How do I dissolve corrosion between upper and lower outboard unit so
to get at the impeller?

I took all five nuts off (125 mercury sal****er) the studs holding the
lower unit on. I had to use penetrating spray to get the washers off.

After whacking the seam dividing the upper and lower with a rubber
hammer till blue in the face I etched/scored/scratched the seam to
allow penetration spray to work in, nothing.

After 24 hours of repeated spraying whacking and more scoring, the two
halves still won't come apart. Short of pounding in a chisel (gouging
aluminum), and prying them apart, anyone have an idea?