The primer is what is corrosive not the powder. The only time I have seen any problems with powder is when you have a single base gun propellant and the stabilizer is depleted. You will get a Redox reaction that will turn the propellant red.
Any red powder, red rust like coating, red gas, or if the powder has become tar like is grounds for disposal of that propellant.
The gas by the way is highly poisonous and should not be inhaled.

Do not keep any old powder if it has any of the above characteristics. The easiest way to dispose of it is to spread it on your lawn. It will make great fertilizer.

As far as reusing old propellant, I would just go purchase new and be safe, not sorry.