How do you clean/remove rust on the inside of chrome bumper?

Don't use Muriatic acid ! If you get any on the chrome it will instantly ruin it .There is an acid you can use not as harsh.Phosphoric Acid .Truck supply places sell it as a truck wash for alum trailer bodies. Even with this milder acid you need to protect the chrome with something like a liquid masking agent or grease and ALWAYS WEAR EYE PROTECTION ! Concrete cleaning products from the hardware store also contain phosphoric acid and can be used.These work faster than vinegar but you need to pay attention to what you are doing