Your mast suffers from galvanic corrosion. The zinc anode would only help for parts which are constantly below water. As mentioned above WD40 helps, good paint would help as well. The idea is to have an insulation between alu and steel. It is also true for aluminium and carbon. In both cases aluminium will corrode. The paint or WD 40 is only needed where both materials came into contact. Scratches in aluminium are not prone for corrosion (if the right alloy is used for your parts. But it is most likely that your mast is from the right aluminium alloy).
Of course best way is to have no galvanic combinations at all, but rarely seen.

Cheers,

Klaus