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topic 30399p2

Black spots on zinc plated parts with trivalent chromate

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A discussion started in 2004 but continuing through 2019

November 23, 2013

Q. Hello all.
I'm having a lot of problems with black spots (around 1-5 mm diameter) after zinc chloride bath in rack.
We have improved through installing more filtration, but root cause is not so clear to me.
Any idea on what is going on?
Thank you.

Bernardo Roque
Process Engineer - Guadalajara, Jaliso, Mexico

February 24, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Sir,

We are getting Black patches and Spots during Salt Spray Test as per ASTM B117 [affil. link to spec at Techstreet] . Parts are Zinc Plated in non-cyanide Alkaline with Tri Chrome Yellow and Blue passivation. Require technical help.

- Jamshedpur, Jharkhand, India

February 2014

A. Hi Arijit. Black spots during salt spray testing are not necessarily a problem, but please attach a photo or two and give us some data like how soon into the test they become evident, or how long the test is. Thanks.


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

February 27, 2014

A. Dr. Sc. Mark Mertens of Enthone R&D, The Netherlands wrote an excellent paper entitled "Visual Degradation of Zinc Surfaces in Salt Spray Tests: Black Spots and White Haze." The paper is available in the Sur/Fin 2007 Proceedings (or you could get it from your Enthone rep). To summarize: Both black spots and white haze are normal characteristics of trivalent passivate behavior in the salt spray test. In my humble opinion, these should not be construed as failure unless the specifier specifically identifies them as such.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
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September 30, 2015

I am facing problem of Black Spots during SST on Alkaline Zinc vat plating with trivalent passivation. Black spots are formed within 24 hours. Please suggest action ... where should I take actions?

Electroplating of automobile parts - Aurangabad , Maharastra, india

affil. link
Trouble in Your Tank: Handbook for Solving Plating Problems
by Larry Durney
from Abe Books
info on Amazon

September 2015

Hi Ramchandra. Thanks very much for giving us the facts of your case, but there are over a dozen previous postings with food for thought here ...

Rony, Khozem, and Tom have shared the knowledge that this probably doesn't represent failure; and Gurumoorthi suggests that the problem may be poor cleaning, poor bath maintenance, or a low-profile chromate. I have twice requested photographs to clarify similar discussions. Hopefully people can speak of the delay for you though. Thanks!


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

September 30, 2015

A. The delay in formation of the "black spots" is indicative of trying to apply typical weak solution trivalent chromate over an alkaline zinc film.

Make up a fresh 1/4%/vol Nitric Dip to neutralize the alkaline zinc, THEN try the tri-valent chromate.

robert probert

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

October 2, 2015

A. Many others, including myself, have seen this phenomenon in trivalent passivated acid chloride zinc and mechanically plated zinc.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
plating systems & technologies banner ad

December 7, 2015

Q. Hi,

We are getting black spots on metal surface from zinc plating and trivalent chromate treatment process. We use barrel zinc plating to plate the metals. Out of 10000 pcs (17 mm diameter) of plated metal gaskets, about 15 pcs were affected by the black spots.

First, we clean the metals by degreasing process (includes acid HCl dipping; 20 kg of 30-40% HCl and 270 l of H2O). Next, zinc plating. Here is the description of the plating process:

ZnO: 42 gm/L
NaCN: 92 gm/L
NaOH: 38 gm/L
Brightener Asnite HT: 0.5 cc/L
H2O: 1800 L

plating time: 50 min
Room temperature
DC current: 250 A/ barrel

Last process is trivalent chromate process treatment (including nitric acid polishing, water rinsing, hot water rinsing, draining and drying)

The black spot is very small in size (about 1/6 of metal size). We are not sure what caused it. Is it because of poor cleaning of metal surface? The plating bath is cleaned every 10 days.

Any suggestion is much appreciated.


Tan Cheng
- Malaysia

January 6, 2016

A. Hi Tan,

It is possible these 10 pieces were stuck to the inside of the barrel during the plating process. Common with flat parts. This might be a high current burn or a barrel mark from iron.

Trent Kaufman
Trent Kaufman
   electroplater - Galva, Illinois

February 2, 2016

A. Good day Tan.

I think Trent nailed the problem, regarding barrel plating. Parts will always "nest" with each other, or adhere to the interior of the barrel.
Either way, you have a 0.15% rejection rate, if your numbers are correct.
That is quite impressive for barrel plating.
I have barrel plated zinc belt buckles with CN Cu, acid Cu (which was a real challenge),Ni, brass.
I have found to add glass marbles to the 60 ft/2 load, as they do provide a measure of fill, to "break" up the load,reduce abrasion of the substrate, and provide a measure of burnishing ( lower brightener consumption), and they do not reduce the ASF requirements/consume metal.
I also modified the barrel, drilling a large "hole"( 6" dia.) in the side of the barrel side plate, and a series of consecutive smaller holes within the entire barrel side wall, to match up with the side plate "hole". I pumped filtered solution through this "hole", to maximize solution exchange, as barrel plating suffers with the exchange. This gave me higher ASF allowances, but the pH of the CN Cu went through the roof! The grocery store did solve the pH problem with an add of 1-2 L vinegar (5% HC2H3O2) daily.
Hope this was helpful.


Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada


February 24, 2016

Q. We are manufacturing U clamps made from En8 material. These are then alkaline zinc plated and trivalent blue passivated. The customer specification is: thickness min 15 microns, and the plated part should pass a SST of 144 hours minimum for Red rust.

One such part was put in SST chamber and the part showed up black spot generation within 24 hours during the salt spray test. Could you please help by answering what could be the reasons for black spot generation and how we can avoid such a phenomenon? Also the customer is worried that he may have to recall the parts from his work site if these start failing on site.

Dhananjay Kadepurkar
- Pune, Maharashtra, India

April 2016

A. Hi Dhananjay. Please review the earlier entries on this thread, because most professionals consider "black spots" to be just an artifact of salt spray testing which does not constitute failure and will not happen in real life.

But I responded before I saw the graphic, and this is not what I think of when I hear of "black spots". I'll have to wait for someone more familiar with zinc plating of EN8 to offer an opinion of what we're looking at :-)
Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

April 1, 2016

Q. Hello,
I am a Quality Technician for an Investment Casting Company and we have had parts stripped and re-plated with new yellow dichromate and there is the appearance of black spots and what appears to be mold looking spots, has anyone encountered this issue or know of the reasons why this has happened? We will now have to blast and re-plate again.

Terry Cisco
- Chattanooga, Tennessee, USA

August 18, 2017



September 5, 2019

Q. We are experiencing the black spots in the Zinc plating with racked castings. They look great when they first come off the line and then the spots develop as they sit and dry/cure. We have 2 different cast suppliers as well as two different zinc platers and all parts end up the same. Can not find a common denominator in this problem other than the high humidity levels this time of year. This problem appeared last year as well but current plating shop changed their process a little and cleaned out the tanks with fresh chemical and problem went away. We determined last year is was silica left in the casting from the cast supplier not blasting good enough but now the problem is back. What are your thoughts and experience with RACK plated parts?

Sheila Onnen
- Iola, Kansas USA

simultaneous September 6, 2019

A. After chromate, the plater must rinse back/forth/in/out/cold water/hot water at least three times to pump the salts that are trapped in the porosity of the casting. Line operators seldom understand this requirement on castings.

robert probert

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

September 6, 2019

A. I believe that the plater needs to leach the parts after plating, prior to the chromate being applied. This appears to be chemicals trapped in the porous substrate that are slowly coming out.

John Johnson
- Jamestown, New York USA

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