Galling

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Galling is a cold welding phenomenon which can occur when uncoated stainless steel or aluminium alloy parts, such as the threads of nuts and bolts, are forced together. These materials owe their corrosion resistance to the ease with which they passivate, forming a thin protective oxide layer. The friction scrapes off this oxide layer from the surface asperities and exposes clean reactive metal. If the mating parts are of a sufficiently similar material, no additional activation energy is needed to cold weld them together.

Galling can occur even if the parts are brought together slowly, and it is prevented by the presence of grease or surface coatings, even if the surface coatings increase friction. It does not occur when joining dissimilar materials (for example threading 18-8 stainless into 17-4 stainless) even though both of those materials are susceptible to galling. Galling does not occur on carbon steel.

In metalworking that involves cutting (primarily turning and milling) "galling" is used to describe a phenomenon which often occurs when cutting soft metal: workpiece material sticking to the cutter. It often occurs with aluminum and is a common cause of tool breakdown. When a soft material sticks to the cutting edges the effective surface area of the cutter increases. To achieve the pressure needed to cut the workpiece, a greater force is needed often resulting in extensive wear or breakage of the cutting tool.