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topic 627

Testing chromate passivation of tin plate


(1996)

Chromate passivation is usually applied to tin plate to prolong its shelf life and to improve its corrosion performance and lacquerability.

Are there any methods available to analyze the effectiveness of a chromate passivation treatment? An ideal solution could possibly allow for analysis of not only the total quantity of chromium plated, but also for the % area covered and possibility for the chemical state of the chromium - is this relevant to the passivation process?

Any help or insight into chromate passivation would be greatly appreciated.

Thanks
Peter

Peter Levey
- South Africa


(1996)

I did not know you can chromate tin, you can passivate tin-zinc alloys, zinc, cadmium.

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 


You can chromate tin, Tom. Not being a chemist, I don't know whether it is a true chromate conversion coating, but references show it being somewhat effective towards reducing tarnish and finger printing.

Peter, free hexavalent chromium is a known corrosion inhibitor, and in the hexavalent gel-like coating it is supposedly 'self-healing'. But I've read that a chromate conversion coating involves hundreds of different chemical compounds when you consider the metals and the oxides that combine with the chromium, the different hydration states, etc. So the bottom line is that, short of salt spray testing, I don't know how to quantify if the chromate conversion coating was successful.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1996)

Tin is terrible to try and chromate. It is a conversion coating and not a plated material. The chromate film helps and I kind of think that it gets a big boost from being a bright dip. I do know that the amount of organics in the tin can make it impossible to do anything but turn it black. Some spec calls out for stearic acid in a xylene solution. When it dries, it leaves a fatty acid that allows it to be soldered without failure for a lot longer time. You have to try numerous storage mediums, because it seems like everything accelerates the corrosion.

James Watts
- Navarre, Florida


(2000)

Chromate passivation is usually applied to tin plate to prolong its shelf life and to improve its corrosion performance and lacquerability. Are there any methods available to analyze the effectiveness of a chromate passivation treatment? An ideal solution could possibly allow for analysis of not only the total quantity of chromium plated, but also for the % area covered and possiblity for the chemical state of the chromium - is this relevant to the passivation process? Any help or insight into chromate passivation would be greatly appreciated.

HAMLAOUI YOUCEF
- Annaba, Algeria


October 7, 2010

I am researching and finding out that various tests are being performed on lowering the pH of the BiChromate solution with Chromic acid may actually slow the corrosion activity after the tinplate has been lacquered.

An increasing of the Metallic Chromium relative to the Total Chromium is also being discussed.

Jim Matysiak
- Ohio, USA



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