Reading a Fastener Coating Callout: ISO 10683 Designations, Salt-Spray Hours and What to Confirm With the Plater

Decoding the callout field by field

The ISO 10683 designation is positional, so each field carries a separate requirement and a missing field is a missing requirement rather than a default ⁵. Take the worked instance 'flZnnc-L-f720': 'flZn' is the zinc flake coating, 'nc' is no chromium (chromate-free), 'L' means a lubricant is included, 'f' is the fine thickness grade at 5–12 µm, and '720' is the minimum 720 hours to red rust in salt spray ⁵. Read that way, the drawing already encodes the corrosion claim the plater has to meet, and it also tells the coater which system to apply ⁵. What the string does not say is as important. If your drawing reads 'ISO 10683 flZnnc-f720' with no 'L', the coating carries no lubricant component and its friction coefficient is uncontrolled; the coating still protects against corrosion, but you get no predictable torque-tension relationship unless you add '-L' or a separate lubricant specification ⁵. That is a decision you make at the drawing stage, not something the plater will infer for you.

What the salt-spray hour figure does and does not commit the plater to

The hour figure is a minimum time to red rust under neutral salt spray per ASTM B117 or ISO 9227, with the parts examined visually for the appearance of corrosion; if they are free from corrosion after the specified test time they have passed, and higher hours denote higher corrosion resistance ¹¹. That is the whole of the commitment: a pass/fail chamber result on uninstalled parts. It is not a service-life figure. Coatings are not permanent barriers — during installation threads cut, bearing surfaces rub and drive recesses deform, and once the coating is damaged local galvanic cells can form at the exposed areas ¹⁵. Variation in thickness, cure or adhesion can also produce different corrosion outcomes across the same production run ¹⁵. No source in this set quantifies how much service life is lost to installation damage or run-to-run variation, and none provides a conversion from salt-spray hours to field years, so treat the number as a process acceptance criterion and keep the joint design in view separately ¹⁵.

Checking the hour figure against the exposure tier

Zinc-flake hour bands are application-tiered rather than one performance number, so the same coating chemistry carries different minimums depending on where the part will live ⁸. The tiers are 360–500 h for indoor and moderate conditions, 720 h for standard automotive, general industrial and typical outdoor exposure, and 1000–1500 h for severe environments such as coastal or heavy road-salt exposure and extended-life requirements in wind energy and offshore ⁸. Use those bands to sanity-check the callout you were handed: a 720 h designation on a part destined for a coastal installation, or a 1000–1500 h callout on an indoor bracket, is worth a conversation with whoever wrote the drawing. The bands are one source's tiering of zinc-flake performance, not a governing standard, so the number you approve should still be the one your drawing names and your test report demonstrates ⁸.

Does the specified route force a hydrogen-relief bake?

The application route decides whether a relief bake is in the process at all. Non-electrolytic coatings — zinc flake, sherardising and mechanical galvanising — introduce no hydrogen during coating and eliminate the de-embrittlement requirement ⁷. Zinc flake is applied by dipping, spinning off the excess and curing with heat rather than by electroplating, which is why zinc-flake coated fasteners carry no hydrogen embrittlement risk ¹². Zinc-nickel is electroplated, so hydrogen embrittlement risk has to be managed on high-strength steel: baking after plating is standard practice, but residual risk cannot be ignored on hardened components ¹⁴. Where the drawing calls for electroplated nuts, the relief bake is specified — Grade 10 nuts at minimum 8 h at 190–230 °C and Grade 12 nuts at 8–24 h at 190–230 °C, both commencing within 4 h of plating ⁷. For fasteners below M6 (or below #10 imperial) in Property Class 10.9 or above, the window tightens to within 1 hour of plating, and mechanical galvanising or zinc flake should be specified where possible to remove the hydrogen source ⁶. The sources give relief times for nut grades only; they do not publish bolt-class-specific zinc-nickel relief times, so if your drawing is a bolt you will need the plater's own schedule.

What to confirm with the plater before you approve

These are the points where the drawing and the plater's process can diverge, and where the sources say geometry should be reviewed early rather than after approval samples are built ¹³.

• Confirm the thickness class on the drawing and the method the plater will use to measure it ²².
• Confirm thread build-up and recess coverage on your actual part geometry — zinc-nickel needs this evaluation on fine-pitch or close-tolerance parts, and zinc flake can distribute less ideally on some internal threads or very tight mating features ¹³.
• Confirm whether the coating thickness forces over-tapped nuts or post-coating thread chasing, as thick coatings such as hot-dip galvanizing do ¹⁸.
• Confirm the adhesion test method and its cadence — under ASTM F2329 the stout-knife test is required for every batch lot with zero-defect acceptance, whereas under ASTM A153 it is only performed when there is visible evidence of a potential adhesion problem ⁴.
• Confirm the lot sampling plan and the certification that will accompany each lot; F2329 references F1470 Table 3 sampling with a default to Sample Size C when certified test reports are unavailable ⁹.
• Confirm that thickness measurement will follow ASTM E376, with multi-location measurement at each end and the middle for articles exceeding five feet ³.

Settling the compliance wording: what 'nc' actually excludes

'nc' in the designation means no hexavalent chromium, which is carcinogenic and has been banned in Europe since 2017 — it does not mean the coating is chromium-free ⁸. The coating may still contain trivalent chromium, which is much less toxic and permitted under REACH/RoHS ⁸. If your application genuinely requires a completely chromium-free formulation, that has to be requested explicitly from the coating supplier, because the 'nc' field alone will not deliver it ⁸.

What the plater can and cannot quote from the coating family alone

The sources establish no relative cost per piece across coating systems at defined volumes, no tooling or fixturing requirements, no minimum order quantities, no lead times and no size limits for coating application. Those have to come from the plater, not be inferred from the coating family on the drawing. The cost figures that do exist are supplier-specific observations rather than market rates. One supplier reports zinc flake at roughly twelve times the cost of phosphate and oil, and attributes part of that to its own much lower zinc-flake volume ¹². A separate source puts ASTM F2329 at a 10–25% cost premium over A153 and recommends it only where critical structural applications, high-strength fasteners or rigorous quality documentation justify it ². Neither figure transfers to your part without the plater's own quote.

Where the sources disagree on salt-spray hours

The hour figures published for the same coating families do not agree, and no source in this set explains the discrepancy. That is why the figure you approve should be tied to a named test standard and a named test report rather than to a remembered number.

FAQ

Related pages

• Surface finishes product page
• Reading a Fastener Datasheet: Which Fields You Can Verify, and Which You Must Demand
• Zinc-Flake Coating for Fasteners: When the Evidence Supports It, and What to Verify at the Coater
• Fastener Material and Coating Choice for Aluminum, Stainless and Carbon Steel Joints in Wet Service
• Zinc-Flake vs Zinc-Nickel for High-Strength Fasteners: What the Evidence Settles on Hydrogen, Salt Spray, Friction and Specs