When the first couple of Supermarine Spitfire fighters were deployed on a British aircraft carrier in the late 1930s, the structure was eaten away by corrosion within 6 months. The sea water spray was enough to cover them in salt, which would quickly reduce the aluminium structure to white dust. Before, Navy airplanes were of wooden construction, using steel for engines or bolts, but no high-strength aluminium.
During the Berlin airlift, table salt could not be transported by the usual fleet of C-47s and C-54s. Even the dust coming from the bags would cause rapid corrosion of the aircraft. So how did they bring the daily ration of 38 tons of table salt to Berlin?
Seaplanes can handle salt much better. Salt and baking powder was brought in by British Short Sunderland flying boats landing on a stretch of the Havel river inside West Berlin. Learning from the early experience with naval Spitfires, corrosion resistant alloys were developed. Typical for marine application are the aluminium-magnesium alloys that belong to the 5000 series. The magnesium oxides enhance and reinforce the protective oxide layer of aluminium whereas copper weakens the oxide layer. Early high-strength aluminium alloys use copper atoms dispersed through the aluminium matrix which locally distort the atomic lattice and strengthen it. This is called precipitation hardening, but is unfortunately exactly what makes the 2000 and 7000 series alloys very susceptible to salt corrosion.
So the answer is to pick the right alloy.
From this report on the subject:
Based on our very long experience of aluminium’s corrosion behaviour in marine atmospheres and in sea water, we can now assert that the service life of aluminium in marine environments will be exceptionally long and can be measured in decades!!! We can truly claim that aluminium is the “metal of the sea” of the modern age.
… but only when the right alloy is used!
While the 5000 and 6000 series alloys will be preferred for their resistance to corrosion, their weldability and their level of mechanical characteristics, other wrought alloys belonging to the 1000 and 3000 series are suitable for use in non-structural applications for decoration, interior fittings etc.
The use of 2000 and 7000 series alloys on the other hand must be the exception in view of their poor resistance to corrosion, and they require special protection when used in a marine environment.