[78-L] Churchill Speech - corroded metal disc

[Tim Huskisson]

Among a batch of 78s I recently bought is an unusual metal disc identified
visually only by a pencil written 'Churchill Speech' near the centre. It's
not an acetate, and I don't think it's made of aluminium, though I could be
wrong. It's very thin and flexible, and looks more like steel. Obviously not
intended for commercial use, I wonder what its purpose was. Would it have
been used by a radio station? I doubt it because the speech is very
incomplete. The reverse side contains a medley of tunes by a dance band
including versions of 'Yes, My Darling Daughter' and 'Let There Be Love'.  
(The speech was made after Churchill's meeting with Roosevelt in
Newfoundland in August 1941 - the 'Atlantic Charter', and is well
documented)

The disc has a lot of corrosion, with white powder speckling. What would be
the best method of cleaning the disc? Any suggestions?
Many thanks,

Tim Huskisson