I got wrapped up in my digressing from the topic - I forgot to add to the casting material and oxidation questions. Up until several years back, one of my customers was Carter in TN, a fuel pump plant. Pressure die cast aluminum, really pure stuff, made mechanical fuel pumps and even AC pumps under license (I used to have a box full of castings from tooling new production lines, AC logos and all). They were very particular about casting density and porosity, inspection dept. rejected castings like you wouldn't believe. I think a lot of the pitting you see is electrolytic corrosion near the steel portions of the pump, plus road grime. I think Fred does a great job with what he has to work with on 50 year old castings -
Regards,
Steve