In the United States and some other countries, the Rust Belt is a region which used to have a lot of manufacturing industry, but whose economy is now in difficulty.
...in the rust belt of the mid-west.
rust belt in British English
noun
an area where heavy industry is in decline, esp in the Midwest of the US
Rust Belt in American English
US
[alsor- b-]
that part of the U.S. comprising many of the states of the Midwest and the Northeast, characterized as an area marked by diminishing population, aging factories, decreasing production as of steel and automobiles, etc.