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How Can I Get Blue Tint With Trivalent Chrome?
My company makes metal hardware for doors. We are converting from hexavalent to trivalent chrome in our plating process for environmental reasons. Our hex chrome has a distinctively (faint) blue tint that we need to match for inventory and field replacement purposes. We have performed a number of tests and tried many combinations of trivalent chrome bath chemistry and parameters, but have continued to have difficulty matching it with our hex chrome. Our chemical supplier has also been involved. We are using a spectrophotometer to obtain L,a,b color values (based on SCE, d65, and 10 degrees meter settings). Our goal is to achieve a b-value in the range of -2.5 to -0.5 consistently. We have conducted several DOE's and found that significant factors appear to be: chrome concentration (lower the better), pH (higher the better), salt concentration (higher the better), and current density (higher the better). Surprisingly, temperature has not been found to be a significant factor. We have been able to get b-values between 0 and +1, but not lower. The regression equation resulting from our latest DOE is: b-value = 4.4 - 1.4*pH + 1.6*[Chrome]*pH - 1.3*pH*current density. However, the R-squared value is only 57%, indicating a poor correlation with the data. Bath operating constraints prevent us from operating where the equation predicts would be best. Question: are there some significant factors that we are ignoring in our testing and/or are there additives, techniques, etc. that would help? Any assistance will be appreciated.
Wayne FurnissQuality Assurance Manager - Madisonville, Tennessee
July 31, 2008
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Hi, Wayne. You are approaching this very professionally, and I hope someone can offer a comment that will help you in your quest.
I will just do my usual soapbox stand of saying I think it is a bad idea to try to make greener chemistry indistinguishable from from the toxic, carcinogenic finish you are replacing.
It is probably unlikely, but by no means impossible, that all hex chromated components in certain classes of consumer items may be recalled at some point because they are after all coated with a carcinogenic jelly.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 1, 2008
I do not understand. All tri-valnt blues give a wider range of blue shades than hex blues !?!? Try adding blue dye.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
August 1, 2008
First of three simultaneous responses -- August 1, 2008
As a supplier I would like to remind you that chromated zinc is a functional finish and color matches are impractical and very difficult.
You do not state what type of corrosion resistance you are desiring. The cobalt fortified products do tend to be less blue than many others which do not offer as much protection. The thick film products tend to me more clear with yellow/green/iridescent colors at the higher concentrations.
One variable which you have not addressed is the drying temperature and the use of a hot water rinse. Heat will aid in the formation of blue tints from those processes that tend to go in that direction. b In hex and tri chromates, some amount of dissolved zinc will assist in giving a bluer coloration, therefore the color will vary with the age of the bath. Oldtimers used to call this break in and would try to avoid it by keeping 10% or so of the old bath to "charge" a new one. Some platers have been known to hand a zinc anode ball in new chromate solutions to break them in. Many chromates will also change colors during their life due to iron buildup and a drag in of other chemicals.
Among some of the solutions to your situation would be for your supplier to adjust the product formulation to yield the coloration you desire. Another would be the use of a liquid dye in the chromate to control the color. This would be more consistent than attempting to control the other factors.
process supplier - Great Neck, New York
Second of three simultaneous responses --
He is talking about electroplating chrome, not a chromate conversion coating.
It is my understanding that you will never get tri color to equal hex color. They have been trying for 20 years to do that.
- Navarre, Florida
August 1, 2008
Third of three simultaneous responses -- August 1, 2008
Like Bob Probert, I don't understand either, but what I don't understand is this - are you talking about chromating zinc plating, or are you talking about chromium plating from a trivalent bath?
If the latter, then you will never exactly match the deposit color from hex. Some tri baths are gray, some more white, but none are the brilliant blue-white of hex. It's a problem you can't solve. Get as close as you can and learn to live with it.
Jeffrey Holmes, CEF
Spartanburg, South Carolina
You should use db value rather than b to match color.
one important factor you did not mention is the coating thickness.
- Singapore
August 4, 2008
Thanks for the responses. I am talking about chromium plating from a trivalent bath - not chromating. This is for a decorative finish that could wear off through handling (door handles, knobs, and escutcheons). Chrome thickness is down around 3 microinches. The underplating is copper, nickel, then chrome. Corrosion resistance target is 200 hours minimum in salt spray. Again, any help will be appreciated.
Wayne Furniss- Madisonville, Tennessee, USA
August 11, 2008
First of three simultaneous responses --
Hi Wayne,
Have you evaluated Cobalt Tin alloys, some suppliers claim this is close to Chrome Plating in Hue & brightness and is being used indoor applications.
- Toronto, Canada
August 13, 2008
Second of three simultaneous responses -- August 13, 2008
Ooops. You know that old saying about he who assumes is...OK So you are looking for a color more like hex from a trivalent bath in decorative chrome flash plating over nickel.
Despite what various suppliers say trivalent chromium plating will not have the blue "warmth" of hexavalent. At best, if you run the bath correctly and remove impurtities continuously, you will get a clean, white deposit.
If practical, you might consider a thin, water based, tinted lacquer.
process supplier - Great Neck, New York
Third of three simultaneous responses -- August 13, 2008
you should do the following DOE:
1.with the thickness increment of hex-chrome, the b value becomes more minus or less minus;
2.with the thickness increment of tri-chrome, where does the b value go?
Then you may find a range of tri-chrome thickness to match your hex-chrome color. of course, this range of tri-chrome thickness maybe out of your 0.3mil spec.
if I am not wrong, your hex-chrome should go thinner and tri-chrome go thicker in order to match them.
- Singapore
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