On dual rear wheel vehicles (also known as dually or DRW), sometimes the factory rear inner is re-used when installing aftermarket wheels and tires.
This factory rear inner can be constructed of steel, while aftermarket rear outer dually wheels are almost always made of aluminum - this raises the question of possible corrosion due to dissimilar metals being in physical contact.
Galvanic corrosion occurs when two dissimilar metals are physically coupled (touching) in a corrosive electrolyte (in this case, water with some salt content).
However, in the case of steel rear inner wheels, they usually have a special coating designed to resist corrosion, since they are already in direct contact with the outer rear wheels from the factory. These OEM outer rear wheels may be aluminum as originally installed, necessitating this coating. Even in the case of the pair of rear dually wheels both being steel, it is likely that this coating is still present.
If there are significant concerns regarding possible corrosion at the mounting surface when aluminum outer and steel inner rear wheels are used in combination, we recommend cleaning that surface of any debris or buildup during installation. An additional recommendation is thoroughly rinsing / washing the vehicle after driving in conditions involving salt water or spray. These conditions are usually found on the beach or when using boat ramps, or during the winter, when de-icing salt is applied to ensure safe road conditions.