like i said i believe they were galvanized to prevent rust not stop it full stop as this is impossible! i may be wrong but it is something i was told a few years ago about differences in the 2 shells.

Although galvanizing will inhibit attack of the underlying steel, rusting will be inevitable, especially due to natural acidity of rain. For example, corrugated iron sheet roofing will start to degrade within a few years despite the protective action of the zinc coating. Marine and salty environments also lower the lifetime of galvanized iron because the high electrical conductivity of sea water increases the rate of corrosion. Galvanized car frames exemplify this; they corrode much quicker in cold environments due to road salt. Galvanized steel can last for many years if other means are maintained, such as paint coatings and additional sacrificial anodes.