Beagle,
Yes the content of fire extinguishers are corrosive, however if you look at the picture on the Boston link, see
BBMF Lancaster Bomber catches fire at RAF Coningsby - Boston Standard
I would say that is them steam cleaning / jet washing the engine and airframe down to remove it.
The huge picture above shows paint gone from the rear of the cowls which tends to indicate the possible area of the fire. I can imagine it being out for a while, as the engine bearers are tubular steel and any heat on them could see those done for, and that does not touch On what else has been damaged.