What characteristic of steel makes it highly susceptible to corrosion?

The characteristic of steel that makes it highly susceptible to corrosion is its tendency to react easily with oxygen. Steel, primarily composed of iron, will readily oxidize when it comes into contact with moisture and oxygen in the environment. This reaction leads to the formation of iron oxides, commonly known as rust. The process involves the electrochemical reactions that occur when steel surfaces come into contact with electrolytes, such as water, leading to a deterioration of the metal.

Other options do not contribute directly to the susceptibility of steel to corrosion. For instance, while the molecular structure and presence of carbon can influence the properties of the steel, it is not the primary factor concerning its corrosion potential. Similarly, being a poor conductor of electricity or being lightweight does not inherently affect the corrosion behavior of the steel. The ease with which steel reacts with oxygen is the pivotal aspect that facilitates its corrosion process.