There are RFID tags which have been designed specifically for mounting onto metal objects, for example for tool tracking in manufacturing, or automatic road tolls for cars. Some tags are even designed to be mounted inside metal objects, such as [the Pico iN Plus from Xeraphy. Incredibly it has a claimed read distance of two meters @ 2W when embedded inside a metal object! They’re not cheap though, at £4 a pop in quantities of 20…](RFID UHF Xerafy Pico-iN Tag)