutely depends on the particular species the steel impacts with. Note also that space is very empty and as such collisions will happen with a very low frequency.
Ultraviolet light will cause photoelectronic emission. As such, the steel will develop a positive charge until the electron work function exceeds the energy of the light that is hitting it. This shouldn't degrade the metal, however very high energy photons (such as gamma rays) can cause crystallographic defects as discussed in this report by N.P. Baumann.
I would probably think that it would depend a lot on where it is and what sort of time frame you are looking at.
At a place where it would be more "susceptible" to the forms of radiation you described (near a star) there are probably more important environmental factors. Not only will be be gravitationally attracted to a star or planet, but it will be slowly eroded by micro-meteor impacts.
Of course at the longest of the long time ranges, you will have to worry about proton decay or more complicated effects like that.
With the chemistry of steel in particular, I don't have any idea.
I will address this problem from a very "classical physics" and engineering perspective. Physicists may offer some failure mechanism for the steel as the universe itself evolves significantly, but as far as I'm concerned it will never degrade, although "degrade" is a very subjective term.
What you're looking observe in the sample is long-term radiation embrittlement as well as swell and some other effects. The specifics will depend on the environment, which differs greatly from one point in space to another. Are we talking about interstellar space? Intergalactic space? Wherever it is, it will experience some radiation damage, and expect for a small volume of exceptions we are talking about a service life entailing:
The first thing to understand is that critical metrics for radiation damage come from the time frame times the radiation energy deposition rate. From there, displacements per atom (dpa) is common metric representing the number of time every atom in the material is displaced from its lattice site on average. Values greater than 1 are common for discussions on this subject, but starting at around that magnitude effects like embrittlement and swell start to become very major. The steel will literally swell and becomes very brittle. If it is a load bearing member, the integrity of the structure it is a part of needs to be carefully considered for the long service life. However, effects like Stress Corrosion Cracking (SCC) will not be a major problem like they are in things like nuclear reactors because that entails a tipple attack of 1) high temperature 2) chemistry and 3) radiation. For the steel in space we have the radiation, low temperature, and no chemistry attack from the environment. Over a long time, self-annealing is also major, but that has an interplay with the temperature, and low temperature may slow that effect.
Generally though, the steel will maintain its structure. You need to ask someone familiar with space travel to quantitatively talk about the radiation environment, but since humans survive in LEO, the radiation in that kind of environment is probably not going to change the material structure of the steel for an extremely long time, at which point it's relatively accurate to say that it doesn't degrade.
