What are the effects of the elements on 3D printed objects made with "infused" PLA
If you use filaments filled with metal particles, some particles will be exposed to the environment. Depending on the corrosion resistance of those metals, yes the environmental conditions will weather the print object. So if it contains iron (and does not contain elements that prevent oxidation like used in stainless steel) and it is subjected to water and oxides, the print will rust. 

This is e.g. also valid for copper filled filament, which you can polish to get a gold shine surface (removing the copper oxides), or bronze filled filament, that if correctly subjected to weathering environments, will give the looks of a very old statue. In both cases this implies that the metal particles have undergone a(n) (de-)oxidation process.