Get fluorine out to corrode metal? (4)

I believe the answer is:

etch

I'm a little stuck... Click here to teach me more about this clue!

'corrode metal?' is the definition.
'etch' can be an answer for 'corrode' (I've seen this before). I'm not sure about the 'metal?' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for etch that I've seen before include "Engrave deeply" , "Make carvings" , "Cut design onto metal using acid" , "Make a deep impression" , "Engrave; fix firmly" .)

I've seen this clue in The Independent.

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