Corrosion
Corrosion is the slow chemical breakdown of metal when it reacts with water, air, or other substances around it.
Think of it this way
Leave a steel wool pad in a cup of water and come back in a few days. The shiny silver metal has turned rough and orange. The water and air reacted with the iron surface and changed it into rust. The metal got weaker because its outer layer turned into a new, crumbly substance.
Explaining corrosion by grade level
Leave a steel wool pad in a cup of water for a few days. When you come back, the shiny metal has turned orange and rough. That orange stuff is rust. The metal mixed with the water and air around it and changed into something new. The metal got weaker because part of it turned into rust.