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topic 30399

Black spots on zinc plated parts with trivalent chromate


A discussion started in 2004 but continuing through 2019

2004

Q. I use zinc plating and blue bright chromating. But there are many black spots on the surface of our product. What should I do? What kind of trouble happened?

Thanks,

Mahfud [last name deleted for privacy by Editor]
automotive manufacturer -Jakarta, Indonesia


affil. link
Zinc Plating
by Herb Geduld
from Abe Books
or
info on Amazon

2004

A. Hi, cousin Marfud. There are many different conditions that people call black spots, ranging from flea-size tiny pits to 1/4" dia. dark spots, so it would be a very big help if you attach some photos so we understand what you mean.

Sometimes, for example, it is iron contamination of an acid zinc plating bath (see our library article Improving Your Chloride Zinc Plating Operation) which results in "barrel print out", a dark spot pattern that echoes the hole pattern in your plating barrels.

But there are many possibilities. What are the parts? What is the alloy? Are they barrel plated or rack plated? Acid, cyanide, or alkaline non-cyanide zinc plated? Just trivalent chromated, or chromated and post-dipped? Has the line just started up, or have you been plating the parts for years? Are you sure the problem starts in the trivalent chromate step rather than the zinc plating step? Please try to give us some more input, and pictures if possible, and we may be able to help. Thanks!

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


2005

A. Black spots after trivalent chromate sometimes can be referred to as "cobalt marking". Japanese industrial standards and an International Organization for Standardization symbol of zinc plating have only criterion about white rust and red rust. Therefore, there isn't a criterion about black spots. It doesn't effect SST but it sure does need a fix. I have some questions about this matter.
1) Is this trivalent chromate new make-up?
2) Zinc plating condition? (Zn concentration, Caustic soda concentration, thickness, impurity concentration)

When thickness of zinc is thin (less than 5 micron), corrosion resistance of trivalent chrome film deteriorates because there is derricking on the surface of materials more than 10 micron and when thickness of zinc is thin, a part remains as pin hole.
3) Drying condition after trivalent chromate?

Rony Nurmawan
- Jakarta, Indonesia



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



2006

Q. Has anyone seen black spots on zinc plating with trivalent clear chromates? We recently converted from hexavalent to trivalent and began seeing what appeared to be a white haze that turns into very small black spots. We use a silicate sealant over the chromate.

Gary Clyburn
Captive shop plating fittings - Dayton, Ohio


2006

A. Is your zinc bath Alkaline or Acid Chloride? Rack or Barrel? Giving more details would narrow things down a bit.

JEFF W LENTZ
Plating Co. - Lancaster, Pennsylvania


2006

A. Gary

There is a wide set of probabilities, which may or may not help. However..

A. If perhaps you are a cyanide zinc fan, the black spots are entrapped cyanide in porosities, bleeding out post-passivate and drying.

B. If you are an alkaline Zinc fan, try beefing up your post-plate nitric acid dip to ensure no caustic carry over into the trivalent passivate.

C. And if you happen to be an acid zinc fan, using a barrel here are 2 probabilities.
   1. Your Current is too High and causes imprints of the barrel holes on the parts which is burning due to inability of solution to carry current. Check Zinc and Total chloride to data sheet levels. Reduce current.
   2. If you run rack, it might be loose zinc particles co depositing and then corroding faster than the regular surface. (In alkaline zinc Rack too, such a possibility exists.)

Now Tell us what you do -- Rack or barrel; Cyanide acid or alkaline?

Regards,

asif_nurie
Asif Nurie [dec.]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016



2006

A. Gary,

One more possibility exists.

Are you applying the Topcoat while the part is wet, or do you dry first, then Topcoat ?

If the second possibility is being followed, the parts will behave as described by you..
Check.

asif_nurie
Asif Nurie [dec.]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016




September 22, 2008

Q. Hi,
We are faced with black spots problem on Zn plated (15~20 microns) & trivalent chrome passivated (bright) parts. Black spots are appearing on surface within 24 hrs in salt spray test. Zn plating is getting done locally. They are following the following process: first components are cleaned with kerosene (scrubbing with saw dust), dipping in HCl, water rinsing, Zinc plating, water rinsing, passivation, water rinsing, drying.
I do not know much about plating. Can anyone help me to get out of this problem? Is trivalent yellow passivation better?

Kunadha Raju
Production Manager - Hyderabad, India


September 23, 2008

A. Hi, Kunadha. I don't think kerosene is an adequate cleaner (plus, of course, it's very dangerously flammable). I think your plating shop should alkaline clean your parts. I'm not confident that that is the direct cause of the black spots (it could be iron contamination, the plating too thin, or something else), but there seems little sense in fine tuning the process until it's been roughly tuned with proper alkaline cleaning instead of kerosene :-)

In the old days of hexavalent chromating, yes, yellow passivation was better because it was thicker and higher in chrome content. But in these days of trivalent passivation, many "yellow" passivates are simply yellow dye added to the clear passivate. Good luck.

Regards,

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


September 29, 2008

A. HELLO SIR,
Are you doing acid zinc or cyanide zinc plating?.And what trivalent passivation you are doing yellow or blue? Tell clearly.

Shoban Kesarapu
Shoban Kesarapu
plating chemicals supplier - Secunderabad, A.P., India



To Reduce the Failure Rate of Zinc Plated Components

September 27, 2008

Q. 1. Most of my Suppliers are using Trivalent Zinc Technology for which sealant is mandatory to generate self repairing function. Now I don't have any procedure to cross check at my end whether the sealant was applied on the part or not. Please guide me how to check the presence of Sealant.
2. Also please reply for if Silicon has been added in Sealant in which form, Silicon-di-oxide or Silicon-tri-oxide.
3. As you know, during post treatment Zinc tends to dissolve in Passivation and the limit of Zinc Impurities has been given as 15000 ppm in Technical data sheet of Chemical Supplier. Now my query is How much ppm zinc dissolves in passivation bath by processing of one sq. ft area having 10 micron thickness of zinc coating.
4. My Plating Spec. is Thickness min. 8 µm and SST Life is white rust: 72 hours, and red rust:- 240 hours. Now please guide me for 4 years warranty: is the mentioned spec. right or do we have to enhance our spec?
5. Why do black spots usually occur before starting of White Rust during SST Testing of Zinc Plating? Is it due to the Co-deposition of Impurities along with Plating? If Yes, then how to control the impurities. Please guide me.
6. Why is Cyanide Zinc Technology banned for Plating. Is it possible to deposit cyanide with Zinc Coating on Components. If it is not, then why do we generally avoid the usage of Cyanide Zinc Bath. Please guide me.

Anil Kaushik
two wheelers purchaser - Gurgaon, Haryana & India


October 21, 2008

A. Hello Mr Kaushik,

1. There is no hard and fast rule that a sealant must be used; most Chromates are of the self healing type with or without topcoats. It looks like you have been reading technical literature of a proprietary process that advocates this. Product Literature serves the interest of the product, I would only suggest that you use corrosion tests as usual.

2. Silica is not always automatically the best sealant material, I have found some polymers work far more consistently.

3. The dissolution of zinc may be up to a micron whilst Passivation; it depends on the total acidity, temperature, and dip time.

4. 72 hours to white rust will get you past one year without trouble in Central areas but not Coastal India. For Coastal areas you should target 240 hours to starting of WR. The mean corrosion rate in Coastal areas is about 3-4 microns a year quite easily.

5. Black spots are not impurities of plating but a natural by-product of chromate break down in a Salt Spray test. It doesn't happen in real world corrosion; only in a salt spray test, these are normally ignored.

6. There is no ban on Cyanide Zinc Plating; It is advisable to restrict use of Cyanide at the source, like it is advisable to restrict Hexavalent Chrome. If the use and disposal of cyanide is managed properly there is no ban that exists.

I conclude that you are meeting too many suppliers of Alkaline Zinc these days!

Warm regards,

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo


thumbs up signWonderful response, KV, and the levity is appreciated. Thanks!

Regards,

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


October 28, 2008

A. Hi Anil Kaushik,

My best suggestion is to use better (long lasting) trivalent chemistry with high iron and zinc tolerance (Zn 25 gm and Iron 300 ppm). This chemistry WITHOUT sealant will produce 160 plus Hrs in blue, 240 plus Hrs in iridescent towards white rust. Imagine that red rust will be over 500+ Hrs.

Black spot is due to poor cleaning, poor maintenance of plating chemistry and again low profile Trivalent chromates.

Yes cyanide is going out slowly due to environmental requirements. But Alkaline Cyanide-free process is available to do the needful -- much better way by various advantages.

Best Regards,

p gurumoorthi
P. Gurumoorthi
    electroplating process chemicals
Chennai, Tamilnadu, India




October 2, 2009

Q. Black spots observed on my Zn trivalent plated steel components along with white rust.

Voufique Qureshi
first timer - Mumbai, Maharashtra, India


October 6, 2009

Thanks for sharing!

Brian Terry
Aerospace - Yeovil, Somerset, UK


October 6, 2009

A. HI,

BLACK SPOT AFTER TRIVALENT CHROME , CHECK YOUR RACK CONTACT MAY BE BROKE AND HOLDING ACID . ANY ALKALINE CONDENSE DRIPPING ON THE PART OR HYDROCHLORIC AICD FUME OR HYDROCHLORIC ACID DRIP ON PART FROM YOUR WORK PROCESS.

popat patel
Popat Patel
    Beacon Park Finishing LLC - Roseville, Michigan



Black spots on Zinc plated brazed/hardened parts

August 24, 2011
57609

Why do black/purplish spots appear on Zinc plated, brazed and hardened parts?

I am a design engineer doing process engineer work, so bear with me as I describe the process:

2 Parts, Leaded cold drawn steel and 1050 steel
Parts oven brazed with copper, no flux
Heat treated to Rc40-Rc45
Hydrochloric acid cleaning, 10 minutes
Alkaline Zinc rack plate
Post baked 4 hours at 375 °F
Trivalent chromate for 40 seconds

The spots appear where the copper braze is, the fillet and areas where excess braze pools. They can be small in size to over a 1/4" (I have tried an acid zinc rack plate, which eliminated the black spots but there was poor adhesion). On inspection the black spots appear to be carbon? Any help? Brent Sandstrom
- St. Paul, Minnesota, USA


August 27, 2011

A. Can you tell me: After Acid cleaning the problem area looks like? Has black powder? And what is the concentration of acid cleaning that you used?

If you have black powder, I suggest you clean it before plating.

As I know if you have good cleaning before plating, you will have less problems.

Sorry for my english if you read my message and not understand.

Kornwit Sriburin
- Bangkok, Thailand


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