A cast iron pan rusts because its surface is largely plain iron, and iron reacts chemically with moisture and oxygen to form iron oxide. The only real shield against this is the thin, black layer of polymerised oil that builds up over time, known as "seasoning".
How does seasoning protect the pan, and when does it fail?
Seasoning is a protective crust formed when oil bonds to the iron surface under heat, keeping moisture from reaching the metal directly. That layer thins, and can strip away in patches, when it is scratched with a metal scourer or abrasive sponge, or when the pan sits soaking in soapy water for too long. Rust tends to start exactly where the seasoning has worn away first.
Leaving the pan wet is just as damaging as scratching it: if a washed pan sits damp in the sink or on a drying rack for hours instead of being dried right away, a thin layer of rust can form within a few days, especially in humid kitchens. Running it through a dishwasher carries the same risk; long exposure to moisture combined with detergent stripping the seasoning makes rust appear much faster.
Does a rusted pan need to be thrown away?
Surface rust is usually a reversible problem for cast iron. Once the rust is scrubbed off with a fine abrasive, the pan is dried thoroughly, rubbed with a thin layer of oil and heated in the oven for a set period to rebuild the seasoning. This process brings the pan back to usable condition much the way seasoning it from scratch would.
The simplest habit to avoid rust in the first place is to dry the pan with a cloth right after washing it, warm it briefly on the stove to drive off any remaining moisture, and store it with a thin coat of oil. That short routine keeps the seasoning intact for a long time.
How deep the rust goes matters too: a thin, orange surface layer cleans off easily, while thick, pitted rust that forms after the pan is forgotten in a damp spot for a long time may need heavier scrubbing and a few repeated rounds of re-seasoning.
Where the pan is stored is part of the picture too; a pan left in a damp sink or a closed cupboard without airflow rusts more easily than one kept on an open, ventilated shelf. Hanging it up or storing it somewhere with airflow is safer than tucking it into a closed cupboard.
Enamelled cast iron cookware largely avoids this problem, since its surface is coated with a ceramic layer and never touches the iron directly; the rust risk applies specifically to bare, unseasoned cast iron. Leaving acidic foods like tomato sauce sitting in a bare cast iron pan for a long time can also wear down the seasoning, leaving a slightly metallic taste and raising the risk of rust, which is why acidic dishes are usually cooked briefly and then transferred to another container.
Cleaning with vinegar calls for the same care; keeping the acid contact brief and rinsing the pan thoroughly right afterwards prevents the seasoning from wearing away unnecessarily.