Unfinished cast metals weather well in most instances. Cast iron, aluminum, and bronze will last many decades with little change if left unfinished. The natural changes in metals due to atmospheric exposure is called patination. A metals patina is actually a thin layer of oxides of the metal, the products of corrosion, that acts to slow down further corrosion.

On cast iron the patina is iron oxide ( to the chemist it's iron oxide to the rest of us it's just rust). Cast iron rusts extremely quickly. In fact cast iron will begin to rust when the relative humidity exceeds about 64%! Unlike steel, however, the rust on cast iron is not invasive but will act like a coating to prevent deep rusting. Therefore rusting on cast iron gratings in no way harms their structural integrity. The patination of cast iron grates goes through a predictable set of stages. The duration of each stage depends on local moisture conditions and the amount of foot traffic. So grates will progress slower in the desert than at the beach but the process is inevitable regardless of location.

 

newly cast iron grate
grates usually arrive on site like this.

Iron grate about three weeks after installation near the ocean. Iron grate after about three years in Phoenix Arizona. Unfinished iron grate after about one year at a California theme park. Grates are hosed down daily.
Natural Aluminum grates get a coating of aluminum oxide. which is very hard. This thin transparent patina keeps aluminum grates looking bright for years. The photo at right shows an unfinished aluminum grate after three years near the ocean in Florida.
Bronze is well known for its patinas. The natural patination of bronze is unpredictable and is part of its charm. Colors arising from the oxidation of bronze range from pale greens and yellows to white to turquoise to dark brown. Most very dark bronze patinas are produced artificially by chemical process.  

Samples of unfinished grates