Question

why plastics do not get corroded?

Answer 1

Plastics and ceramics as well as composite materials are poor conductors and are not susceptible to electrochemical corrosion as metals. They corrode but not as readily, and the corrosion occurs when unstable molecules in the environment react chemically with the molecules making up these non-metal materials.

Answer 2

As such metals other than the above corrode. Plastics, more accurately called polymers, are not metallic so they don't corrode as such. However, the do suffer from environmental degradation. Polymers degrade in the environment because some agent causes the covalent atomic bonding in them to break down.