Some, but not all, metals are biodegradable. Metals are considered to be biodegradable if they are broken down by their environment; a common example of which is iron being broken down into rust by oxygen.
Another factor in biodegradability is the environment. Iron- and magnesium-based metals biodegrade within the human body. Another environment in which metals can biodegrade is seawater. According to the Center for Microbial Oceanography: Research and Education, it takes a tin can 50 years to completely biodegrade in seawater, while it takes an aluminums can 200 years to do the same.
When metals biodegrade in an outdoor environment, they react with elements or moisture in the air. These chemical reactions slowly break the metals down, biodegrading them. One of the most commonly seen reactions is the rusting of iron. Rust is actually an iron oxide created by the reaction between iron and oxygen. These oxides can wind up back in the soil, where they can be absorbed by plants, according to the NIH's National Center for Biotechnology Information.
Other metals do not biodegrade much or at all. According to Newton's Ask a Scientist web page, gold and platinum are examples of such metals. Other metals, such as aluminum and magnesium, only react with oxygen until they create a protective layer that prevents further corrosion, so these metals are also not considered to be biodegradable.