You can also see superficial corrosion of supply line connections.  

I believe the current fix is to crimp on pigtails to all ends and replace the old switches and outlets... varies but think it is running $2500 to $5500 per house to correct... aluminum vibrates when it gets a little loose causing resistance and then heat.. hence the problem.lots of homes have black pipe or steel pipe water lines used a lot in older homes and it is OK but it rusts inside and then as the corrosion continues the pipe gets weak at the joints mostly and often can fill up  and restrict water flowall this being said I have seen extension cords run behind finished walls,  I have heard people removing 40amp breakers and replacing with 50amp breakers because the 40amp keeps tripping.  

You also need to blow out the dust/corrosion in that panel & add some anti-corrosive/connector gel.I replaced several 100 amp breaker panels over the last 12 months for this reason & to remove random sub-panels that were not to code. 

Rust/ corrosion was observed at the panel box or breakers.

Plumber went today and explained pipes are blocked, and some corroded (expected with the age of the property), but unable to say where the block/corrosion is to quote properly until they "start to dig". 

All cities have their own problems...Las Vegas desert water cause a lot of pipes corrosion and water leaking issues.  

Maybe would try to investigate with through the fill pipe with a borescope for some corrosion holes and cleanliness of the bottom to understand potential spill potential ($25 on amazon).Good luck!

If that isn't fixed correctly, the corrosion will get worse over time until you get to the point that the mesh no longer serves its purpose and begins to detach from the house.

Bleach is extremely corrosive to humans and has cumulative affect when exposed.

Also reduces corrosion and buildup on the pipes.Did the occupied side of the duplex have a softener installed?