DIN 50979 — the German designation system for zinc and zinc-alloy coatings with Cr(VI)-free passivation

Scope — and what it does not cover

DIN 50979 covers electroplated zinc and zinc-alloy coatings on iron and steel with chromium(VI)-free passivation, and above all it is a DESIGNATION SYSTEM: it tells you how to write a coating requirement so that substrate, coating metal, thickness, passivation and sealing are all fixed in one string. It also pairs each system with corrosion requirements expressed as a minimum time in neutral salt spray. It does NOT specify mechanical properties, does not cover hot dip galvanizing (ISO 10684) or zinc flake systems (ISO 10683), and does not certify service life in a real environment.

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

• 1The code reads left to right as substrate // coating metal and thickness // passivation // sealing. Fe//Zn8//An//T2 is steel, 8 µm minimum local zinc, type A passivation, with a sealer.
• 2The thickness figure is a MINIMUM LOCAL thickness on the significant surface — not a bath average and not the mean of a handful of readings. Which surfaces are significant is the drawing's job, not the plater's.
• 3The passivation letter describes the film, not the colour of the tank: An is the thin transparent to clear-blue film, Cn the thicker iridescent one, Fn the black one. The "n" marks the Cr(VI)-free chemistry the standard exists for.
• 4T0 means no sealer; T2 means a sealer or top coat is applied. A sealer changes friction as well as corrosion behaviour, so it belongs in the torque-tension conversation and not only the corrosion one.
• 5Zinc-alloy systems (ZnFe, ZnNi) use the same grammar with a different coating field, which is why an unfamiliar code can still be read field by field.
• 6The standard pairs each system with minimum salt-spray times to white corrosion and to red rust. Those are acceptance figures inside a defined laboratory test, not a prediction of years in service — read them from the edition in force and treat them as a test requirement.

What to write on the drawing or PO

• The full code, written out, plus the same requirement in words: coating metal, minimum local thickness in µm, passivation type, sealer yes or no.
• Which surfaces are significant, and where thickness is to be measured on a threaded part.
• The corrosion requirement with the test standard and the criterion — white corrosion or red rust — because those are two different acceptance events.
• Whether de-embrittlement baking is required for the strength class involved, with the temperature, the hold and the maximum delay after plating.
• If friction matters, the coefficient band and the test method, because a T2 sealer moves it.

What goes wrong when it is cited loosely

• Copying a code from an old drawing that still carries a Cr(VI) passivation designation and expecting a Cr(VI)-free shop to match it exactly. The chemistry changed; the code has to change with it.
• Reading the colour instead of the code. Clear, blue and iridescent describe appearance; An, Cn and Fn describe the film and its requirement.
• Specifying a thickness the thread cannot physically accept and discovering it at the gauge. The thread allowance sets the ceiling, not the coating standard.
• Treating the standard's salt-spray hours as a guarantee of field life. They are a laboratory acceptance criterion inside a defined chamber, nothing more.