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Trivalent Chromium vs. Hexavalent -- what are the differences?

Q. I am using hexavalent chrome for coating. Now the waste generated are converted into stripping solution by HCl which I treated with NaOH and keep pH 9 and then by one polymer I get chromium oxide as precipitate. Now question is why hexavalent chrome changed to chromium oxide? Second question is separated water during process has high salt content so do chromium oxide. What can be done to get pure chromium oxide?

Farhan Patel
Student - Alberta, Canada
July 26, 2022

A. Hi Farhan. I do not understand" "the waste generated are converted into stripping solution by HCl" :-(

But if you are not talking about your plating solution waste or its rinsewater waste, and instead you are saying that you stripped metallic chrome from the parts with HCl, that does not generate hexavalent chromium. Chromium metal is valence/oxidation state zero regardless of the fact that the plating solution contained hexavalent chromium. And stripping chromium metal with HCl generates trivalent chromium not hexavalent chromium.

However, what you have precipitated is unlikely to be "chromium oxide"; it's more likely to be a hydrated chromium hydroxide sludge. As a student, we encourage you to keep trying to convert all your wastes to clean, usable, chromium oxide because thousands of plating shops generate tons of chromium hydroxide sludge and presently have little choice but to landfill it.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Yes you are right. I used HCl to strip chrome and that made trivalent chrome. Now I have chromium hydroxide but my plan is to make pure chrome oxide so it can be further used for other purposes. But while evaporating chromium hydroxide, the chromium oxide contains a lot salt content; I want pure chromium oxide as a product.

Farhan Patel [returning]
- Canada

↓ Closely related postings, oldest first ↓

Q. A client has interest in a high nickel material that is presently regarded as a characteristic haz-waste due to the chromium levels. I have informed the generator that there is an exclusion under the Hazardous Waste standards that if you are able to rule out that the process does not generate hexavalent Cr, that the waste is typically managed in a non-oxidizing environment and that the Cr in the waste is exclusively trivalent then you can exclude the waste from being considered a haz-waste.  My question is what is the difference of trivalent and hexavalent other than understanding that hex leaches much quicker and is considered haz where trivalent does not leach. How does tri become hex? Or hex become tri?

Brandon Smith
- Cleveland, Ohio

A. Hi Brandon. I need to preface this by saying that I am not sure that I full understand what you are talking about :-)

"A client has interest in..." unfortunately has multiple meanings: I don't know if you mean he is talking about buying it or is already saddled with it. "A high nickel material" is open-ended too, and I don't know whether it's stainless steel chips, or a pickling waste, or a sludge from the wastewater treatment of plating effluent. And I don't know what "the process" is.

With that out of the way, trivalent chromium is chrome ions that are in the +3 oxidation state, hexavalent chromium is chrome ions in the +6 state, and chromium metal (such as in stainless steel) is neither. Hexavalent chromium becomes trivalent through a chemical reduction process conducted as a part of the wastewater treatment, where sulfur dioxide or sodium bisulphite is added as a reducing agent. The reason for reducing it like that is so that it can be precipitated; the reason hexavalent chromium leaches is that it is soluble at normal pHs. Hexavalent chromium can be produced by an oxidizing process, for example when chrome plated parts are stripped in an alkaline electrolytic cleaning process.

If you can be a little more specific I may be able to answer your questions more directly. Thanks.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. In addition to all of the Correct information already supplied, there was an article published about 10+ years ago by Dr. Peter Klos, in which he indicated yet another species of Chrome ion. He mentioned the existence of a Cr+2 ion in Trivalent Passivate Baths. If you can get a copy, it makes good reading.

ed budman eb sig
Ed Budman [deceased]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018

thumbs up sign  I'm sure that Ed fully realizes his understatement regarding that seminal paper :-)

Many believe it to be the breakthrough that made modern trivalent conversion coatings and the RoHS requirements possible.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

thumbs up sign Oddly my great grandpa's name was Peter Klos, I've never seen the name Klos anywhere else before. He worked on the railroad 1800-1900s, great big handsome fellow, skips generations. Anyway, came across this web link looking for the quality differences between Hexavalent & Trivalent triple plated chrome, for a restoration. Near as I can tell copper has more corrosive resistance properties than straight nickel, though the nickel process seems proven. I'm in dry conditions regardless; heck, this will hardly see water. Speaking from a real novice here. Nice website, I've learned some things about other types of finishes as well, thank you.

Anthony r gould
- Carson City, Nevada
July 29, 2021

A. Hi. I guess Peter Kloses do great things :-)

Please see our "Intro to Chrome Plating" for quick coverage of all the issues you raised.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Can you tell us hazardous difference between trivalent and hexavalent chromium? I mean which compound is less harmful and how?

Ritesh shah
- valsad, gujarat , India
March 1, 2015

Chromium picolinate

A. Hi Ritesh. Most governments consider hexavalent chromium to be carcinogenic (certainly by inhalation, probably by touch, and possibly by ingestion), and everyone recognizes it as toxic. Trivalent chromium is not considered toxic; in fact health food stores sell chromium picolinate as a supplement.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Can trivalent chromium get changed to hexavalent chromium in nature or in human body?

Q. Recently I was having chromium supplements to reduce extra weight. The product uses chromium piconilate. However, recently I noticed that the product is approaching its date of expiry. I Wonder, knowing the potential carcinogenic effect of hexavalent chromium, that if there is a chance of alteration to hexavalent form.

Dipanjan Mazumdar
researcher - Salt Lake, Kolkata, India
August 3, 2015

A. Hi Dipanjan. You and I don't know what the basis is for the expiration date so we probably shouldn't conjecture about the safety of expired products.

I personally don't think either your medicine shelf nor your body will turn chromium picolinate into hexavalent chrome; and nature doesn't often do so either. Still, I can't rule out the possibility, and I think you should seek research papers on the subject rather than relying on conjectures that have no experimental data to support them :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. If you put some chromium picolinate in a salt spray cabinet it will generate enough hexavalent chromium to register as positive using the 1,5 diphenylcarbohydrazide [affil link] test.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad

Q. First a little background...
I have been obsessed with metallic finishes since the first time I saw a Tarn-X [affil link] add on UHF channel 17 in (1970-Something) where the dipped the blackened silver coin and antique sterling spoon in and it instantly glowed like a new mirror! Since then, everything I owned, or was left unsupervised around, (i.e., a brass or chrome household fixture, engine, jewelry, car, truck or Bike Part) and polishing compounds were reasonably obtainable, my stainless & nickel plated firearms, flawless, always; my W.R. Case & Sons, Buck, Puma, and a few handmade knives always draw admiration from friends who usually ask, 'hey, did you get a new knife?' I appreciate it when someone else validates or acknowledges the results, but I do it as an intensely gratifying activity. After the compliments, I still see areas that could use a little more TLC (Tom's Longterm Commitment).

I am about to commit to a high end (intensely scrutinized QC) cosmetic plating operation, I am not afraid to go hungry, because I don't eat when there's more prepping or polishing to do, and have been through a thorough process of elimination from cost efficiency to personal preference conflicts, precise target market demographics as well as being truthful to myself about my weaknesses and recruiting outside help where needed as part of my list of realistic goals.
-- Now for the question!
Is it your professional opinion, If I choose to plate using the latest trivalent chromium process, that someone with my OCD will be satisfied with if the planets are all in alignment?
I know I know, it depends on what I'm plating, but I absolutely if the materials are available, can polish a turd! Thats if the prep is on point, believe me that anything I take upon as a job or barter will be100% immaculate or I will never allow it to pass QC Inspection, and double verification will be implemented until established standards are consistent and reliably problem free)
What Im actually asking, will the characteristics be as aesthetically pleasing as Mirror Stainless, or Ye ole fashioned Cr-6 or close enough to stand behind?

{My plan so far is Magnetron SVD/TFD Vacuum Sputtering, and Plasma Flame/Arc Spraying & media tumbling}

And the final question is the aluminum substrate process any easier?
I can't find any specific literature.

Tom Conner
Consulting Arborist (Tree Care Specialist) and uber tinkerer! - Brooksville, Florida U.S.A
April 2, 2018

A. Hi Tom. Although trivalent chromium is sometimes said to be darker than hexavalent chromium, it has found widespread commercial use by professional platers for decades now, and I think you're on the right track offering it instead of hexavalent chromium regardless of how high your standards are. But that's my personal opinion not a professional opinion.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. I always find that trivalent chrome has a slightly iridescent blue tinge to it, perhaps more so than hexavalant chromium...but as you say it depends what you are plating.

I don't think you'll have any trouble with the finish, so long as you don't need to plate any thickness.

Oliver Gwynne
- Bradford, West Yorkshire

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