ISO 4042 — electroplated coating systems for fasteners, and hydrogen embrittlement relief

Scope — and what it does not cover

ISO 4042 specifies electroplated coating systems for threaded and non-threaded fasteners: which coating metals and passivation or sealer combinations may be applied, how the system is designated, how much coating the thread tolerance can physically accept, and the requirements for reducing the risk of internal hydrogen embrittlement on high-strength parts. It does NOT specify mechanical properties (ISO 898-1 and -2), non-electrolytic zinc flake systems (ISO 10683), hot dip galvanizing (ISO 10684), or corrosion service life — salt-spray requirements are agreed separately and tested to ISO 9227.

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

• 1Coating thickness on a thread is limited by geometry, not by preference: plating grows the effective pitch diameter by about four times the coating thickness, so the usable maximum is roughly the thread's fundamental deviation divided by four.
• 2If the specified thickness will not fit inside a 6g thread, the answer is to produce the thread to a smaller tolerance position (6e or 6f) before plating, or to tap the nut oversize — not to plate thin and hope.
• 3Hydrogen-embrittlement relief applies from roughly 320 HV / 1 000 MPa upward: class 10.9, case-hardened parts, springs and hard cold-formed parts. Baking starts as soon as possible after plating, typically within 1–4 hours.
• 4Very high hardness — about 390 HV and above, i.e. class 12.9 — is treated as high risk, and electroplating such fasteners is discouraged unless the risk is accepted in writing.
• 5Baking is normally done before passivation, because bake temperatures can damage a conversion film that is already on the part.
• 6A coating designation is a code that has changed between editions. Write it out in words as well: metal, minimum local thickness in µm, passivation type and colour, sealer or lubricant yes/no.

What to write on the drawing or PO

• Coating metal and the MINIMUM LOCAL thickness in µm on the significant surfaces — and which surfaces are significant on your part.
• Passivation type and colour (for example trivalent Cr3+ clear/blue, iridescent yellow, black) and whether a sealer or lubricant top layer is required.
• The pre-plate thread tolerance class, so that thickness and gauging are compatible.
• Whether de-embrittlement baking is required, with the temperature and time window, the maximum delay after plating, and whether records are issued per lot.
• The corrosion requirement as a separate line: ISO 9227 NSS, hours, and whether the criterion is white corrosion or red rust.

What goes wrong when it is cited loosely

• "Zinc 12 µm, thread 6g, M5 × 0.8" — arithmetic that cannot be satisfied. At 0.8 mm pitch the 6g allowance leaves roughly 6 µm.
• Citing ISO 4042 and assuming baking is automatically included on every part. The requirement is triggered by hardness and strength; above the trigger it must be stated and recorded.
• Baking a lot two days after plating to tick the box. The window is hours, not days.
• Copying a coating code from an old drawing without checking which edition of ISO 4042 it belongs to.