According to the forming process of cold-extruded internal thread，the microstructure at the surface of cold-extruded internal thread was investigated. The corrosion resistance was analyzed from the changing perspective of surface status of cold-extruded internal thread. The results showed that the fibrous structure of the metal surfaces on the root of thread was pressed and refined to distinguish the grains difficultly. The metal structure appeared to be streamline type and distributed along the normal direction of the tooth surface. The hardening layers of cold-extruded internal thread had an adverse effect on corrosion, and it reduced corrosion degree at the surface of the sample in the same corrosive environment, and mostly avoided producing corrosion pits to cause local stress concentration. Cold-extruded internal thread had better corrosion resistance.
