You can either use sheet metal screws or machine screws. I prefer to tap the hole and use machine screws. In my opinion, the coarser the machine screw threads are the better - 20 better than 24, 24 being better than 32, because coarser threads are deeper.

You are right to be concerned about corrosion. The screws you use will be stainless steel and the mast obviously is aluminum - a set up for disaster. The screws will lock and eventually break or pull out if not properly set with a corrosion inhibitor.

You should use a corrosion inhibitor on the screw before being screwed in. On page 572 of the West catalog you will find "Tek Gel" What I have used is USS Tef-Gel which I got from West but don't see it listed anymore. Perhaps Tek-Jel and Tef Jel are the same. But, that is the type of stuff you should use, and it works fabulously.