This picture concerns me. It looks like the mast is starting to crack from the rivet hole, which could quickly grow and result in a mast failure & de-masting. I would at a minimum drill stop the ends of the two or three visible cracks and weld a thicker aluminum plate on both sides of the mast, under the compression plates. You will need a longer sleeve to go through the mast once you do this.
It looks like you could have some corrosion issues under the spreader bar anchor plate as well, which could be just as concerning as the compression plates at the base of the mast.
As far as preventing the corrosion goes, you need to isolate dissimilar metals, however you can. If you can get either a coating or a plastic between the metals, you should be able to prevent further corrosion. You could get a thin piece of plastic behind the compression plates, or bed them with an epoxy. The tricky part is isolating the rivets from the aluminum of the mast.
For more, you should read up on galvanic corrosion. In any application where you have two dissimilar metals in contact with eachother in water or a humid environment, you'll encounter galvanic corrosion. The problem is exacerbated by salt water and/or electrical current, but even in fresh water and in the absence of a regular electrical current, the corrosion occurs. The idea is that one metal acts as an anode, and the other as a cathode. The anode deteriorates, while the cathode experiences no corrosion whatsoever. This is why ships have aluminum, zinc or magnesium anodes attached to their hulls - they protect the hull of the ship from corrosion, by sacrificing the metal of the anodes (which are regularly replaced). It's also part of the reason why electrical leads are often made of gold, as gold acts as a strong cathode.