This problem is often due to thread “galling” or seizing, which happens when the surfaces of mated parts are more abrasive. Galling only occurs with metal fasteners and is more likely when external threading is cut as opposed to rolled because when thread cutting is used, the machining process creates a rougher surface. Surface oxidation may also lead to galling when particular materials are used. Galling refers to when parts are joined and small particles break free from the surface and lodge between the connected parts which results in the parts sticking together. Thread galling can become so severe that the mating parts completely seize up, making it virtually impossible to disconnect them.
Galling should be considered when fasteners are designed and can be avoided by producing mating parts of different material compositions and hardnesses. Lubricants can also be added to the threads to avoid galling as well.