Galling is a cold welding phenomenon which can occur when uncoated stainless steel (or aluminium alloy parts), such as the threads of the case and case back 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.