What is it?
Bronze is an alloy of Copper, Zinc and Tin. The tin acts as corrosion inhibitor keeping the zinc in solution and restricting the oxidation of the copper to the surface only. Brass is similar but has no tin thus allowing corrosion to continue with pitting into the metal until the metal is eaten away completely.
The oxidation on the surface of the bronze is often called verdigree or patina, this forms a natural protective layer that ensures that bronze will last forever. This is why Arrow Bronze warrants that the inscription, borders, emblems and motifs will last forever. Regardless of what coatings are placed on bronze, eventually they will break down and the natural patina process will take place.