Why are Stainless Bolts Prone to Galling? 

Stainless, aluminum, and titanium fasteners form thin protective oxide films on their exposed surfaces that prevent corrosion. In addition, this coating reduces friction and prevents direct metal to metal contact during fastening. The protective oxide film can be rubbed or scraped off under the pressure and movement of the fastener being tightened. When this happens these relatively soft metals come into direct contact. Friction increases and the chance of galling increases significantly.  The protective oxide film can be rubbed or scraped off under the pressure and movement of the fastener being tightened. When this happens these relatively soft metals come into direct contact. Friction increases and the chance of galling increases significantly.

What is Actually Happening? 

Thread surfaces have microscopic high points that can rub together during fastening.  In most cases this does not present a problem as the points slide over each other without damage.

Under certain conditions however, the surfaces will not slide past each other. The high points will then shear and lock together, greatly increasing friction and heat. 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 destroyed and the fastener will no longer turn in either direction.

How Do I Avoid Galling? 

The best way to avoid galling is to avoid excessive heat and pressure during installation. That means only tighten using hand tools - nothing electric that spins fast enough to generate unnecessary heat. Also, avoid over-tightening the nut. Once the lock washer is compressed, you've tightened things enough. If the nut starts to stick, stop tightening and slowly work the nut back and forth until it moves smoothly.