Corrosion was always a problem, particularly in the early days when nearly all the skinning was of magnesium alloy. This made the helicopter that bit lighter, but was a Westland con in that the maintenance costs far outwade the benefit of weight saving. As the years went by much of the skinning was replaced (often by MARTSU) with L72 Alclad, but once again if a magnesium skin butted/overlapped an alclad one there would be trouble ahead, as you suggested dissimilar metal problems. In the early days as part of a 25 hour servicing and before the advent of WD40, we used to have to wipe the external skin with OM-15 hydraulic oil for corrosion protection - this made for an interesting surface finish in sandy places like Khormaksar!