Generally, if you cannot avoid dissimilar metals, you want the larger part (Al. mast) to corrode preferentially to the smaller part (S/S fastener)*.
is generally less active (cathodic & more noble) and higher on the the Galvanic series than is aluminum (anodic & less noble). This is an acceptable solution, especially if an isolating coating is used on the connection.
* To avoid corrosion, avoid a small anodic area relative to the cathodic area.
- Use LARGE ANODE AREA (mast) & SMALL CATHODE AREA (fastener).
The larger the relative anode area, the lower the galvanic current
density on the anode, the lesser the attack.
- The amount of galvanic corrosion may be considered as proportional to the Cathode/Anode area ratio.
Design for a SMALL Cathodic/Anodic Ratio (CAR). (When designing, remember your small CAR.)
- The same metal or more noble (cathodic, higher number in the table) metals should be used for small fasteners and bolts.
➥ Galvanic Corrosion Series Chart*of Metals Seawater