Corrupt Walls are naturally-generating background walls found in the Corruption. These naturally occurring walls can be destroyed with a hammer, but will drop nothing. These walls also do not stop enemies from spawning and cannot be used for proper housing.
Some cave walls can be converted into corrupt walls: Mottled Stone Walls become Corrupt Pustule Walls, Fractured Stone Walls becomes Corrupt Tendril Walls, Worn Stone Walls become Corrupt Growth Walls, and Stalactite Stone Walls become Corrupt Mass Walls. Using a Clentaminator with Green Solution is the only way to reverse the conversion.