Coatings are chosen on three axes: how long the part must last, whether the thread must still assemble, and whether the steel grade is sensitive to hydrogen. Optimizing one axis at the expense of the others is the usual cause of coating complaints.
The four common finishes
BLACK OXIDE: a conversion coating 1-2 µm thick. Decorative, mildly oil-retentive, salt-spray life measured in hours. Use it indoors, for dry storage, and where dimensional change must be zero.
ELECTRO-GALVANIZING: 5-25 µm of zinc, uniformly applied and thread-friendly because thickness is predictable. Salt-spray life roughly 100-300 hours depending on thickness and passivation. The default for sheltered outdoor use and for parts that must assemble freely.
HOT-DIP GALVANIZING (HDG): 45-85 µm, metallurgically bonded, salt-spray life above 1000 hours in rural and urban atmospheres. The correct choice for permanent embedment — anchors, base plates, prestressed bars left in the structure — and for coastal sites. The cost is dimensional: coating builds on threads and requires tapped-oversize nuts or re-tapping after coating.
ZINC-LAMELLAR (e.g. Dacromet type): 8-12 µm of zinc-aluminium flakes in a binder. Salt-spray life around 500-1000 hours, no hydrogen embrittlement because the process is not electrolytic, and no thread blocking. The standard answer for class 10.9 and above where HDG is impractical.
The hydrogen embrittlement trap
Any acid-cleaned, electroplated high-strength part absorbs hydrogen. Class 10.9 and above must be baked within a defined window after plating — typically within four hours, at 190-220 °C for four hours or more, depending on the standard — and the processor must record it. Without the bake, delayed cracking can appear at 24-72 hours. If you buy electroplated 10.9 fasteners, require the baking record. If the processor cannot produce it, change the finish.
Thread fit arithmetic
A 60° metric thread has two flanks, so coating on both flanks consumes pitch diameter at roughly four times the coating thickness. A 60 µm HDG coating therefore reduces effective clearance by about 240 µm on pitch diameter — enough to make a 6g/6H assembly impossible. The usual fixes: tap nuts oversize after coating, allow a coating-specific clearance in the class, or specify a coating that does not build.
Selection rules
Indoor, dry, frequently reused: black oxide or thin electro-galvanizing; nothing that blocks threads.
Sheltered outdoor, temporary works, frequent assembly: electro-galvanizing 12-25 µm.
Permanent embedment, coastal or high-humidity site, long design life: hot-dip galvanizing 45-85 µm with matched oversize nuts.
High-strength steel (10.9+) where coating is required: zinc-lamellar or mechanical plating; avoid electroplating.
Mixed assemblies: one coating system per joint. A galvanized rod with a bare steel nut creates a galvanic couple; the nut corrodes preferentially and the joint loosens.
Specification language that prevents disputes
Write the finish as a standard plus a measured range, not as a word. "Galvanized to ISO 1461, minimum local thickness 45 µm, average 55 µm" is auditable. "Galvanized" is not, and it is the phrase most often at the centre of a rejected shipment.