1990 Accord bad battery...altenator?

my car is powering down (lights dim) when I put it into neutral. I
suspected that I need a new battery so I just took it in to a local
mechanic - he agreed that I need a new battery, but the battery leaked
acid which now has corroded the altenator. Can a battery corrode the
altenator? Do I just replace the battery and not the alternator? Or
do I replace it all....we're talking $459.00 and the car has over 156
thousand miles

 

Battery can corrode whatever is directly underneath it. When it boils over,
a corrosive acid bubbles out the caps and drips off underneath.

Just because there is acid corrosion on the outside casing of the alternator
is not a good enough reason to replace it, unless of course it doesn't work
anymore. The real question is what caused the battery to leak?
Overcharging? It might be that it was a bad alternator that fried the
battery in the first place. If so, then you will cook the new battery in
short order if you stay with the old alternator.

The right answer is to replace the battery and have the charging system
fully evaluated to see what else needs to be replaced/repaired.