Galvanized bolts: coating, thread fit, embrittlement
A galvanized bolt keeps its full coating, so the nut is tapped oversize after galvanizing to run on it. Pick the thread and grade to see how far, what coating the bolt needs, and what the standards say about hydrogen embrittlement for that strength.
The bolt is galvanized at its normal thread dimensions and keeps the full coating. ASTM A123-24 (6.3) does not allow external threads to be cut, rolled or finished after galvanizing unless the purchaser authorizes it. ISO 10684 requires nut threads to be tapped after galvanizing and does not allow retapping.
The bolt keeps the zinc, the nut is tapped oversize
The male thread is galvanized at its normal dimensions and keeps its coating, which protects the engaged thread in service. The nut is galvanized and then tapped oversize, so it runs freely on the coated bolt. ISO 10684 gives two oversize positions for metric nuts: 6AZ and the larger 6AX, marked Z or X after the property class. ASTM A563 Table 5 gives one diametral allowance per size for nuts used on bolts galvanized to F2329; its metric values are the same as ISO's 6AX.
An oversize-tapped nut carries less thread engagement, so the assembly can lose strength. ISO 10684’s informative annex gives the cure: with 6AZ nuts, use nuts one property class higher (an 8.8 bolt with a class 10 nut) or the taller style 2 nut; with 6AX nuts above M10, a higher class still, and some national standards require two classes higher. To keep the clearance working, the coating on the bolt should not exceed a quarter of the minimum clearance, which is the maximum shown above.
Hydrogen embrittlement. ASTM A143 puts the concern above about 150 ksi (1100 MPa) tensile strength, or after severe cold work before pickling. Heating to 150 °C after pickling and before galvanizing usually drives the hydrogen out. Blast cleaning makes no hydrogen at all; the flash pickle after it only removes the last of the blast media. ISO 10684 asks for inhibited acid, alkaline or mechanical cleaning on parts heat treated or work hardened to 320 HV or more, covers bolts only up to property class 10.9, and keeps class 10.9 bolts of M27 and above out of high-temperature galvanizing.
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Questions
How much are galvanized nuts tapped oversize?
For metric nuts, ISO 10684 Table 1 gives two positions after galvanizing: 6AZ, for example +350 µm on M20, and 6AX, +530 µm on M20. ASTM A563 Table 5 gives one diametral allowance per size for nuts used with bolts galvanized to F2329, for example 0.018 in. for 1/2 in. and 0.024 in. for 1 in.; its metric values equal ISO's 6AX column.
What coating thickness does a galvanized bolt need?
ISO 10684 requires a local thickness of at least 40 µm and a batch average of at least 50 µm. ASTM A153 Table 1 gives Class C for fasteners over 3/8 in. (9.52 mm): average 53 µm, any individual specimen 43 µm; and Class D for 3/8 in. and under: 43 µm average, 36 µm individual.
Can class 10.9 and 12.9 bolts be hot-dip galvanized?
ISO 10684 covers bolts, screws and studs up to property class 10.9, so 12.9 is outside it. Class 10.9 has a minimum tensile strength of 1040 MPa, just under the roughly 1100 MPa (150 ksi) above which ASTM A143 says hydrogen embrittlement becomes the concern, so it needs the precautions: inhibited acid, alkaline or mechanical cleaning, a minimum pickling time, or blast cleaning followed by a flash pickle.
Why use a stronger nut with an oversize-tapped thread?
Tapping the nut oversize reduces the thread engagement and therefore the strength of the assembly. ISO 10684's informative annex recommends a nut one property class higher, or a style 2 nut, with 6AZ threads, and a higher class still with 6AX threads above M10.