Over-sizing nuts to compensate for coated bolts

Q: We have a corrosive application and we are debating on using coated carbon steel bolts and nuts or a solid corrosion resistant alloy. Obviously the coatings cost a lot less but we are unsure about how the coatings affect mechanical strength.

A: Coated bolts and nuts are typically undersized to compensate for the coating thickness. Most of the time if there is a bolt and nut pair, and all of the under-sizing could be done on the nut. This creates a weakened joint and failure can occur in the threads when the thread pitch isn’t fully engaged. If these parts were gauged they would most likely be considered an out of spec part. As a result the clamping strength of a coated part will be reduced. Obviously for solid alloy parts this is not a concern. Furthermore great care needs to be taking when considering coatings for corrosive applications as any pin hole could undermine the base material.

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