You're exposing a metal foil to an oxygen plasma =atomic oxygen=highly reactive oxidizing species. Therefore it's rather logical that it gets oxidized.

Hi Jürgen Weippert , thanks for your response - I understand the basic principles of oxidation. My question was: why did my samples burn to black at low plasma etching conditions that are commonly used for backside CVD graphene removal? Could the reason behind the unexpected burning be that the samples were already partly oxidised before I treated them with oxygen plasma?

Thanks in advance.

Plasma conditions can affect a sample in vastly different conditions depending on the chamber geometry. If you have a "sample-in-plasma" geometry, the sample will be strongly heated in addition to the oxygen while if you have a "remote plasma", the identical conditions will lead to only very little heating and therefore the thickness of a potentially forming oxide layer will most likely be restricted to the first few nanometers.

That distinction is very helpful - thank you. I was using a “sample-in-plasma” (cylindrical chamber) geometry which, following your logic, could be the cause of my issue. I will try a remote/localised plasma set-up for my next attempt. Thanks again for your insight and time!