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topic 11728

Chrome Passivation Reaction


 

Hello,

Please explain the passivation reaction of hexavalent chrome on galvanized steel. What initiates the conversion of hex-Cr to trivalent-Cr? I understand that tri-Cr is not as effective as hex-Cr, but what would accelerate the conversion to tri-Cr after the galvanized strip is coated with a chromate passivation solution.

Also, I have heard that a 4-5% chromium concentration initiates the desired passivation reaction, but are higher concentrations less effective?...more effective?

K. A. Hermann
- Fort Wayne, IN, USA


 

Please explain the passivation reaction of hexavalent chrome on galvanized steel. I believe you are referring to the formation of a conversion coating, which proceeds when an acid solution of some complication, and of the correct pH, and salt content, dissolves some of the zinc, raising the pH of the solution adjacent to the zinc surface, I don't know what species loses electrons to the hexchrome, but zinc metal is being oxidized from the metal, so that may be a possible source. Some of the salts and metals in the solution precipitate as a result of the rising pH. A gel is formed at the surface and before it get washed off, it hardens.

I understand that tri-Cr is not as effective as hex-Cr, but what would accelerate the conversion to tri-Cr after the galvanized strip is coated with a chromate passivation solution. I'm not sure what point you are getting at, but nowadays, there is some evidence that "trichrome" conversion coatings have equal (or better!) corrosion protection to the older formulations containing hexavalent chromium. It is a complicated chemistry, so we really shouldn't talk only about "tri" or "hex".

Also, I have heard that a 4-5% chromium concentration initiates the desired passivation reaction, but are higher concentrations less effective?...more effective? You should talk about a passivating solution with all the components, rather than, say, a 5% sodium chromate solution. I don't think that anyone uses such a simple solution, because better results can be had by using blends of chemicals.

pooky
tom pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania



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