IC pin corrosion

Out of interest, are any particular types of environment harsher for IC leg corrosion than others?

I've not seen that much corrosion about, other than surface stuff that can be cleaned off - but I'm currently trying to get an old Acorn System Three running, and about half the ICs in it are leaving some of their pins behind when removed from sockets as they've corroded right through :-( The PCBs are otherwise in very good condition, but for some reason a lot of the chips (most notably DRAMs and LS logic) are in a right state, and it would seem that *something* has been eating away at them - yet *some* of the ICs are totally corrosion-free (as are all the passives, IC sockets etc.) Luckily all the dead parts (so far!) are ones that I've got spares for, but it's just made me wonder what's accelerated the decay in some of the chips but not anything else (unfortunately I only picked this machine up last week so don't know its storage history, but I can ask the previous owner). Judging by the pretty good condition of the rest of the machine, I wouldn't say that it's been in a particularly high moisture environment.