Microbial corrosion, also called microbiologically influenced corrosion (MIC), microbially induced corrosion (MIC), or biocorrosion, is when microbes affect the electrochemical environment of the surface they are on. This usually involves building a biofilm, which can lead to either an increase in corrosion of the surface or, in a process called microbial corrosion inhibition, protect the surface from corrosion. 
With every surface that is in some way exposed to the environment also exposed to microbes, microbial corrosion causes trillions of dollars in damage around the globe annually.
Microbes act by either producing byproducts from their cellular processes that corrode metals, or preventing normal corrosion inhibitors from functioning and leaving surfaces open to attack from other environmental factors.