What is ‘Galling’ and how to prevent it

Galling [or ‘Cold-Welding’] as it’s often known, is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.

Threads of a stainless bolt destroyed and all chewed up by ‘Galling’

Fastener thread surfaces have microscopic high points that can rub together during assembly and in most cases, this does not present a problem as the points slide over each other without damage. Under certain conditions and with fasteners such as stainless steel with protective oxide films, the surfaces will not slide past each other; protective oxides rub off at the high points, exposing the base metals and causing them to weld together.

The high spots will then shear and lock together, significantly increasing friction and heat due to Low Heat dissipation at the high points. As tightening continues the increased pressure results in more material being sheared off the threads, this cycle continues with even more shearing and locking until the threads are obliterated with fasteners will no longer turn in either direction.

Galling is most commonly found in metal surfaces that are in sliding contact with each other and can occur quite frequently when stainless steel fittings such as bolts [especially M8 and greater diameter hardware] are tightened onto a stainless Nyloc nut.

Galling does not normally occur with smaller diameter [M6 and smaller] hardware.

Although it may seem a bit odd, a small amount of lubricant is all that’s required to alleviate galling. Lubricant on the Nyloc nuts helps eliminate the problem, and the Nyloc will still actually do the intended job. We’ve used ‘WD40’ or even ‘Alumslip’ with great results

Download our info sheet on ‘Galling’ [Cold-Welding] here.