A Practical Guide to Trivalent Chromium Passivation on Zinc

Since RoHS and ELV regulations pushed hexavalent chromium out of most supply chains, trivalent passivation has become the standard conversion coating over zinc. Done right, modern Cr(III) chemistry meets or beats the old yellow hex-chrome numbers — but it is less forgiving of sloppy operation.

How it works

A trivalent passivate dissolves a microscopic layer of the zinc surface and redeposits a chromium(III)-zinc mixed oxide film. Film thickness — and therefore corrosion protection — depends heavily on pH, temperature, immersion time and zinc dissolved in the bath. The classic failure mode is an exhausted bath run 'because it still changes colour'.

Hitting salt-spray targets

Blue passivates typically deliver 24–48 hours to white rust; thick-film yellows 72–120 hours; add a silicate or polymer top-coat sealer and 200+ hours is achievable. If your parts fail audit, check bath age first, then rinse quality — dragged-in acid or alkali destroys films fast — then drying temperature, since trivalent films cure and should not exceed 60–70°C.

JSM Traders stocks trivalent blue, yellow and black passivations plus top-coat sealers, with guidance on bath make-up and maintenance schedules. Share your salt-spray specification and we will match a system to it.