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What galvanic potential is used for chromate conversion coated aluminum?

 

Ciao,
Does anybody know what electrogalvanic potential value is correct to use for the Aluminum after utilization of Alodine 1200?
Do I still use the Al value, or is it more correct to change it to the Cr value?

Thank you,

Giovanni

MR GIOVANNI MUMOLO
- TURIN, ITALY
March 30, 2010

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April 2, 2010

Dear Mr Giovanni Mumolo,

I'm afraid there's no easy answer available to your question. The old fashioned passivation types of chromate passivation leaves mixed films of Chromium, Chromium oxide and Aluminium phosphates or chromium phosphates and aluminium oxides (still in debate) on the surface, The layers are very thin, prone to pitting corrosion and easily mechanically damaged, but in general offer quite an improvement in chloride environment compared to the unpassivated (anodised) Aluminium. But where to place them with their potential compared to (anodised) Aluminium is an interesting question mark which is only solved by investigations in the medium you intent to use.

See also "Studies on the passivation of aluminium in chromate and molybdate solutions" C.B. Breslin*, ī, G. Treacy* and W.M. Carrollī
Corrosion Science Volume 36, Issue 7, July 1994, Pages 1143-1154.
And:
"Monitoring the corrosion behaviour of chromate-passivated aluminium alloy 2014 A-T6 by electrochemical impedance spectroscopy during salt fog exposure"
G. M. Treacy, , G. D. Wilcox and M. O. W. Richardson
Surface and Coatings Technology
Volume 114, Issues 2-3, 12 May 1999, Pages 260-268

Best regards,

Harry van der Zanden
- Budapest, Hungary