Corrosion layer (7)

I believe the answer is:

tarnish

(Other definitions for tarnish that I've seen before include "Stain or blemish as with silverware" , "Stain, metal or reputation" , "Besmirch" , "Stain or discolour, say metal" , "Take the gloss off" .)

I've seen this clue in The Star Tribune.

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