The Leicas are made from brass and were nickel plated. Brass is used because it is easy to machine. The nickel plating allows the cameras to take paint easily. Brass has an added advantage in that it is self lubricating preventing the lenses from binding to the lens mount.
While aluminum is light it has the disadvantage in that it tends to bind with other metals. So even though a camera body may be made from aluminum the lens mount is usually made of brass. The binding problem also explains why cheap filters using aluminum tend to bind to lenses. Aluminum also corrodes easily.
Metals parts that are intended for chromium plating are first plated with nickel. Failure to do so causes the chromium plating to eventually develop pits.
An alloy called german silver composed of 60% copper, 20% nickel and 20% zinc is also useful for camera parts and was widely used in the early 20th century. It has a silvery appearance, takes a nice polish and does not tarnish.