A2K encodes the look. Zn5/An encodes the process.

The sources for this page are the free previews of ISO 4042:2018 and ISO 4042:2022, which run to pages 7 and 9 respectively, and the ISO catalogue entries. Clause 9, the designation system, sits on pages 17 to 21. Annex F sits on pages 54 to 56. Both are outside the preview, and this page quotes neither.

Three vocabularies, all still in circulation

A distributor’s cross-reference table will happily put three strings in one row for what it calls the same finish: A2B, Fe/Zn5/A, and Zn5/An. They come from three different documents. The first is ISO 4042:1999. The second is the general electroplating vocabulary of ISO 2081 and DIN 50979. The third is ISO 4042 from its 2018 edition onwards.

The 2022 edition cut one of those loose. Its foreword lists, among the main changes, that all references to ISO 2081 and ISO 19598 have been removed because ISO 4042 is a self-containing document for the purpose of fasteners. The introduction gives the reason: the latest editions of those two general standards are not adequate to cover the requirements for electroplated fasteners, especially with regard to hydrogen embrittlement and baking, so the references were deleted to avoid contradictions.

You can watch that happen inside a single table. Table 1 of both editions lists coating metals against the ISO standard that covers each one, and it is in the free preview of both.

Zinc, the coating most fasteners actually carry, went from pointing at two general electroplating standards to pointing at a dash. That is not a gap in the table. It is the standard saying that for a fastener, it is now the only document you need, and that the older cross-references were pulling in requirements written for parts that are not fasteners.

What the 1999 code was describing

The old designation had three positions: a letter for the coating metal, a digit for the thickness, and a letter for the finish. Zinc was A, cadmium B, copper C, nickel E, zinc-nickel P, zinc-iron R. The digit indexed a thickness in micrometres.

The third position is the one that matters here. Its table is organised by gloss level crossed with chromate colour — matt, semi-bright, bright and high-bright, each against colourless, bluish, yellow to yellow-brown iridescent, olive, and black. Sixteen or more letters, and every one of them is a description of what the finished part looks like.

That was a reasonable thing to encode in 1999. A chromate conversion coating based on hexavalent chromium announced itself by colour: the yellow iridescent one was thicker and protected better than the clear one, and you could sort a bin by eye. Colour was a usable proxy for process.

What 2018 replaced it with

The foreword of the third edition lists the change in five words: new designation system for all coating systems. The introduction says what forced it. The document was completely revised to take into account new developments related to hexavalent chromium free passivations, the application of sealants and top coats, requirements for functional properties, and research aimed at minimising the risk of hydrogen embrittlement.

The scope of the current edition, which is free to read on the ISO browsing platform, lists what a coating system can now be built from. It applies to zinc and zinc alloy coating systems and cadmium, with or without conversion coating, with or without sealant, with or without top coat, and with or without lubricant, whether that lubricant is integral or added afterwards.

Four optional layers on top of the metal. The new designation has a position for each, which is why a real one runs to four or five fields rather than three. The heading of 9.3 names the target directly: examples of designation of hexavalent chromium free electroplated coating systems for fasteners.

Two things the clause list gives away for free

A coating designation and a fastener designation are not the same string, and Clause 9 keeps them apart. 9.2 is the designation of the coating system for the order. 9.4 is the designation of the fastener with its coating for labelling, and the normative references list ISO 8991, Designation system for fasteners as the document that governs the second one. So a purchase order and a box label are being told to carry different strings, built to different rules.

The other thing sits in a note at the end of the scope, and it is the sort of line that gets skipped. Electroplating, it says, is also applied to stainless steel fasteners, for example for the purpose of lubrication in order to avoid galling. Plating is not only a carbon steel problem, and the reason a stainless fastener might need it has nothing to do with rust — it is the way stainless seizes against itself.

Why colour had to leave

Once the passivation is Cr(VI)-free, colour stops sorting anything. A published corrosion-protection table from a fastener distributor makes the point with three rows that all name the same five micrometres of zinc.

Same metal. Same thickness. Eight hours to white rust, then seventy-two, then a hundred and twenty. The thickness digit — the only quantity the old code carried besides the metal — is the field that changes least across the range those three rows cover.

And the aspect column cannot separate the first two. Clear to bluish, clear to iridescent: both are the sort of thing an inspector writes down as “clear”. The nine-fold difference in white rust sits in the letter that names the passivation, and in the letter that says whether a sealant went on afterwards. Neither is visible.

The one row in that table that still relies on hexavalent chromium is the black chromated one. It is also the only row whose red rust figure is a dash.

The annex is still there, and it is informative

ISO did not delete the old codes. Annex F carried them through the 2018 edition at page 51 and carries them through the 2022 edition at page 54. Three pages both times. It did not get shorter.

What it is, though, is informative. In an ISO document that word is not decoration: an informative annex is not part of the requirements. A drawing that says ISO 4042 – A2K is quoting a document that files that string under obsolete, and quoting it from a section that specifies nothing.

The practical consequence lands when an old drawing gets revised. The temptation is to translate the code: find the old string in Annex F, copy across whatever sits next to it, and move on. The three rows above are the reason that is the wrong move. The old string fixed a metal, a thickness and an appearance. The new one fixes a metal, a thickness, a passivation type, and whether there is a sealant. Going back through the appearance to recover a passivation and a sealant is going back through the one property that stopped being informative.

The question to take back to whoever owns the drawing is not which new code matches the old one. It is which hours the part has to survive, and whether anything about the joint depends on the finish being a particular colour.

Two footnotes on the standard itself

ISO 4042:2022 is the fourth edition, published June 2022, fifty-seven pages, from ISO/TC 2/SC 14, Surface coatings. The third edition, ISO 4042:2018, is marked withdrawn. If a supplier document in front of you cites the 2018 edition, that is worth noticing rather than worth panicking about. The technical handbook quoted further up does exactly that, in a 2025 revision.

There is also an amendment, ISO 4042:2022/Amd 1:2026, published April 2026. It is one page. This page has not read it and does not say what it changes.

One more thing worth keeping straight, because the same Chinese and English word does two jobs. The passivation in a zinc coating designation is a conversion coating put onto the zinc. The passivation of a stainless fastener is a subtraction — iron being taken off the surface, with nothing added. Same word, opposite direction.

Where this sits among the coating codes

Every fastener coating standard this site has read writes its designation differently, and the differences are not stylistic.

Two of the three describe operations. The one that describes appearance is the one that was replaced. That is the direction coating designations have been moving, and it is worth knowing which side of it a drawing was written on.

None of this touches the two things a plated fastener drawing gets wrong most often, both of which have their own pages: the coating is a dimension and it eats the thread tolerance, and the plating process is where hydrogen gets in. Where the zinc goes on without any current at all is a different standard again.

This page covers step 5, the finish. The whole order is substrate, thread, head, drive, finish, documentation, and why doing it out of order is rework rather than a tweak is in specifying a screw.

Common questions

Is the code A2K still valid under ISO 4042?

It is still printed, in Annex F of ISO 4042:2022, whose title is “Obsolete designation codes for electroplated coating systems on fasteners according to ISO 4042:1999”. That annex is informative, which in an ISO document means it carries no requirements. A drawing can still say A2K and everyone will know roughly what is meant, but it is not a designation to the current requirements of the standard.

What do the fields in ISO 4042 / Zn5 / An / T0 stand for?

The coating metal and its thickness in micrometres come first, then a field for the conversion coating, then a field for the sealant. The scope of ISO 4042:2022 lists the layers a coating system can have: conversion coating, sealant, top coat and lubricant, each optional. The clause that specifies the notation is Clause 9, on pages 17 to 21, which is outside the free preview, so this page does not quote it.

Why did ISO 4042 change its designation system in 2018?

The foreword of the 2018 edition lists “new designation system for all coating systems” among the main changes, and the introduction says the document was completely revised to take account of hexavalent chromium free passivations, the application of sealants and top coats, requirements for functional properties, and research on minimising hydrogen embrittlement. The older code identified a finish partly by its chromate colour, and colour stopped indicating what had been done once the hexavalent chromates went.

Does the same thickness of zinc always give the same corrosion protection?

No. A published distributor table shows three systems that all specify five micrometres of zinc reaching white rust at eight, seventy-two and a hundred and twenty hours, the difference lying in the passivation type and whether a sealant was applied. Those are guiding values from a vendor handbook citing the withdrawn 2018 edition, quoted here to show the shape of the difference rather than as requirements.

Is ISO 4042:2018 still current?

No. It is marked withdrawn. The current edition is ISO 4042:2022, the fourth edition, published June 2022 and running to fifty-seven pages, with an amendment, ISO 4042:2022/Amd 1:2026, published April 2026. The amendment is one page and this page has not read it.

Why does ISO 4042:2022 no longer reference ISO 2081?

Its foreword states that all references to ISO 2081 and ISO 19598 have been removed because ISO 4042 is a self-containing document for the purpose of fasteners, and the introduction explains that the latest editions of those general electroplating standards are not adequate for electroplated fasteners, especially with regard to hydrogen embrittlement and baking. The effect is visible in Table 1, where the row for zinc lists two standards in the 2018 edition and none in the 2022 edition.

References

• ISO 4042:2022, Fasteners — Electroplated coating systems. Fourth edition, June 2022. Foreword, Introduction, Clause 1, Table 1 and the contents page
• ISO 4042:2018, third edition, withdrawn. Foreword, Introduction, Clause 1, Table 1 and the contents page
• ISO 4042:2022/Amd 1:2026, published April 2026, one page

The two ISO 4042 editions were read from their public previews, which end at page 7 of the 2018 edition and page 9 of the 2022 edition, together with the ISO catalogue entries read in a browser. Clause 9, the designation system, occupies pages 17 to 21, and Annex F, the obsolete codes, occupies pages 54 to 56. Both are outside the preview and this page quotes neither. The correspondence between old and new codes is what Annex F contains, and this page does not reproduce it; a third-party full-text copy of the 2022 edition is indexed publicly and was not used as a source. The designation fields and the corrosion figures shown above are transcribed from named vendor publications, Bossard’s fastening technical section dated 01-2025 and a public tech-info page for the 1999 code structure, and are labelled as such in the text: they are not presented as the standard’s own wording, and the vendor handbook cites the withdrawn 2018 edition. The 1999 code structure came from a machine translation with visible OCR damage, so only the shape of the code was taken from it and none of its numeric tables. This page gives no thickness recommendation, no salt spray requirement, and no mapping from an old code to a new one. What the amendment changes is not stated because the amendment was not read.