I have a metal planter on my patio. Why do squirrels come up and chew on the sides of it? Are they sharpening their teeth?
It is most likely that instead of sharpening their teeth, they are actually shortening them.
Like all rodents, squirrels' incisor teeth grow continuously, and need to be worn away, which is achieved by gnawing on things like wood, or in your case metal. Soft metals like tin are suitable for this purpose because they are hard enough to wear away the teeth, but not so hard as to cause damage to the teeth.
I suggest that you get a sacrificial block of wood such as hickory, oak or ash and place it in your garden. The squirrels are likely to transfer their chewing habits to the more suitable wood.