Can I use a mechanically galvanized nut with a hot-dip or electroplate galvanized bolt (or vice versa)?

No, mixing and matching different types of galvanized products is not advised.

From ASTM A563/A307/A354 and F1554:

“4.7.4 When no preference is specified, the supplier may furnish either a hot-dip zinc coating in accordance with Specification F2329, or a mechanically deposited zinc coating in accordance with Specification B695, Class 55. Threaded components (bolts and nuts) shall be coated by the same zinc-coating process and the supplier’s option is limited to one process per item with no mixed processes in a lot.”

From ASTM A325:

“4.3.5 When no preference is specified, the supplier shall furnish either a hot-dip coating in accordance with Specification F2329, a mechanically deposited zinc coating in accordance with Specification B695, Class 55, or a Zinc/Aluminum Corrosion Protective Coating in accordance with Specification F1136, Grade 3. Threaded components (bolts and nuts) shall be coated by the same zinc-coating process and the supplier’s option is limited to one process per item with no mixed processes in lot.”

From the Research Council on Structural Connections/American Institute of Steel Construction:

“Both the hot-dip galvanizing process (ASTM F2329) and the mechanical galvanizing process (ASTM B695) are recognized in ASTM A325. The effects of the two processes upon the performance characteristics and requirements for proper installation are distinctly different. Therefore, distinction between the two must be noted in the comments that follow. In accordance with ASTM A325, all threaded components of the fastener assembly must be galvanized by the same process and the supplier’s option is limited to one process per item with no mixed processes in a lot. Mixing high-strength bolts that are galvanized by one process with nuts that are galvanized by the other may result in an unworkable assembly.”

Mixing and matching different types of galvanized parts is not advisable based on these specifications. The primary difference between these two types of galvanized nuts is that hot-dip galvanized nuts are tapped after galvanizing, while mechanically galvanized nuts are tapped prior to galvanizing.

Our in-house tests revealed that different types of galvanized parts will fit together, however, the RCSC indicates that the mixed and matched parts may not work together properly. These different types of galvanizing have different friction coefficients and may not develop the desired tension after the torque is applied. The torque and tension relationship affected is especially important for structural steel connections. You can learn more about torque and tension from this FAQ. These different friction coefficients also mean that the surface of the male/female threads may interact in such a way that results in stripping or galling of the galvanized surface, effectively rendering the galvanizing defective.

Portland Bolt has an in-house hot dip galvanizing line and millions of nuts and washers to match your custom hot-dip galvanized bolts. Please contact one of our team members with any further questions or if you would like a quote.

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5 comments

    Thank you Dane. For this particular application, A563 Grade A is acceptable.

    My follow-up to this question, is there any adverse effect that would occur using a zinc plated nut instead of a plain nut when the nuts are used for an anchor rod embedded in concrete? Since no coating is actually required, would implementing the zinc plated nuts cause more harm than good? The only thing that comes to mind is that the zinc is going to rapidly attract electrons from the plain steel, but after the zinc is fully consumed, the previously coated nuts would behave as plain (black) steel, which is what is originally specified.

    @Michael- You are correct that zinc is more anodic and will corrode first when in contact with steel. We are far from metallurgical experts, so it would not hurt to confirm with one, but we don’t think the zinc will be detrimental to the overall anchor bolt assembly.

    Does the restriction of mixing and matching of coatings apply to zinc plating / plain finish combinations, as well? I have a question from a contractor that wants to use A563 zinc plated nuts with plain finish A325 bolts. Is this permissible?

    @Michael- The reason for the restriction on mixed coatings is that some coatings like hot dip or mechanical galvanizing have different overtapping allowances for the nut. Since zinc is a very thin and mostly aesthetic coating that doesn’t require any overtapping, you shouldn’t have any issues using a zinc nut with a plain bolt. That said, I would confirm with the contractor that they are using the proper structural grade of A563, as most of the structural grades don’t normally come zinc plated, whereas the low strength grades like grade A commonly do.

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