Very often, though not always, a corroded metal can be identified by the characteristics of the corrosion/patina. Regarding Acaldwell32's finds, I know of two metals which show the characteristic I call "snowflake" corrosion, meaning, blotchy white spots (kinda like snowflakes) on the grey metal. The two metals are zinc and aluminum. The way to tell those two metals apart is by their comparative weight. Zinc weighs almost exactly the same as cast-iron (whose "Specific Gravity" rating is 7.1), and aluminum weighs only 40% ("Specific Gravity" rating is 2.7) as much as zinc and cast-iron do.
In other words... if Acaldwell32's find weighs the same as a cast-iron object of the same size weighs, it is made of zinc. If his find is a lot lighter, it is made of aluminum.