Confronted with this problem on a salt-water Alpha, I used a sturdy ball joint puller to push the lower pin out, hoping to shear the remains of the cotter key on the way out, as mentioned above. Although this has worked for all-steel vehicle suspensions, it did not work here -- the pin drug through the fiber lower bushing and tore out a little bit of the aluminum casting. Both of these could have been dealt with (replace bush, live with small aluminum nibble) but I broke the lower yoke in the end, and replaced the casting with a much nicer one shipped over from Minnesota (land of fresh water lakes) Although costly, this actually solved a plethora of corroded-related damage for me. In hindsight, I would recommend the tip to drill access holes in the housing to drive the pin out first, then check not to use excessive force driving the pin out.
On a related note, I made short work of the super-corroded top pivot by using a torch to heat the rusty cross-bolt and top nut and the exposed end of the pivot pin (on the bench, lots of access). Fortunately, the last did not appear to ruin the ($400 OEM) pivot pin for reuse. Various pentrants and "easy-out"-type drivers barely work for me but heating to dull red works every time.
On a related note, I made short work of the super-corroded top pivot by using a torch to heat the rusty cross-bolt and top nut and the exposed end of the pivot pin (on the bench, lots of access). Fortunately, the last did not appear to ruin the ($400 OEM) pivot pin for reuse. Various pentrants and "easy-out"-type drivers barely work for me but heating to dull red works every time.