Welcome, Peterr!
I could imagine (just a hypothesis) that the metal takes the bromine away from the carbon to build a metal bromide.
I had a similar case when using a freon containing chlorine, maybe trichlorotrifluoroethane, I don't remember exactly. Nice as a grease solvent, but I wanted to clean a satellite with it, including aluminium surfaces. After wiping the aluminium 10 times, 20 times with a special ultra-clean white cloth (for semiconductor processing) and freon, the cloth was still grey - until the seller told me this Freon corrodes aluminium.