Yes, katanas can become dull over time with regular use or exposure to various elements. The cutting edge of a katana can wear down, resulting in a loss of sharpness and cutting ability. Several factors can contribute to the dulling of a katana:

1. Contact with Surfaces: When a katana comes into contact with hard surfaces, such as other blades, armor, or solid objects, it can experience micro-damage or blunting of the cutting edge. Repeated impacts or improper technique can accelerate the dulling process.
2. Cutting Exercises: Katanas are often used for cutting exercises, where they come into contact with various materials such as bamboo, tatami mats, or rolled straw mats. While these exercises can be beneficial for training and skill development, they can also cause the blade to dull over time due to the repeated cutting and impact forces involved.
3. Oxidation and Corrosion: If a katana is not properly cared for or stored in a controlled environment, it can be susceptible to oxidation and corrosion. This can cause the blade to lose its sharpness and develop surface imperfections, affecting its cutting performance.
4. Wear and Tear: Regular use of a katana, even with proper technique and maintenance, can gradually wear down the cutting edge. Over time, the blade may require maintenance and sharpening to restore its sharpness and cutting ability.

To maintain the sharpness of a katana, regular maintenance is necessary. This includes proper cleaning, oiling, and storage to minimize the effects of oxidation and corrosion. Additionally, periodic sharpening by a skilled professional is essential to restore the blade's sharp edge and cutting performance.

It's worth noting that katanas are typically made from high-quality steel and possess excellent edge retention properties. With proper care, maintenance, and occasional sharpening, a katana can maintain its sharpness and cutting ability for an extended period.