Advice to Prevent Thread Galling
What is Galling?
Galling is a form of cold-welding that occurs in threaded components under high pressure, and may cause the threads to seize.
Galling is caused by the asperities (high points) of one component puncturing the protective oxide layer of another, and then transferring material between the two. Stainless steel and Inconel have impressive corrosion resistance, but come with one critical drawback: galling.
Many materials that have decent corrosion resistance (aluminum, stainless steel, Inconel, titanium, etc.) do so because of a passivated oxide layer. That is, the surface of the material has already reacted with oxygen and formed a thin layer all over the exterior of the piece. This thin layer is enough to prevent further reactions, and therefore makes the material more resistant to detrimental corrosion.
The Gauls in Rome, Alphonese de Neuville. No relation to galling. 
However, under the “right” (or “wrong,” galling is usually bad) set of conditions this oxide layer can be punctured and leave the underlying material exposed and likely to transfer. All materials have an inherent roughness, and threaded fasteners (nuts and bolts) are no exception.
If you're looking for some more information on thread pitch, check out our article on Pitch and its Relation to Screws and Gears and for a helpful guide.
Contact Between Threads
When screwed in, these threads are in close, high-pressure contact - any imperfection or roughness on one piece may act like a shovel, and break through the oxide layer on the opposite piece. The freshly exposed material is dug out to the surface and often creates a new high point on either the original piece or it gets transferred to the other piece and becomes a high point there.
This new high point continues the digging action and accelerates the material transfer. In this process, the exposed material is in close contact before an oxide layer can form, which allows it to cold-weld from one part to the other.
Stainless steel fitting showing signs of severe galling from over-tightening. 
Cold-welding can bind two pieces together as one, and cause them to seize up and ruin the threads. The unavoidable roughness and high-pressure environment of a threaded fitting may make galling seem inevitable, but all is not lost!
Here are some simple tricks to reduce galling:
Lubricate your threads
- Using a high-pressure lubricant, like Never-Seez, can help create a barrier between the components and reduce the “shoveling” effect of the high points and asperities. Lubricants also help reduce friction and dissipate heat, which further reduces galling.
- Despite its name, cold-welding occurs faster at elevated temperature. By slowing down the installation, you allow more time for heat to dissipate, resulting in less galling
Use smoother surfaces
- While surface roughness is inevitable, using a smoother thread (like a rolled thread instead of a cut thread) will buy you some more time before galling becomes severe.
Use materials of different hardnesses
- The most galling occurs between components of the same material. Changing the material, or even just the hardness, of one component can reduce the rate of galling. In general, harder materials tend to gall less than soft materials.
Use bigger threads
- Larger and more coarse threads have greater clearances and can resist more galling before they seize up.
We've all walked into a hardware store looking for the exact nut and bolt combination for our project at hand, only to be looking at a long wall of thousands of different fasteners. It may seem overwhelming at first, but the only reason we have standard gauges is because of a Joseph Whitworth who Standardized Threads and made everyones lives a lot easier.
If you're looking for some more information on thread pitch, check out our article on Pitch and its Relation to Screws and Gears and let us know what you think.
© Evolvent Design, 2020
 de Neuville, Alphonse, The Gauls in Rome. 1870s. Retrieved from Wikimedia.
 Kees08, External Thread Galling. 2018. Retrieved from Wikimedia.