Well, not quite. Rust eats into iron because oxidised iron is larger and much more brittle than unoxidised iron, physically ripping itself out of place.
Many oxides arent that much larger than their base metals and form a nice patina protecting the metal underneath, like in aluminium.
Other oxides destroy the structural integrity of the metal and eat into it, forming corrosion. Rust is just corrosion specific to iron.
Well, not quite. Rust eats into iron because oxidised iron is larger and much more brittle than unoxidised iron, physically ripping itself out of place.
Many oxides arent that much larger than their base metals and form a nice patina protecting the metal underneath, like in aluminium.
Other oxides destroy the structural integrity of the metal and eat into it, forming corrosion. Rust is just corrosion specific to iron.
I didn’t know of other type of oxides that eat into the metal like rust does…
But it’s true that a “rusted crown” implies that it is iron-based, so the cantrip should work!