Strangely beautiful pattern of rust and dirt
Corrosion is part of the natural process of decay. You can read more about how it happens here.
Also, my twitter pal Patrick Baty writes a lovely and informative blog about historical paints, and he has an article about corrosion in bituminous finishes. It has pictures of unwrapped mummies! And old-timey paint bottles! Do check it out.
Exiled Stardust 