[Jayasudha]

Hi all...

pls answer for the following question if anyone know

How Hexavalent chromium converted to trivalent chromium?
What the conversion technique?
Which one best hexavanlent or trivalent?

[eazye1334]

You need to provide more information and context.

What is the hex chrome for? What are you doing with the tri chrome? This dictates what you can do to reduce it.

Define "best". What are you using it for? Do you mean for passivation? Health?

[Jayasudha]

oh sorry...i meant it for passivation....just i would like to know the conversion....
how hexavalent reduced to trivalent....

[eazye1334]

Are you doing it for wastewater treatment? Or are you just trying to reduce the chromate to use trivalent instead of hexavalent?

Hex chrome can be reduced using sodium metabisulfite in acidic conditions. The color will turn from yellow to green. The now-trivalent chromium will precipitate out if you raise the pH, which is why I ask about waste treatment as this is the method commonly used.

In the case you're talking about reducing your hex chrome to tri chrome and using that as a passivation, I wouldn't recommend it. You'd be better off just starting with a trivalent solution instead.

Answering your last question, hexavalent is undoubtedly better, but carries a lot of health concerns with it. In the United States, very strict limits have been set on the use of hexavalent solutions in electroplating. Trivalent works for some people, but it all depends on what metal you're plating and what environment it is going in to.

[Jayasudha]

how could you say hexavalent better than trivalent in passivation?which one give longer life?
we only doing trivalent passivation obviously its converted from hexavalent...
how Cr6+ element converted to Cr3+....any cracking process involved...
just i want to know the mechanism for my future study...

one more question
how to find the passivated component is hexavalent or trivalent?

[eazye1334]

Hexavalent has been know for many, many years to be the best available passivation for most electroplating/electroforming. We used to use it where I work, but I've developed an alternative due to the health concerns. Hexavalent should always hold up longer, and there are some theories that trivalent passivation actually oxidizes to hexavalent under the right conditions.

I don't know the mechanism and I kind of doubt you're converting it in-house. Trivalent passivation baths usually come purchased as they are very proprietary mixtures, not nearly as easy as making a hexavalent bath. The only way I know how to convert it without using very expensive and sophisticated equipment is by the method I described.

Usually you can tell by the color, as hexavalent will have a yellowish tint. The only other way to test it that I've heard is through this standard: http://engineers.ihs.com/document/abstract/KTZOPAAAAAAAAAAA

I've never done it and don't have it to even tell you what is involved, I just know this is the method used.

[Jayasudha]

thanks for your help...am not prepare any of the chemical in-house...i red that trivalent gave more salt spray
life than hexavalent....and one more information
diphenyl carboside chemical used for identifying the component whether hexavalent or trivalent passivated

[eazye1334]

Ok, yes, I've heard of that test now that you mention it.

Salt spray is actually what I was talking about when I mentioned tri chrome oxidizing back to hex chrome. There have been a couple papers done on the process that indicate the trivalent layer at the base converts to hexavalent and the rest of the trivalent passivation does nothing. Whether this is totally true or not, I have no idea, as my industry is quite different from yours or normal plating.