Galling is adhesive wear between two metal surfaces in sliding contact. On stainless steel threads, it progresses from surface roughening to material transfer to cold-welding in seconds. Often during the installation that was supposed to take five minutes.
The mechanism is specific to austenitic stainless grades: 304 and 316. The passive chromium oxide film that provides corrosion resistance gets disrupted at thread contact points during engagement. Under the contact pressure of thread advancement, the disrupted film cannot reform fast enough. Oxygen access at the thread interface is restricted. Bare metal contacts bare metal. Austenitic stainless is ductile at the microscopic scale. The material at asperity peaks deforms and transfers to the mating surface rather than fracturing cleanly. This transfer is the galling event. Once material has moved across the interface, the raised deposit increases friction on the next rotation. The failure self-accelerates.
Three material properties of 304 and 316 combine to make them more susceptible than carbon steel:
Low thermal conductivity. Frictional heat at the thread interface dissipates slowly. Temperature at the contact zone rises. This increases material ductility and adhesion tendency. Carbon steel conducts heat roughly twice as fast, keeping peak contact temperatures lower.
Work hardening. As the thread flanks cold-work during engagement, the surface layer hardens rapidly. Harder material generates more friction on subsequent contact, not less. Carbon steel work-hardens far less during the same deformation. Softer surfaces deform rather than transfer material.
No natural oxide lubrication. Stainless steel relies on a passive chromium oxide film for corrosion resistance. The passive film does not lubricate. Once disrupted under thread contact, the bare metal underneath is adhesion-prone. There is no lubricating backup.
