Dear student,
Corrosion of Iron in the presence of oxygen and air leads to the formation of brown solid known as rust. This process is called rusting due to the formation of rust.
4 Fe + 3 O2 + 2x H2O → 2 Fe2O3.xH2O
The process does not take place in the molecule of the plastic as the Fe is not present in that.
Therefore, the plastic does not rust like iron.
Regards