So.... Bright and early this morning I went to a door and windows place right down the street from me. I brought my phone and showed the guy the pictures I posted here. He told me he has seen this type of corrosion a lot in this area. It can be a combination of many things. Mostly it's a chemical reaction with the chemical mixture in the concrete, which conducts electricity through the Aluminum track.
Add ground moisture and time, and this is the result. The solution is a new slider, which I expected. He sells Milgard, which is what I had when I replaced the slider in my other house in Glendale. They have several different grades and sizes. For me it will run around $1,400.00 and change for something with halfway decent quality.
He's coming in a couple of weeks to do some exact measuring, then he'll order it and I'll be done. Most all of the models he sells are Vinyl, so this can't happen. (Vinyl doesn't conduct electricity, and is far more corrosion proof). I'm not worried about direct sunlight exposure because it will be protected by the patio roof.
When he's done, it will be done. No painting, stuccoing, trimming, or spackling, on my part. For that price I'm seriously thinking about letting him do both of them. That way they'll match. The screen door on the one in my dining room is a POS that doesn't line up right. He told me it could take 6 to 8 weeks because of Covid. Everything is back ordered, but he told me they are shipping now quite regularly.