"Become rusted" is a phrase that describes the corrosion of metal over time. There are several synonyms for this phrase that can be used interchangeably, including corrode, oxidize, tarnish, and rusticate. Corrode refers to the chemical breakdown of metal due to exposure to water or air, while oxidize refers to the reaction of metal with oxygen. Tarnish describes the dulling or discoloration of metal due to exposure to air or other elements, and rusticate specifically refers to the formation of rust on iron or steel. All of these synonyms can be used to describe the process of metal becoming rusted or discolored over time.