Simon, part of the problem is the mixed metal that you mention above, steel, zinc, stainless steel, aluminium, add in some mixed metal fixings too and it's a cooking pot for bimetallic corrosion and even some potential galvanic rot as the metalwork is often used as the neutral (earth return), add some moisture then increase it's conductivity with salt & BOOM ! Well not quite, but you can almost hear it fizzing away
