Q: Are galvanic corrosion and galling the same?

A: Galvanic corrosion and galling are actually very different. One causes accelerated corrosion while the other cause the threads to seize.

Galvanic corrosion is when two dissimilar metals are brought in contact with a solution or moisture – most commonly, salt water. In these conditions, these two metals will create an electric current which will accelerate corrosion. This chart can help you avoid galvanic corrosion as it compares how different metals will react with one another.  For galvanic corrosion to occur, all three elements all must be in play:

1. Two different metals
2. Direct metal-to-metal electrical contact
3. A conductive electrolyte solution, most commonly water or even moisture.

Galling occurs not because of dissimilar metals, but from the process of fastening the bolt. Here the friction and pressure put on the bolt when torquing, causes the threads to seize or “cold weld” together. When galling occurs, you can only remove the fastener by cutting it or breaking the nut. To avoid galling you can:

1. Use a bolt with a coating – like silver plating or dry film lubricants
2. Apply a lubricant
3. Be careful not to over-torque the bolt
4. Use dissimilar materials (but be careful about galvanic corrosion!!)

Q: I’m having issues with thread galling. We’ve switch to coarse threads and tried to slow down installation speed/over-tightening. Any other ways we can prevent this?

A: There are a few other things to consider in reducing thread galling. First, when possible use dissimilar metals with different hardness. Having material with slightly different properties usually helps. Another suggestion would be to us a lubricant or coating. This can be one of the most effective and easiest solutions. Molybdenum disulphide is an excellent high temperature lubricant for many industrial applications. As an example, the Extreme Anti-Seize MoS2 lubricant can be used from -65°F to 2400°F in temperature range. In addition, there is also dry film lubricants and tribological coatings which are also can be successful in preventing thread galling.