Corrosion of Aluminium mast in region of stainless screw. This is exactly galvanic corrosion from dissimilar metals being in electrical contact.
New Stainless fitting sealed with zinc chromate paste (green) in order to avoid such galvanic action.
Corrosion of aluminium mast at step. This is likely to be poultice corrosion which occurs in a wet bilge.
After surface preparation aluminium should be primed within about 20mins with etching primer. Steel brushes should never be used for surface preparation because they can leave small particles which make galvanic corrosion. Stainless steel hardware must be electrically isolated from the aluminium mast with suitable paste for instance zinc chromate or pads of various materials.
Still even if you have done all that work there is potential for crevice corrosion to occur when sea moisture gets under the paint and excludes oxygen. It will enter at cracks in the paint, typically these occur around hardware where the paint edge is exposed.
Drilling of holes for hardware may be done after the mast is painted. Thereby creating an edge of paint. Also even if the mast is painted after the holes are made, there can be cracks or just thinness of the coating at the hole which allows moisture to enter.
Crevice corrosion under paint of aluminium mast. 2 years old.
Another example. The corrosion spreads under the paint.
Big Sparcraft Mast
34 year old anodising still in good condition. Do you really want to paint your mast?
Galvanic corrosion of aluminium cross beam of a catamaran in region of anchor track support. Yes it is going to detach soon.
Mast aluminium extrusion is damaged at deck step. The mast is condemned. Acceptable repair would be to shorten the mast but ideally to replace.
