ISO 2081 — electroplated zinc coatings on iron and steel with supplementary treatments

Scope — and what it does not cover

ISO 2081 is the general specification for electroplated zinc on iron or steel: thickness grades, the supplementary conversion (passivation) treatments and sealers that may follow, the designation system, and the tests used to verify thickness, adhesion and corrosion behaviour. It is written for parts in general — for screws, nuts and bolts the fastener-specific requirements of ISO 4042 sit on top of it. It does NOT cover zinc alloy coatings such as Zn-Ni or Zn-Fe (ISO 19598), hot-dip or mechanically deposited zinc, coatings on non-ferrous substrates, or any promise of service life.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1Thickness is a LOCAL MINIMUM on the significant surfaces, not a batch average. On barrel-plated small parts, recesses, internal corners and thread roots see the least current and are exactly where the requirement is tested.
• 2The designation carries four separate decisions: substrate, zinc thickness grade, conversion coating type, and sealer or topcoat. Omit one and the line cannot be quoted.
• 3Common grades are 5, 8, 12 and 25 µm, which map closely onto ASTM B633 SC1–SC4 — the reason the two documents are so often cross-cited.
• 4Trivalent (Cr3+) passivation with Cr(VI)-free sealers is the normal answer for RoHS and ELV markets. Colour is not proof of chemistry; the process specification and, where required, a declaration are.
• 5Thickness is verified by XRF (ISO 3497), microsection (ISO 1463) or coulometric methods (ISO 2177). Name the method, because the methods disagree on curved and threaded surfaces.
• 6For steels around 1 000 MPa and above, embrittlement relief applies; ISO 2081 points at those requirements rather than restating them.

What to write on the drawing or PO

• Thickness grade in µm as a minimum local value, with a note or sketch defining the significant surfaces.
• Passivation type and colour, sealer or topcoat yes/no, and whether Cr(VI)-free is mandatory.
• The thickness measurement method and where on the part it is measured.
• The corrosion test (ISO 9227 NSS), the hours, and whether the criterion is white corrosion of the coating or red rust of the base metal.
• For high-strength parts, the de-embrittlement requirement, referenced to ISO 4042.

What goes wrong when it is cited loosely

• Specifying "zinc plated, yellow" and nothing else. That is a colour: thickness, passivation chemistry and sealer are all still undefined.
• Measuring thickness on the bolt head and assuming the thread matches. Current density differs, so significant surfaces have to be stated.
• Assuming thicker zinc always survives longer in salt spray. Passivation and sealer dominate time to white corrosion; thickness dominates time to red rust.
• Reading ISO 2081 as a fastener standard on its own.