Galling is a form of wear where two metal surfaces seize together and tear material from one another. It happens when metals slide against each other under pressure without adequate lubrication, causing them to cold-weld at microscopic contact points. The result is visible damage: rough, raised lumps of transferred metal on one or both surfaces. Once galling starts, it tends to get worse rapidly, and in severe cases it can lock components together permanently.
How Galling Works at the Surface Level
No metal surface is truly smooth. At a microscopic scale, every piece of metal has tiny peaks and valleys called asperities. When two metal surfaces press together and slide, these asperities collide. Under enough pressure, the thin oxide layers that normally protect metal surfaces break down, exposing bare metal underneath.
Once bare metal contacts bare metal, the atoms at each surface bond to each other in a process called adhesive transfer. Chunks of material literally weld to the opposing surface and rip away from where they started. Researchers have confirmed this mechanism by sliding two slightly different stainless steel alloys against each other and tracking where the material ended up. Even at relatively low pressures of about 50 MPa (around 7,250 psi), large shear stresses and adhesive transfer occurred. This transferred material then acts as a raised bump that digs into the opposing surface, creating more damage with every further movement.
This is what separates galling from ordinary friction wear. Normal wear gradually removes thin layers over thousands of cycles. Galling can cause catastrophic damage in a single motion.
Metals Most Prone to Galling
Soft, ductile metals with similar compositions are the most vulnerable. Stainless steel is the classic offender. Austenitic stainless steels like 304 and 316 are particularly galling-prone because they’re relatively soft, they work-harden under friction (which sounds helpful but actually increases the force at contact points), and their oxide layers are thin and easily disrupted.
Aluminum and titanium also gall readily. The common thread is that these metals form strong metallic bonds when their protective oxide films are breached. Harder metals and dissimilar metal pairings generally resist galling better, because the asperities are less likely to deform and interlock.
Where Galling Causes Real Problems
Threaded fasteners are the most common place people encounter galling. Stainless steel bolts and nuts are notorious for seizing during tightening. The combination of high contact pressure on the thread flanks and rotational sliding creates ideal galling conditions. Fastenal, a major fastener supplier, specifically warns that stainless steel fasteners “tend to gall, especially with long run downs, prevailing torque fasteners, impact drivers, and lack of lubrication.” Their torque specifications for stainless steel are calculated at only 40% of yield strength specifically to avoid galling.
Beyond fasteners, galling damages valve stems and seats, pump components like wear rings and lobes, chain-drive systems, pins, bushings, and roller bearings. Any application where metal parts slide or rotate against each other under load is a candidate.
How Galling Is Measured
Engineers use a standardized test called ASTM G98 to rate a material’s galling resistance. The test presses two cylindrical specimens together and rotates one against the other at progressively higher loads until galling occurs. The result is a single number called the threshold galling stress (TGS), which represents the maximum contact pressure the material pair can handle before galling begins.
The measurement is binary: galling has either occurred or it hasn’t. There are no degrees. The TGS is calculated by averaging the highest pressure that didn’t cause galling with the lowest pressure that did. A related standard, ASTM G196, uses a slightly different specimen geometry with an annular contact area, but the goal is the same. These numbers let engineers compare materials and choose the right pairing for a given application.
Galling-Resistant Alloys
Not all metals gall equally. Specialty alloys are designed specifically to resist it. The best-known example is Nitronic 60, a stainless steel alloy with high manganese (7 to 9%) and silicon content. It resists galling through two mechanisms: it forms a thin, tightly bonded oxide film on its surface, and it has a high strain-hardening capacity that supports this film under pressure rather than letting it crack and expose bare metal.
In standardized testing, Nitronic 60 withstood over 50,000 psi (345 MPa) of contact stress without galling, which was the maximum the test could apply. For comparison, common stainless steels like 304 and 316 gall at a small fraction of that pressure. Nitronic 60 also has nearly twice the yield strength of 304 and 316 at room temperature, and it costs significantly less than cobalt-bearing or high-nickel alloys that offer similar galling resistance.
How to Prevent Galling
The most practical defense is lubrication. Anti-seize compounds are the standard choice for threaded connections. These pastes contain solid lubricant particles, typically molybdenum disulfide or graphite suspended in a petroleum carrier, along with rust inhibitors. The solid particles sit between the metal surfaces and prevent direct metal-to-metal contact even under high pressure. For stainless steel fasteners, applying anti-seize and using a slow, steady torque rather than an impact driver dramatically reduces galling risk.
Surface coatings offer another layer of protection. Physical vapor deposition (PVD) coatings like titanium nitride and diamond-like carbon (DLC) create a hard, low-friction barrier on the metal surface. DLC coatings in particular combine extreme hardness with a very low friction coefficient. Surface finish matters too: smoother surfaces have fewer and smaller asperities to interlock, so polishing after coating further improves galling resistance.
Material selection is the most fundamental prevention strategy. Using dissimilar metals for mating parts reduces the tendency for adhesive bonding. If both parts must be the same alloy family, choosing a galling-resistant grade like Nitronic 60 for at least one of the two surfaces can solve the problem. Hardening treatments that increase surface hardness also help, since harder asperities resist the deformation that initiates cold welding.
Slowing down the assembly speed, reducing contact pressure where possible, and keeping surfaces clean and free of debris all contribute. In fastener applications specifically, limiting installation torque to 40% of yield strength is a practical rule of thumb that balances clamping force against galling risk.