corrosive,
It depends on the flux.
see, Some fluxes are corrosive over time, although not generally the ones you's normally use.
Even good flux, if contaminated can become corrosive - I used a flux ago,that was OK for a while, but then started corroding things a few days after application.
Some types will also absorb moisture, so if you have high impedance stuff it can cause long-term problems.
I often , adviced to clean your PCB after soldering with isopropanol alcohol because
1.) the solder flux could oxidize the copper in the long run
2.) the solder flux is not as high-ohm as a cleaned PCB and can create high-ohm connections on the PCB
it is much better to NOT clean the PCB from (pure rosin!) solder flux, because
1.) Isopropanol is aggressive to some plastics and can intrude into parts creating undesired effects.
It can e.g. dissolve polystyrene capacitors AFAIK
2.) By this dissolving of other materials a potential low-ohm film is spread over the PCB while the rosin itself is very high-ohmig when left untouched at the solder joints.