In a plumbing system, which material is at the highest risk of corrosion when used underground?

Galvanized steel is at the highest risk of corrosion when used underground due to its composition and the nature of its protective coating. The zinc coating applied to galvanized steel offers some level of corrosion resistance; however, when it is exposed to moisture, especially in soil that may contain salts, acids, or other corrosive elements, the zinc can degrade over time. Once the zinc layer wears away, the underlying steel is exposed and becomes susceptible to rust and corrosion.

In contrast, copper also suffers from corrosion, particularly in acidic or low pH environments, but it is generally more resistant than galvanized steel in most conditions. Plastic materials are not subject to corrosion in the traditional sense, as they do not rust or oxidize, making them a better choice for underground applications. Cast iron, while it can corrode, typically has a different level of durability and resistance to environmental factors, especially when protected with a coating or lining.

Thus, galvanized steel, due to its reliance on a thin protective zinc layer and vulnerability to corrosive elements found in soil, stands out as the material most likely to corrode in underground plumbing systems.