But why would you need a new wrench in the first place? It's already rusty; it means you never needed it, you never used it, but suddenly you want a new shiny wrench that can easily cave in a part of a human skull. Why?
Usually a rusty wrench is caused by moisture setting on the wrench. Even in a toolbox a wrench can get rusty, even if it is used a fair amount. Though the main reason for replacing it would be safety during use. If you turn a wrench and the rusted edge of the wrench cuts you then you'll be needing a tetanus shot. I've had to get a few myself from that exact circumstance when I worked in an automotive shop.