Stainless Fastener Galling: Causes and Anti-Seize Solutions

Stainless Fastener Galling: Causes and Anti-Seize Solutions

A stainless bolt threaded into a stainless nut often seizes just before it seats: it will neither tighten nor come out, and forcing it snaps the shank. The phenomenon is galling, or cold welding, and is common on austenitic grades such as 304 and 316. It is not corrosion or thread stripping; no rust appears, yet local metallurgical bonding has already occurred.

Galling is direct metal-to-metal contact plus plastic flow. The passive film on austenitic stainless is continuously ruptured by friction, and the freshly exposed metal is highly active, bonding to the mating surface before it can re-oxidise. Stainless work-hardens strongly, so sheared asperities form new peaks, friction climbs and thread clearance is consumed.

Four typical triggers

Matching materials come first: a 304 bolt in a 304 nut galls most readily, while a hard-soft pairing behaves far better. Assembly method comes second: power tools at high speed heat the contact in seconds, while hand wrenching rarely fails. Surface condition comes third: mirror-polished threads are not automatically better, since low roughness removes oil-retention space, and cut threads gall more than rolled ones.

Four ways to prevent it

Dry-film lubricants such as molybdenum disulphide, wax-based or fluoropolymer coatings are the most effective measure, stabilising friction and keeping metal surfaces apart; passivation does not help because the film is rubbed away. Material pairing matters as well: pair 316 with 304, A2 with A4, or use bronze or nylon-insert nuts. Coarse threads and slightly larger clearance reduce risk, and assembly should limit speed, cap torque and avoid repeated re-use.

Selection and service advice

For bolts above M16 and joints exposed outdoors, specify coated product with dry-film lubricant; the cost increment is small compared with rework at height or offshore. Seized joints should not be forced in reverse: apply penetrating oil with light back-and-forth effort, and cut the fastener out if it will not move.

Conclusion

Galling can be designed out through material pairing, coating and assembly discipline. Zhuhai Jiali Hardware (FASTKING) supplies stainless bolts and precision screws with dry-film lubrication options. See the product center or contact us for samples.