Radiation will send high energy particles that may displace metal atoms from their lattices. This may cause weakening, embitterment etc. Also shooting high energy particles at atoms will cause atoms to get into chemical reactions that they would have never gotten into before. This may cause new and unusual forms of rustlike behavior.

It is not predictable because with radiation you are deep in quantum physics territory, and a lot of things depend on chance. Things may be perfectly fine if only isolated atoms are kicked out of their lattices, but if you are unlucky some atoms along a line get kicked out and you get a crack.

Similarly you are not sure exactly what atom will get energized by some high energy electron and what other atom it will react with. Once you get into high energies, you can get a lot of chemical reactions that seemed impossible otherwise, or that nobody has ever thought of. There again usually you get some isolated strange compound. But if you are unlucky you get something that can act as a catalyst for further reactions and spread around like rust.