The air conditioner drip pan is a shallow tray located under the evaporator coil that is designed to collect and drain away any water that condenses on the coil. It is typically made of metal and can be prone to rusting over time. There are several reasons why the air conditioner drip pan might be rusted, including:

1. Age: As the air conditioner drip pan ages, it can become more susceptible to rusting due to wear and tear.
2. Moisture: The air conditioner drip pan is constantly exposed to moisture, which can cause it to rust over time.
3. Corrosive substances: If the air conditioner drip pan is exposed to corrosive substances, such as chemicals or cleaning agents, it can become more prone to rusting.

If the air conditioner drip pan is rusted, it is important to address the issue as soon as possible. A rusted drip pan can weaken and eventually fail, which can lead to water damage and other problems. To fix a rusted air conditioner drip pan, you can try the following steps:

1. Clean the drip pan: Use a mixture of water and mild detergent to clean the drip pan and remove any dirt or debris that may be contributing to the rust.
2. Sand the rust: Use fine-grit sandpaper to sand away the rust from the surface of the drip pan.
3. Coat the pan with rust inhibitor: Use a rust inhibitor, such as a rust-inhibiting paint or rust converter, to protect the drip pan from further rusting.
4. Replace the drip pan: If the drip pan is severely rusted or damaged, it may be necessary to replace it with a new one.

Overall, it is important to address a rusted air conditioner drip pan as soon as possible to prevent further damage to the system. Regular maintenance and cleaning can help to prevent rusting and prolong the life of the drip pan.