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topic 47325

Zinc plating with clear chromate has yellowish discoloration



A discussion started in 2007 & continuing through 2017

(2007)

Q. I am Quality Technician of a machine shop in Austin, Tex. We machine components parts for the semiconductor industry. We are currently having an issue with our customer in China with our parts being rejected for a yellowish appearance on arrival of the parts. I would like to know if this discoloration is caused during shipment overseas or is there a flaw in our supplier special process.

Zinc plate with clear chromate per ASTM B633 [link by ed. to spec at TechStreet] , Type 3, SC 2
Sheet is CRS 1010-1020, .060 thick

Tommy Garza
Quality Technician - Austin, Texas , USA


TUTORIAL FOR NEWBIES:

Steel components are often zinc plated for corrosion resistance and sacrificial (cathodic) protection. Because zinc is a more active metal than steel, it will preferentially corrode; that is, when corrosive forces are at work, "stealing" electrons from metals and causing them to corrode into solution as positively charged ions, zinc will donate its electrons to the steel, sacrificing itself to protect the steel.

To deter the zinc from too rapidly corroding away itself, zinc plating is almost universally chromate conversion coated -- a process which passivates the zinc and slows the corrosion. Historically these chromates were usually based on hexavalent chromium, which is inherently yellow, because hexavalent chromium did the job better than anything else. People would sometimes try to leach the color away through immersion in hot water before the chromate cured; but others have felt if there is no color there is no hexavalent chromium, so the approach wasn't valuable. And proprietary formulations of clear or blue chromate did not come even close to matching the performance of yellow chromates.

Hexavalent chromates are toxic & carcinogenic, and efforts to reduce their use have been perennial, but in recent years the European Union issued RoHS (reduction of hazardous substances) standards that effectively prohibited most use of hexavalent chromates in Europe, and all car makers around the world began adopting those standards for everywhere. At the same time, a new generation of high technology trivalent chromates (hexavalent-free) became available that can match the corrosion resistance of hexavalent chromates. Most of these are colorless, so today there is no categorical difference in the corrosion resistance of chromates based on their color, and most chromate is trivalent.

So some of the ideas in this 2007 thread are not very applicable anymore: it is possible to specify clear or blue trivalent chromates that are just as corrosion resistant as the yellow chromates.

(2007)

A. Lots of problems there, Tommy :-)

1). The people commenting on the appearance are thousands of miles away from each other; although everyone hates 'sample boards' for good reason, appearance standards can't be enforced without them; 2). Politics may be involved regarding whether this rejection is an attempt to erect an artificial trade barrier; 3). The people whose opinions you are enlisting here can't see the parts and don't know if it's rusting, the chromate too thick and streaky, or yellow chromate applied accidentally.

If you can attach really good photos maybe we can at least fully understand the complaint and narrow it down. Thanks, and good luck.

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


(2007)

A. Ted is perfectly right, it's not possible to diagnose such troubles with any certainty without seeing the parts in question.

I will say that when I have seen this problem, it has generally been a result of drag in contamination of the bright dip, because of chronically inadequate rinsing after the plating bath. Cutting corners, in other words.

I will add my completely personal opinion in the form of that old bromide...YOU GET WHAT YOU PAY FOR.

Good luck.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


(2007)

Q. Here are the attached photo's you requested. I just wanted to know if the discoloration appearance of these parts could change during shipment overseas. We keep having this issue even after all parts are 100% inspected.

47325-147325-1

Tommy Garza [returning]
Quality Technician - Austin, Texas , USA


(2007)

A. It is either a bad picture or that is poor (bad) plating. I would not call it a clear chromate. You might want to go to a very light blue chromate to avoid the yellow. Your chromate vendor can help you with both the yellow and possible solutions / alternatives.

James Watts
- Navarre, Florida


(2007)

A. I suspect the chromate pH is high due to zinc solution drag-in. In buckets set up a Nitric Acid 1/4%/vol. for neutralization followed by a fresh made chromate. Then cheat a 1/4%/vol. Soluble Oil in the hot water rinse. Then consider that everything that goes thru the Panama Canal does temperature and humidity cycle, so do not wrap in sulfur bearing brown paper.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como



(2007)

A. One way that some shops attempt to do a "clear chromate" is via a quick dip into a yellow chromate tank followed by leaching the color out in a final warm water rinse. This can easily lead to the issue I see in the photos. Your vendor should have a legitimate clear chromate tank and the first thing I'd probably do is ask him if he does.

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


(2007)

A. I agree with all the previous comments although, of course, I would like to add some of my own.
The yellowish discoloration could be due to the parts being processed in an alkaline non cyanide zinc solution that uses older (non polymer) brightener technology. This would be typical. The "fix" is a 0.5% nitric acid pre-dip before the chromate, use of a "very blue" trivalent chromate and close control of the chromate pH.
With trivalent blue bright chromates, increases in the pH that lead to values greater than 2.3 (1.8 to 2.0 being optimal for most processes) results in a thicker, more protective film but the refraction of light off this film will result in a yellowish cast to the part.
The simplest explanation could be just plain old poor rinsing so the part was stained.

The first thing you need to do is talk to the plater and have good communication with them to help solve this problem. The plater may chose to ask his vendor(s) for advice. Now you have knowledgable (I hope) people working to solve your issue. Of course none of this does any good if you also have a communication problem with your customer!

Gene Packman
process supplier - Great Neck, New York


December 25, 2014

A. Hi Tommy,
Your blue/clear chromate parts yellow at the edge , I think your nitric acid dip tank is contaminated; make a new 0.5% by vol. nitric acid dip and try again.

popat patel
Popat Patel
    Howard Finishing
Roseville, Michigan



March 12, 2008

Q. Is there a difference between Clear vs. Clear, blue bright?
We are trying to come up with a standard finish note that is ROHS compliant.

FINISH: CLEAR, CHROMATE PER Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil]F, TYPE II - CLASS 3.
or
FINISH: CLEAR, BLUE-BRIGHT, TRIVALENT CHROMATE PER MIL-DTL-5541F, TYPE II - CLASS 3.

Would this be saying the same thing or would this be 2 separate notes, stating different colors?

Richard Johnson
Designer - Wilmington, Massachusetts


March 13, 2008

A. You are the one who is deciding what you want, Richard, so you must assure that if someone follows your spec that you will get what you want. And the question then is, if parts come in somewhat bluish would you consider them satisfactory? If they come in with no coloration at all would you consider them satisfactory? If yes, then either spec can be used. If you have an appearance standard in mind, you can make sample boards showing what is acceptable and what isn't. Good luck.

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



Leaching conversion coatings on Zinc plating

January 15, 2014

Q. I do a small amount of zinc electroplating at it applies to auto restoration in a hobby-only setting. Is there a simple clear chromate conversion formula for treating the parts after zinc plating? I plan to use a sodium di-chromate/ sulfuric acid formula as a conversion coating, but I need a clear finish on some parts, not yellow/gold. Will a leaching dip of some sort remove the yellow/gold/iridescent color, yet leave the protective conversion coating? I came across a note somewhere that vinegar could be used to remove the coloration but retain the protection of the conversion coating, is this true?

Always fun coming here, reading through posts by professionals, trying to understand what the heck you guys are talking about :)

Thanks,
JohnnyB

John Branson
Hobbyist - Saratoga Springs, New York, USA
  ^- Privately contact this inquirer -^


January 17, 2014

A. Hi John. Last item first: we've started putting a "Tutorial for Newbies" sidebar on some our pages to try to address the problem of the threads being hard to follow. With 50,000 threads it will take forever, but at least it's underway.

Leaching will reduce the yellow saturation and the amount of hexavalent chromate and the corrosion resistance. Vinegar is bad stuff to put on zinc, as it is a mild acid and zinc plating is not at all acid resistant. Professionals use only proprietary trivalent chromates these days, and I'd suggest you try to do the same. Good luck.

Regards,

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



Clear chromated parts are yellowish, especially at the edges

December 9, 2014

Q. Hi there. I work at a plating shop as well and I have a similar problem: I process the steel pieces and when I'm ready to clear chromate them I tried several different immersion times, and I get yellow all over especially on the edges. What could it be if temperature is good and pH is good ?

Luis Melendez
- haverhill, massachussetts


June 5, 2016

A. Sorry, but many of the answers above are not convincing enough about yellowing of clear chromate Zinc parts.
A number of things can happen.
1.- Discoloration is a result of over chromating times 10-15 secs., depending on pH.
2.- Tank too deep so chromating solution too long in contact with parts.
3.- Blue Chromate too low pH.
4.- Drag out solution over-concentrated, or Rinse tank too high pH.
5.- Poor Zinc plating coating

Answer. Best way of tackling this problem is by testing all areas of the process separately and step by step. Small set of containers (5 liters)each will allow you to test all your processes with clean rinsing water, new chromates, timed dipping times and so on & so forth. If you are an electroplater, that is easy and effective, so you can produce regular top quality product finish consistently. Otherwise refer to your chemist for assistance. Hull Cell can also help.

jenaro Minchola
Electroplating - Melbourne, Australia



Purple Hue on Clear Chromated Parts

July 17, 2017

Q. I work at a plating company and we use Dipsol 444 products and we are seeing purple hue.
Do you know what causes this and how to fix it.
Lori

Lori Laddusaw
Quality Personal - Barney North Dakota USA


August 7, 2017

A. Hi Lori,

Purple hue is indicating you a low thickness chromate conversion layer. Colors range from red (the thinnest) to green-yellow (the thickest). Blue is in the middle, and that iridiscence is very common in trivalent conversion coatings.

You can modify that hue modifying your operational parameters in your trivalent conversion coating bath, or applying some sealer. If I may, I would ask your vendor how to manage a clean bright color without hue or with a blue hue, if you don't need high corrosion resistance.

Hope this helps you! Regards,

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina



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