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Chromate for Tin-Zinc Alloy Plating

DVD / Blu-Ray:
"Erin Brockovich"
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Quickstart:
     Tin-Zinc electroplating -- like zinc plating and other zinc alloy platings -- invariably requires a chromate conversion coating to deter rapid white rusting. In earlier times such chromate conversion coatings were usually based on chromic acid or sodium dichromate, but both of these are the 'hexavalent chromium' that made Erin Brockovich famous ⇨
and are eschewed by the European Union, all automobile manufacturers, and many others. Today, most chromating is done with far less toxic, trivalent, RoHS (Reduction of Hazardous Substances) compatible formulations.
     But most such trivalent formulations are proprietary; and because tin-zinc plating is very high in tin content, typical trivalent chromates for zinc are probably not optimal. Presumably, proprietary chromate formulations optimized for tin-zinc are available.
     Read on, join in ...

"Electroplating of Zn-Fe, Zn-Co & Zn-Ni"
by V Narasimhamurthy
zincAlloyPlating_2020

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"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber

( Rarely avail. for purchase, but see library holdings on Worldcat.org)
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Eastwood Tin-Zinc Plating System
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Q. Dear All,
I am currently looking for a suitable passivation process for tin-zinc coatings with a composition of approximately 70-90% tin and 10-30% zinc.
In this context, I would like to inquire whether standard passivation systems designed for zinc can be applied, or if there are specific passivation processes developed exclusively for tin-zinc coatings.

Thank you in advance for your support.
Kind regards,

Alex Jimenez
- Germany
April 27, 2026

A. Hi Alex

Some years ago I was involved in the tin-zinc re-plating of the thousands of square feet of ironwork on Philadelphia's William Penn tower (city hall). We used an electrolytic chromate on it, but that was hexavalent, and probably not acceptable for your EU needs.

Biestek & Weber suggests that a heavy chromate can be obtained with 20 g/l CrO3 at 80°C -- but this would not be RoHS compliant.

Readers are encouraged to help you if they can, but I think you'll need to go to one of the major chromate suppliers for a RoHS-compatible formulation designed for tin-zinc.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.

Hello Ted,

Thank you for your recommendations and your support.

Best regards
Alex

Alexander Jimenez [returning]
- Germany

A. Acculabs has had some recent success with a clear passivate for tin-zinc. We found from R&D that our traditional zinc passivates were not a direct replacement for tin zinc and needed to be reformulated.

Jose Rodriguez
ACCU-LABS

Chicago, Illinois

May 20, 2026

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