Brass/Copper Conversion Coating 1

[snapper3]

I work for a company that provides brass and copper machined and assembled end products. For the past several years we have treated those products with a brite-dip and then chromate conversion coating called Luster-On 70. The Luster-On 70 was a powder that we could add to our small dip tank (70 gal) to replentish & increase the concentricity of our bath.

Since Luster-On 70 was a HEXAVALENT chrome powder, it was discontinued by the manufacturer. They offered an equivalent liquid which we tried, but this generated an increased amount of hazardous waste since liquid had to be removed from the tank in order to reestablish correct concentricity. Treatment of that waste is costly.

We think that if we use a TRIVALENT chromate conversion coating that the treatment of the waste will be easier. Is there a trivalent chromate conversion coating available for brass & copper? If so, who makes it?

Thanks!

 

[CoryPad]

I would contact MacDermid. They are experts in surface treatment of metals, including copper alloys.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[aspearin1]

Trivalent Chromate turns to Hexavalent Chromate in the presence of oxygen. CrO6 (hexaChrom) is the fully oxidized state of chromium. That is to say, if there is a trivalent form, it WILL turn to hexavalent in liquid, unless it is in the presence of a strong reducing agent additive or in a fluid purged completely of oxygen. Not sure how cost effective this would be either.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[kenvlach]

Lest aspearin1's comments create unnecessary fears of hexavalent chromium, note that trivalent chromium is stable in air and natural waters.

Converting Cr⁺³ to Cr⁺⁶ requires a strong oxidizer such as ozone or chlorine. Even hydrogen peroxide cannot oxidize Cr⁺³ at low pH. In analyzing the new trivalent chromate baths for aluminum, in order to use the Cr⁺⁶ titration procedure, the Cr⁺³ in a sample must first be converted to Cr⁺⁶. This entails making the sample alkaline with NaOH, adding hydrogen peroxide and boiling (boiling is continued to remove residual hydrogen peroxide which would interfere with the titration).

 

[aspearin1]

I cannot comment completely on kenvlach's response as my experience with the chromate is limited to environmental analysis, working with contaminated soils and water, not process solutions. I should speak in less absolute terms regarding Cr(III) converting to Cr(VI), though. This is an equiilibrium reaction, the balance of which is influenced by the factors kenvlach has mentioned, and many others. Below is a link to a summary which claims that even Cr(III) can be converted naturally in the body to Cr(VI).

http://www.stanford.edu/~jpc/Chapter6.htm#_Chromium_Supplements_May

As far as stability is concerned, kenvlach may be dead on. Cr(VI)is reportedly dangerous at thousandths of a ppb which is still well below California's 50-microgram per liter (or 50 ppb) Max Contamination Level allowed in drinking water. It only takes a little, but then again, hopefully no one is drinking your waste.

Getting back to the real question, "Is there a trivalent chromate conversion coating available for brass & copper?" I don't know. In either case my experience leads me to believe some level of Cr(VI)is unavoidable in the waste stream without treatment. Proof of the contrary would be encouraging, though.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[kenvlach]

OK, snapper3, I'm not aware of any trivalent chromium treatment as effective as hexavalent for copper. Have you considered using benzotriazole? One source, wirh info:

http://www.metalprotection.com/benzotriazole.htm

"Pearson's (1974:302) procedure of mixing 3% benzotriazole in the ethanol wash as ...the benzotriazole inhibitor (Incralac) is a good procedure to follow."

https://www.denix.osd.mil/denix/Public/ES-Programs/Conservation/Underwater/5-CU-AG.html

[for mass production, e.g., continuous strip or sheet, benzotriazole is applied in hot water after cleaning]...

Also, the International Copper Research Association some years back came up with a lacquer containing benzotriazole suitable for outdoor use (statues & stuff). Called Incralac.

http://www.cda.org.uk/megab2/elecapps/pub122/sec59.htm

aspearin1, I read the paragraph 'Chromium Supplements May be Carcinogenic.' "Now Australian chemists have shown that chromium(III) can be transformed into chromium(VI) by oxidizing chemicals in the body such as hydrogen peroxide and hypochlorite (the chemical found in bleach and also generated by the body’s immune system)."

-- Vitamin/mineral tablets contain the FDA's 'Daily Value' (formerly RDA) of 120 micrograms Cr (trivalent). Hence, to play it safe, I avoid imbibing large quantities of hydrogen peroxide & hypochlorite. In one's stomach, conditions are highly acidic (maybe pH 1) anyway, so any hypochlorite is destroyed by the HCl and any hydrogen peroxide is non-oxidizing to the Cr⁺³ at such low pH. N
ow in the bloodstream, (my understanding is) the hemaglobin molecules have an Fe⁺³ in the arteries (carrying oxygen) and Fe⁺² in the veins. This Fe⁺² will reduce any Cr⁺⁶ produced by postulated processes. If you are in Australia and have recently been preyed upon by vampires, then maybe something to worry about.

If advice is wanted for chemical reduction + precipitation of Cr⁺⁶ in wastewater, I can help.
Cheers,
Ken

 

[DedalusEnviro]

As per Ken's caveat, I am cutting way back on my Clorox chugging habits, as well as hanging a braid of garlic cloves in my bedroom.

Seriously, a lot of what one thinks about this depends on what one has seen. In the context of hazardous waste landfill leachates, I have seen studies that document that Cr3+ can be converted to Cr6+ under certain conditions, in groundwater. I don't recall numbers, but the levels were high enough to trigger concern, in groundwater.

In the context of metal finishing wastewater treatment, the strictest Cr6+ limit I have had to work with is 0.2 mg/l. Meeting this was never a problem, and I was dealing with wastes generated by hexavalent chromium plating and hex conversion coating. The likelihood of the conversion of Cr3+ to Cr6+ to the extent that would make it a concern to a regulator seems very low, to me.

"correct concentricity" puzzled me a bit, OP...you mean, the correct concentration? You could try gently heating a portion of the solution to reduce the volume so that the make up liquid would fit in the tank. It may also be possible to extend the life of such a coating solution by plating the copper out of it. It might be necessary to use a divided cell to avoid oxidizing the Cr3+ to Cr6+; or, a shrewd selection of anode materials might do. Talk to Luster-On, there's a lady there by the name of Kim Price who has always been helpful to me.




Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.