Yes, samurai swords, including the traditional katana, can rust if not properly cared for and maintained. Samurai swords typically have a carbon steel blade, which is susceptible to rust and corrosion when exposed to moisture or humidity.

Carbon steel blades, although known for their durability and sharpness, require regular cleaning and maintenance to prevent rust. When a sword is used or exposed to moisture, the blade should be thoroughly cleaned and dried to remove any moisture or contaminants that may cause rust.

To protect the blade from rust, it is common practice to oil the sword with a thin layer of oil, such as mineral oil or sword oil, after cleaning. The oil acts as a protective barrier against moisture and helps to prevent oxidation.

Additionally, proper storage is crucial to prevent rusting. Samurai swords should be stored in a dry environment with controlled humidity levels. Ideally, they should be kept in a scabbard (saya) or a sword bag to provide additional protection from moisture and dust.

Regular inspection of the blade is also important to detect any signs of rust or corrosion early on. If rust spots are found, they should be addressed promptly with appropriate rust removal techniques to prevent further damage to the blade.

It's worth noting that while rust can occur on a samurai sword, it doesn't necessarily mean that the sword is of poor quality. Rusting is a natural process for carbon steel blades, and with proper care and maintenance, it can be minimized or prevented.

In summary, samurai swords, including katana, can rust if not properly cared for and maintained. Regular cleaning, drying, oiling, and proper storage are essential to protect the blade from rust and maintain its quality and appearance over time.