That is some nasty corrosion you got there fella.
When my Rad was leaking and I ended up replacing it with an Aluminium one, I did a heap of searching on here to find out what I could.
Being the eternal Noob that I am, I am thinking perhaps it has been caused by:
a) not enough (if any) anti-freeze / anti-boil in the cooling system. There is anti-corrosive additives in it to stop rust etc.
b) Old coolant loses its ability to work properly and therefore can lead to corrosion of the system.
c) Using tap water in the system instead of distilled water. The impurities in the water cause corrosion.
d) air bubbles in the system.
e) stray electrical current in the coolant (caused by bad earthing) can cause electrolysis which will eat away at metals and make them corrode away, especially when there is different metals involved. Brass or aluminium radiator with alloy head, cast iron block and perhaps whatever metal that pipe was made of.
As stated, I am the eternal Noob and some of this info may be a little blurrrred (correct me if I'm wrong), but I reckon there is a good chance that some or all of these things may have killed your pipe...