The Stainless Steel Galling Problem

Stainless steel is specified for its corrosion resistance. But it has a dirty secret: stainless fasteners are extremely prone to galling — a form of cold welding where threads seize permanently during tightening. A bolt that was fine going in suddenly locks up and snaps off when you try to remove it.

This isn’t a defect. It’s a material property. Understanding why it happens is the first step to preventing it.

Why Stainless Galls

Three properties of austenitic stainless steel (304, 316, 303) make it galling-prone:

  1. Self-healing oxide layer — The chromium oxide film that prevents corrosion is very thin (~5nm) and easily disrupted by friction. Once broken, bare metal contacts bare metal.
  2. Work hardening — Austenitic stainless work-hardens rapidly under friction. As threads slide, contact points harden and create abrasive high spots that tear into the opposing surface.
  3. Poor thermal conductivity — Stainless conducts heat about 1/3 as well as carbon steel. Frictional heat concentrates at contact points instead of dissipating, accelerating the welding process.

The combination is devastating: friction strips the oxide, work hardening creates abrasive points, and poor heat dissipation lets temperatures spike at the thread interface. Micro-welding begins, and within another quarter-turn the fastener is permanently seized.

Risk Factors

Galling probability increases with:

  • Same alloy on same alloy — 316 bolt in 316 nut is the worst case
  • Fine threads — More surface contact per unit length = more friction
  • Larger diameters — More thread surface area, more heat generation
  • Fast installation — Impact drivers and power tools dramatically increase galling risk
  • Rough thread finish — Burrs and tooling marks create friction initiation points
  • High torque — More force pushing surfaces together
  • Dry threads — No barrier between mating surfaces

Prevention Strategies

1. Lubricate — Always

The single most effective prevention. Apply anti-seize compound to all stainless fastener threads before assembly. Copper-based anti-seize is the standard choice. See our anti-seize guide for types and torque adjustments.

2. Slow Down

Hand-start every stainless fastener. Run it in by hand until finger-tight, then use a wrench for the final tightening. Never use impact drivers or power tools on stainless unless the fastener is lubricated and you’re controlling the speed.

3. Use Dissimilar Alloys

Pair different stainless grades to reduce galling tendency:

  • 303 nut + 316 bolt — 303’s free-machining additives (sulfur/selenium) act as natural lubricants
  • 304 bolt + 316 nut — Different hardness levels reduce cold welding
  • Bronze or brass nuts on stainless studs — Eliminates same-material galling entirely

4. Specify Galling-Resistant Alloys

Some stainless alloys are engineered for galling resistance:

  • Nitronic 60 (UNS S21800) — The gold standard for anti-galling stainless. 5× to 8× more galling-resistant than 304/316.
  • 303 stainless — Sulfur and selenium inclusions reduce friction
  • Waxed or PTFE-coated fasteners — Surface treatment rather than alloy change

5. Surface Treatments

  • Silver plating — Common for high-temperature stainless bolting (refineries, turbines)
  • PTFE/Teflon coating — Reduces friction dramatically, available as dry-film or spray
  • Nitriding — Hardens the surface layer, reducing work-hardening galling mechanism
  • Passivation — Thickens the oxide layer, providing slightly more protection (but not a primary galling solution)

6. Thread Quality

  • Use Class 2A/2B fit (standard) — NOT Class 3A/3B (tight). Tighter fits increase galling risk.
  • Inspect threads for burrs, cross-threading damage, and tooling marks before assembly
  • Roll-formed threads (cold-formed) gall less than cut threads due to smoother surface finish

What To Do When It’s Already Galled

If a stainless fastener has seized:

  1. Stop immediately. More force will only make it worse — or snap the bolt.
  2. Try penetrating oil and let it soak. PB Blaster or Kroil may free a partially galled joint.
  3. Apply heat cautiously — Thermal expansion can break the micro-welds, but too much heat on stainless causes carbide precipitation (sensitization).
  4. If it’s truly seized: Cut it off. Drill it out. Install a thread insert. Don’t try to be a hero.

Specification Callout

For drawings and specifications, add a note:

“All stainless steel threaded fasteners shall be assembled with copper-based anti-seize compound per MIL-PRF-907 or equivalent. Reduce torque values 25% from dry specification.”

This one line on a drawing prevents 90% of stainless galling problems in the field.

Bottom Line

Stainless galling isn’t bad luck — it’s bad practice. Lubricate, slow down, and specify dissimilar alloys or galling-resistant grades. The cost of prevention is a tube of anti-seize. The cost of failure is a seized, broken bolt in an expensive assembly. Choose wisely.