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Hex Chrome or Trivalent Chrome

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I plate brass pipes with 'watts' nickel followed by the conventional chrome plating.

My Chrome bath uses
1) Chromic Acid - 250 g/l
2) Sulphuric acid - 2.5 g/l

As far as I know, Chromic acid exists in the Hexavalent form but actual deposition onto the plated items are trivalent chrome. Can somebody shed some light on this matter i.e. whether chrome plated on an item from a conventional chrome bath is actually Hex.Chrome or Trivalent chrome?

I recently had a customer insisting to stop plating all his articles if Hex.Chrome are plated on his parts.

Please, anyone!

Cheah Sin Kooi
- Hong Kong


First of two simultaneous responses -- +++

Actually, plated chrome metal is neither hex or trivalent. It's valence state is zero.

totter James Totter, CEF
- Tallahassee, Florida


Second of two simultaneous responses -- +++

You are using hexavalent chromium. The term hexavalent or trivalent refers to the electrolyte, not the deposition process. The actual chromium deposition process is very complex as the Cr(VI) has to be reduced to Cr(0). The mechanism has been extensively studied by Hoare, who published a brilliant paper on the subject some years ago. This should be compulsive reading for any chromium plater. As I recall, the mechanism actually goes through an unstable intermediate of Cr(II), so using your theory, you are using a divalent chromium bath. Hexavalent chromium is a known carcinogen and there is wide concern, especially in the USA, about residual hexavalent salts that can get into the human body, as well as its environmental impact. There are numerous supply houses who can supply trivalent baths, but they are limited in their versatility. However, at the next AESF conference in early February, there are quite a few papers on trivalent chromium, so it may be worth your while looking out for their publication.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


First of three simultaneous responses -- +++

It's a good thing if you can get a trivalent chrome bath chemistry available. But I'm sure that it's the leachable chrome that your customer is after and not the decorative chrome on the parts.

khozema Khozema Vahanwala
Saify Ind
 
Bangalore, Karnataka, India


Second of three simultaneous responses -- +++

This issue is confusing a lot of people, hopefully not me :-)

Hexavalent chromium is used in two common applications in the plating industry. The first is as a chromate conversion coating for zinc plated and aluminum parts. In this application, if you do the conversion coating process with hexavalent chromate you deliberately leave hexavalent chromium on the parts for corrosion resistance. The European Parliament has effectively banned this process from use in the automotive industry, and these days most such parts are treated with proprietary trivalent chromate conversion coatings instead.

The second application is chrome electroplating. In this case what is reduced onto the parts is metallic chrome. Truck bumpers, toasters, irons, sink faucets, and decorative knives have had this finish for decades. As long as the parts are adequately rinsed, I don't think chrome plated surfaces are of concern to anybody -- you can eat off of them.

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


Third of three simultaneous responses -- +++

OK. Although the reaction mechanism during the reduction of Hexavalent Chromium to chromium metal has not been fully understood, 3 reactions occur.When current is applied, the first reaction to occur is Hexavalent Chromium being reduced to Trivalent Chromium. This is followed by reduction of hydrogen ions to hydrogen gas and then the deposition of chromium metal itself. If this is the case, can I assume that almost pure chromium metal is deposited on the cathode in the conventional Chromium bath? I may agree that traces of hex chrome may be present on the plated article due to inadequate water rinses after plating.

Cheah Sin Kooi
- Hong Kong


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If your bath formulation is: Chromic Acid - 250 g/L and Sulphuric acid - 2.5 g/L, so your "BATH" contains mostly hexavalent chromium in the formulation. The chromium metal that is plated on articles is not hexavalent or trivalent chromium but is chromium metallic Cr(O). The reaction simplified form can be represented as follows: Cr(VI) + 6e => Cr(O). The bath that has mostly trivalent chromium in the formulation eliminates problems that the chemistry of hexavalent chromium has, like toxicity, but the metal that is plated on articles is the chromium metallic too Cr(O). The reaction simplified form can be represented as follows: Cr(III) + 3e --> Cr(O)

Jean Vicente Ferrari
- São Paulo, São Paulo State, Brazil


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Mr. Kooi, yes, I have seen "hard chrome" (engineering chrome) plated components which, to my way of thinking, were inadequately rinsed and might contain hexavalent chromium. But I don't know how you could produce a bright, shiny, stain-free nickel-chrome plate with poor rinsing that left chromic acid on the parts.

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

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In all the great discussion and explanation above, we are missing one point: is the customer objecting to hex chrome in the plating process or trace amounts of hex chrome on parts? If it is hex chrome on parts, then James Totter has the answer above. I hope the customers knows what he/she is looking for :-)

Mandar Sunthankar
- Fort Collins, Colorado


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Ted,

My Nickel/Chrome plating plant has 2 drag out tanks followed by 3 water rinse tanks. The 3 rinse tank ,there is counter flow of water passing in the opposite direction of the work with the last tank having water spray. There are traces of Cr+6 because I believe the 3 water rinses could merely be diluting those chromic acid solution coming out from the last drag out tank. Moreover, as some of the brass pipes that I plate have internal diameters of 2.2 mm with racking pins of 1.2 mm, chances are, some chrome solution may be trap inside and carry over to the rinse tanks .

Cheah Sin Kooi
- Hong Kong


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Off onto a tangent, Cheah, when stainless steel is to be electropolished it must be processed in highly poisonous concentrated sulphuric acid solutions. Despite this exposure to poison, most dairy equipment and food processing equipment is required to be processed that way. Naturally it must be adequately rinsed. Even washing dishes in a dishwasher exposes them to dishwasher detergent which is poison in concentrated form. The most common disinfectant, bleach, is certainly poisonous.

Similarly, conventional chrome plating involves exposure of the material to poisonous hexavalent chromium, but that doesn't render the plated item hazardous. It just means that it needs to be adequately rinsed. I don't know how much dragout your pieces and racks involve, and whether two dragout tanks, plus three counterflowed rinse tanks and a top spray constitutes adequate rinsing at the flow rates you employ. But the principal is that you must rinse adequately, but you do not need to forswear chrome plating.

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

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What if I change one of the rinse tank into some sort of 'reduction tank ' with addition of sodium bisulfate at low ph ,say 2.5 .All Hex.Chrome will be reduced to Cr+3.Hmmmmmmmm . . .

Cheah Sin Kooi
- Hong Kong


First of three simultaneous responses -- +++

Mr Kooi,

I am getting a bit lost in this thread. I think that we now all agree that if you are using a chromium plating bath based on chromic oxide (CrO3), then the bath is a hexavalent one, but the metal deposit is of zero valence. Your original question mentioned that your customer asked you to stop using "hex chrome". As I see it, you are trying to find a way of ensuring there is no residual hexavalent chromium on your finished products - this is obviously highly commendable, but what does your customer say about it? Why is your customer so opposed to having "hex chrome" and will he agree to having his parts processed using hexavalent chromium systems? Perhaps he has environmental issues. Can you please enlighten us on your customer's thinking and reasoning. Furthermore, will your customer accept products that have had hexavalent chromium reduced to a lower, but not necessarily Cr(0), oxidation state? Hearing the customer's point of view could help other chromium platers who face similar problems.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Second of three simultaneous responses -- +++

Ted,

I accept your line of reasoning.For example, if stated 'Cadmium-Free', it means there is some sort of limit,say, 5 ppm or whatever. Rinsing requirements should be adequate to meet that limit. Back to Hex.chrome,is there such limits? Can Hex.chrome be tested on a chrome plated article?

Cheah Sin Kooi
- Hong Kong


Third of three simultaneous responses -- +++

That's not really a new idea, Cheah. Dr. Leslie Lancy used to call a variation of this "Integrated Treatment", and installed hundreds if not thousands of such systems. In those days we used hydrazine rather than bisulfite in the integrated treatment systems, but exactly why is buried too deep in my memory banks for recall.

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

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Cheah, if you are so concerned about trace amounts of hex chrome on chromed parts, then you have to find it yourself what the numbers are. I suggest you run an EDS scan under SEM to quantify, then run all different rinsing/neutralizing experiments and find what works best. I believe, the rinsing you are describing, if done right, will show trace amounts below detection limit. Make sure to run this method periodically to keep process under control. One thing for sure, anybody else's advice and method has no meaning unless you count the trace on your part yourself.

I am sure there are other detection methods I can't think of.

Mandar Sunthankar
- Fort Collins, Colorado


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Another Tangent,

If his customer is objecting to just the Hex as a bath, or let's say his customer has read where overseas they are limiting Hex chrome (Chromate) on Zinc or total chrome by weight, there may need to be some education needed here, I ran into a problem like that just a while ago.

Chris Snyder
plater - Charlotte, North Carolina


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Hi all,

My question on Chromating of Zinc plated parts.

I understand from the discussion is chrome on parts will be Cr(0).
Platers use Diphenyl carbazide to check presence of Hex chrome on part, when chrome on parts is Cr(0) Q1:why are we checking for Cr(VI)?

Q2:Are we checking for any residual Cr(VI)?

Q3:How can we check for the Cr(VI) content (i mean ppm levels) in the parts?

Q4:If any waxing done after hex chromating, does the test detect the presence of Hex Chrome?

Q5: Can anyone brief the reaction involved in the lab analysis using diphenyl-carbazide?

Q6: Does diphenyl detect the presence of any hex chrome on parts when chromated with Cr(III)?

I thank you in advance for your consideration and advice.

Javed
Automotive Sourcing - Hyderabad, AP, INDIA


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In answer to Javed,

Q1 Hexavalent chromium is toxic.
Q2 Yes, residual hexavalent Cr is sought with the test.
Q3 Agitate a part in water, then test the water.
Q4 Don't know for sure, but I think the wax could mask the presence of Cr(VI). I would recommend testing before waxing.
Q5 A colored complex is formed.
Q6 Yes, diphenyl carbazide still reacts with Cr(VI). Cr(III) is not detected. If total Cr (both VI and III) are sought, the Cr(III) must be oxidized with KMnO4.

For an overview of this test, see the book "Colorimetric Determination of Traces of Metals" by E.B. Sandell. Interscience Publishers, NY NY 1959

Roe Hicks
- Wichita, Kansas, USA


April 23, 2009

Cheah, better reduce his CrVI to CrIII with hydrazine or acidic sodium bisulfite. The toxicity question is always with the CrVI and besides the parts that are plated with any limit of CrVI in the rinse water had better be disposed of without any CrVI in it.
Because CrIII and CrVI exist together normally Cheah and Javed it better be oxidized to Chromate and precipitated with a high solubility product like Lead PbII and the precipitate disposed of properly.
Any ideas on how to do that, get the CrIII and CrVI out of the rinse water, off the product and precipitated then disposed of properly?
Anyone ever hear of Erin Brockovich movie, there the rinse water was the problem? Or the National Beef/Prime Tanning CrVI disposal in fertilizer sludge across Missouri, and the health hazards at Cameron, MISSOURI?

CG Gebhart
- Oregon, Missouri


October 25, 2010

Yes, CG, all plating shops that use chromic acid treat their effluent to first reduce the hexavalent chromium to trivalent, and then precipitate it out. Sodium metabisulfite is the most common reducing agent, but sulfur dioxide, iron, and other materials have been used. But I have never heard of lead being used for such treatment and think the cure would be worse than the illness :-)

Regards,

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