Galling is a form of severe adhesive wear where two metal surfaces literally bond together under pressure and friction, tearing material from one surface and depositing it onto the other. It creates raised lumps and rough patches that can permanently damage parts or lock them together entirely. If you’ve ever tried to unscrew a stainless steel bolt and found it completely seized in place, with the threads visibly torn and smeared, that’s galling.
How Galling Happens at the Surface Level
Metal surfaces that look smooth to the naked eye are actually covered in tiny peaks and valleys at the microscopic level. When two metal parts slide against each other under load, those tiny peaks make contact first. At these contact points, the pressure is concentrated into an extremely small area, generating intense friction and heat.
That localized pressure and heat strip away the thin protective oxide layer that normally coats most metals. With the oxide gone, the bare metal atoms of both surfaces are exposed to each other. At this point, something counterintuitive happens: the atoms from each surface bond together in a process sometimes called cold welding. The metals don’t need to be heated to melting temperatures. Under enough pressure, bare metal atoms will simply stick to one another.
Once bonded, continued sliding rips material away from one surface and welds it onto the other, creating raised lumps called protrusions. These protrusions dig into the opposing surface on the next pass, tearing away even more material. The damage feeds on itself. Plastic deformation at the contact points can reach extreme levels, with strain values between 8,000 and 10,000 microstrain. Left unchecked, the surfaces seize completely and the parts become impossible to move.
What Triggers Galling
Several conditions make galling more likely, and they often work together:
- High contact pressure. Galling typically begins at pressures around 1,000 kilopascals (roughly 145 psi) at the actual contact points. Over-tightening a bolt is one of the most common ways to reach this threshold.
- Poor or absent lubrication. Lubricant creates a barrier between surfaces that prevents metal-to-metal contact. Without it, those microscopic peaks grind directly against each other.
- Speed of assembly. Driving a bolt in quickly with a power tool generates more friction and heat than threading it slowly by hand. That extra heat softens the surface and accelerates the bonding process.
- Surface finish. Rougher surfaces have taller peaks that concentrate pressure more aggressively. But extremely smooth surfaces can also gall because they make broader contact, increasing the area where adhesion can occur.
- Similar metals in contact. Two pieces of the same alloy are far more likely to gall than two different metals, because matching crystal structures bond to each other more readily.
Which Metals Are Most Susceptible
Stainless steel is the most notorious galling offender. Its protective chromium oxide layer is thin and, once disrupted, the underlying metal is soft enough to deform and bond easily. Stainless steel fasteners are used across marine, automotive, oil and gas, chemical processing, energy, defense, and construction industries, and galling is a persistent problem in every one of those sectors. Thread galling in stainless steel bolts, where the bolt threads seize to the nut or tapped hole, is especially common.
Aluminum galls easily for similar reasons. It’s soft, and its oxide layer, while harder than the base metal, is brittle and cracks under pressure, exposing fresh aluminum underneath. Titanium is also highly prone to galling. Its combination of high strength, relatively low hardness, and strong atomic bonding tendencies makes it stick to itself and other metals under sliding contact.
Hardened carbon steels and cast iron are much less susceptible. Their harder surfaces resist the plastic deformation that starts the galling cycle. Nickel-based superalloys can also gall, particularly at elevated temperatures where oxide layers break down faster.
How Galling Differs From Normal Wear
Regular abrasive wear slowly removes material from a surface over time, like sandpaper gradually smoothing wood. Galling is different in three important ways. First, it transfers material rather than just removing it, building up lumps on one surface while gouging the other. Second, it can happen almost instantly, sometimes on the very first assembly of two new parts. Third, the damage is self-accelerating. Each protrusion created by galling makes the next pass worse, whereas normal wear tends to slow down as surfaces polish each other.
The result of galling is also distinctive to the eye. Instead of smooth, polished wear marks, galled surfaces show torn, rough patches with visible material buildup. The damage often looks like someone dragged a rough tool across the surface, leaving smeared ridges of metal.
How to Prevent Galling
The most effective prevention strategy is to make sure the two mating surfaces are different from each other in some meaningful way.
Hardness difference is the single most reliable factor. The Australian Stainless Steel Development Association recommends a hardness gap of at least 50 Brinell points between mating parts. For example, if you’re using a stainless steel bolt, pair it with a nut made from a different alloy or one that’s been hardened through heat treatment or cold working. This gap means the softer surface can’t as easily deform and bond to the harder one.
Lubrication is the next line of defense. Anti-seize compounds containing molybdenum disulfide, graphite, or copper particles create a physical barrier between surfaces. Even a light coating of oil or wax on bolt threads significantly reduces the chance of galling. For applications where liquid lubricants aren’t practical, dry film coatings can serve the same purpose.
Surface coatings and treatments also help. Nitriding (a process that hardens the surface layer of steel) or applying coatings like chrome plating, phosphate conversion, or ceramic films all change the surface properties enough to interrupt the galling mechanism. Some specialty fastener manufacturers produce stainless steel bolts with silver or wax coatings specifically designed to resist thread galling.
Installation practices matter too. Slowing down your assembly speed reduces friction heat. Avoiding cross-threading eliminates the misalignment that concentrates pressure on a small area. And backing off a bolt slightly if you feel increasing resistance, rather than forcing it, can prevent the damage from cascading.
How Galling Resistance Is Measured
Engineers use the ASTM G98 standard to compare how well different materials resist galling. The test presses a rotating button of one material against a flat block of another under increasing loads until galling occurs. The load at which visible surface damage first appears is recorded as the threshold galling stress for that material combination. The test is relatively quick, requiring as few as two specimens, and gives designers a concrete number to compare when choosing materials for sliding or threaded contact.
Threshold galling stress values vary enormously between materials. Some stainless steel combinations gall at stress levels below 7 MPa (about 1,000 psi), while hardened or coated materials can withstand ten times that before damage begins. These numbers let engineers choose material pairings and coatings based on the actual loads their parts will experience in service.