The necrodermis is "living" metal, so theoretically it could get "sick" like a former of skin cancer that would manifest itself as rust
There you go. Nurgle would have a field day.
The word "living" here is used to describe its properties, its ability to repair damage unlike ordinary metals. But it is no more living than any other metal in actuality, as it has been revealed to be based on nanotechnology. It simply appears to regrow as the nano cells realign themselves to their designated configuration.
It's really an interesting question.
Living metal indicates a type of repair based off of an extremely high output of energy that requires a connection to a hub. We know that to be the case with the Necrons.
As we know it, components are drawn back to the receiver so the components can be reformed. Based on our primitive science, that would indicate magnetism by way of Iron composite alloys, all of which rust.
The fact that Necrons appear to be both completely energized and bathed in ozone stands to reason that in the open world environment, they won't rust. In stasis, one can assume there is 0% humidity and low voltage present so really the only time a Necron would theoretically rust is when it is blown to pieces and cut off from repairing.