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.
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.
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