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"Black spots in zinc plating"

Current question:

April 2, 2021

I am getting black spots on threaded area of nuts, I have changed anodic cleaner and increased acid concentration of pickle -- please help.

32077-4a   32077-4dtl   32077-4b  

ABHISHEK KALIA
- Ludhiana Punjab
^

none
adv.
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April 10, 2021

A. PLEASE CHECK YOUR PRE-TREATMENT AS OIL RESIDUE LEFT IN THREADED PORTION LEADS TO THIS TYPE OF DEFECT.
DID YOU DEGREASE IN TUMBLER OR BY JUST DIPPING.?
IF YOUR SUPPLIER IS USING THICK OIL YOU HAVE TO DO FLASH PLATING AND DEPLATE AND FOLLOW YOUR REGULAR PROCESS.

RAHUL V POTPHODE
- PUNE MAHARASHTRA INDIA
^




Previous closely related Q&A's starting in:

2000

Q. Black spots appears on our barrel acid zinc plated parts, can anyone help us on this matter?

Process:

Thank you for your time

andrew kaw
- manila, philippines
^


2000

A. This sounds like it might be a classic case of iron contamination. See "Improving Your Chloride Zinc Plating Operation". or send pics.

Regards,

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


2000

A. Dear Andrew,

Firstly (a) do read the article Mr Mooney has suggested. Meanwhile if after you treat the bath for iron and the problem still persists then
(b) try reducing the Voltage. If efficiency drops, then increase anode area.

If the reduced voltage has helped without any problems, then
(c) sigh with relief. If not then
(d) check the barrel rpm which may be too low. Increase the barrel rpm between 14 to 16 rpm whatever suits the hole size and current with giving your parts black spots.

smileyIf all the above efforts fail, then do the opposite of everything above and charge more for the black spots without giving them away for free!

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

saify logo
^



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 :-)



2004

Q. I WANT TO UNDERSTAND THE REASONS FOR THE GREY / BLACK SPOTS IN THE ZINC PLATING.

SANJAY NARULA
FASTENERS - ROHTAK, HARYANA, INDIA
^

----
Ed. note: Sorry, we can't post answers to question 2. (why?)


2004

A. Sanjay,

You don't include enough information, but sounds like corrosion. Are you chromating? If you let the part(s) sit long enough I'm guessing the dark spot will turn to "white rust".

Trent Kaufman
Trent Kaufman
   electroplater - Galva, Illinois
^


2004

A. Dear Sanjay,

Black spots in barrel acid chloride zinc plating is a common problem there are many reasons for that some of the important reasons are Iron contamination, low metal content, higher content of brightener "R" if you are using Indian Chemistry. If there is no iron contamination in the bath, check & adjust metal content above 30 gms/litre.

N K Praveen Kumar
- Secunderabad, ANDHRA PRADESH, INDIA
^



2004

Q. Dear Praveen,

I would like to know more about Iron Contamination in details.

Kindly respond to the same.

Sanjay Kumar
- Bangalore, Karnataka, India
^


2004

A. Sanjay

These are possible causes of Black spots in Barrel Zinc plating from acid Zinc electrolyte:

1) Dissolved iron from dropped parts in solution.
2) lack of carrier or Wetter sold as "M" in India.
3) Lack of Chloride or low Chloride./ maintain 125 to 140 Gms/liter.
4) lack of Zinc as  metal . maintain 25 to 28 Gms/lit.
5) Excess brightener too sometimes causes Blackish deposits.
6) Recast Impure Zinc anodes containing lead or Copper cause black deposits.
7) using Local impure "A" salt ..
8) using Local impure "B" salt.
9) Proper pretreatment is essential.Clean parts plate good.
10) Continuous filtration is essential. Dirty Solutions don't plate well.
11) Passing higher volts than the solution can carry.
 

Like we said above this is a short list of POSSIBLE causes.

Good luck with your plating .

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

^


2004

A. Dear Sanjay ,

Following are the some of the reasons for Iron contamination in Acid Zinc Plating (barrel):

1. Parts falling into the tank during loading, unloading or during the course of plating.

2. As the current density in barrel plating will be very low, parts to be plated are exposed to acidic solution and hence Iron gets dissolved into the solution.

Apart from the above poor quality of anodes and chemicals also result in iron contamination.

Removal : Depending on the extent of contamination you should treat the solution with Hydrogen peroxide 100 vol, @1-2 ml/liter. Then filter the solution after settling the solution overnight, and discard the sludge.

Note brightener "M" should be added after the above treatment @ 2-3 ml/litre.

N.K.Praveen Kumar
- Secunderadad, India
^



White patches after Barrel Acid Zinc Plating, Clear Trivalent passivation

February 6, 2011

Q. We are doing plating on steel balls after polishing them in vibrator. We are plating them in barrels for about 15 mins. It is acid zinc solution. Trivalent Clear passivation is used after plating. White patches are observed after the parts are placed for drying on table (we also do the drying process by rotating the parts in dryer for about 5 mins). Are these patches due to incomplete drying of parts or due to some other reason? Kindly help.

Raja Rupinder Singh
Machinist - Ludhiana, Punjab, India
^


February 8, 2011

A. Hi, Raja

We need to do a thousand things right to get satisfactory plating, and just one thing wrong to get unsatisfactory plating :-(

So it is often not possible to discern which of those thousand things is wrong based on a description of the unsatisfactory plating. My guess is that there is a problem with the brightener in the zinc plating bath, but it's only a guess ... but that is not to discredit the need for total dryness. Good luck.

Regards,

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


March 19, 2011

Dear Mr Raja,

You need to improve the plating thickness -- 15 minutes is too low since the steel balls have too much of surface area and my guess is that in a conventional plating system you will need an hour at least to achieve about 5 to 10 microns which should be a minimum.

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

saify logo
^


thumbs up sign Thanks for getting to the heart of the matter, Mr. Vahaanwala, which I completely missed: you can't do proper barrel zinc plating in 15 minutes :-)

Luck & Regards,

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



May 10, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We are doing Zinc plating with trivalent yellow chrome passivation on Zamak 3/5 (Zinc alloy) casting.
In some cases black spots appear on plated casting.
What are these black spots? Why these black spots appear with time? Why not this problem is 100%?

Vikram Puri
- Noida, India
^


May 10, 2012

A. Hi Vikram.

We appended your posting to a thread about black spots in zinc plating which may help you. We have many other threads about the subject, which you can find if you search the site. But people will probably have trouble offering assistance tailored to your situation until we know more details: whether your parts are cyanide zinc plated? Spotting out of cyanide is common. Are your parts barrel plated? Black spots from iron contamination of acid zinc solutions are common. Please tell us whether you are doing barrel vs. rack plating, cyanide vs. alkaline vs. acid zinc plating, whether there are any pretreatments before the zinc plating, whether the castings are vacuum impregnated, whether the processes you use are from reputed suppliers or are home-brew, etc. And please send pictures because different people visualize very different problems when they hear 'black spots'. Thanks!

Regards,

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


May 14, 2012

Q. We are doing barrel plating and the parts are alkaline zinc plated.The Zamak castings are not vacuum impregnated.
We are doing pretreatment before plating.The pretreatment includes Soak cleaning, cathodic cleaning, water rinsing, Acid pickling and water rinsing.
If you require I can send you the control plan of the complete process

Regards
Vikram

Vikram Puri [returning]
- Noida, India
^


May 14, 2012

A. Hi, Vikram.

Asif Nurie offers some good possibilities in letter 30399. First guess is that the current density is too high, causing burning in the areas exposed to the barrel holes. Do you have any evidence of such a pattern?

Regards,

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



July 7, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi experts,
I'm running acid zinc barrel plating and currently facing the barrel holes mark on my plated parts.
I suspect this problem is caused by the barrel rotation speed! Is it true?

barrel hole mark on plated part

Lovely thanks for help.

Sng Horng
- Penang, Malaysia
^


July 2013

A. Hi Sng. We appended your question to a similar thread. This looks like a classic case of iron contamination in your acid zinc plating tank, not a problem caused by the rotation speed. Good luck.

Regards,

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


July 22, 2013

A. Black marks in barrel plating of acidic zinc are a results of too high divalent iron in the solution.

You have to oxidise the iron (by hydrogen peroxide or air) and filter the sludge.

sara michaeli
Sara Michaeli sara michaeli signature
chemical process supplier
Tel-Aviv, Israel

^



August 19, 2013

Q. Are Black spots caused by any spillage of chemicals.? What chemical spillage will cause Black spots except Pickling Chemical.

MOHANAVELAVAN Elangovan
AUTOMOBILE INDUSTRY - CHENNAI, INDIA
^


August 21, 2013

A. Hi cousin MOHANAVELAVAN. Please invest a couple of paragraphs in your question. Unfortunately your question is so brief that I have no idea what you are talking about :-)

Are you saying that you have zinc plated some parts, and they were perfectly acceptable, then somebody spilled something on them but you don't know what it was, and now they have black spots and you are trying to figure out what the chemical was, based on the black spots it creates on zinc plated parts? We do have a thread on line here that says a spill of Dr. Pepper soda discolored plated parts. Good luck.

Regards,

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



February 8, 2018

Q. Hello all

We are Automobile components, and our supplier performs Acid Zinc plated on Cast iron Surfaces.
We are facing the Issue of Black spots on the plating surface after 2 or 3 days. It is very difficult to judge whether current parts of Stock were OK, or if in the future they will get Black spots at a later time.

Can anyone suggest what are the precautions to be taken to get rid of Black spots and how to ensure that it doesn't occur.

R Baskar
- Gurgaon, Haryana, India
^


February 2018

A. Hello R.,
Please consider retaining a consultant to visit your supplier's shop and resolve this. Alternately demand that the shop solve the problem or you'll take your business elsewhere; or send the parts out for salt spray testing, or require salt spray test results from your supplier. Best of luck! Although Asif offers 11 possible causes, and although we can sometimes help a plating shop solve their problems, it usually takes a lot of back & forth, plus good access to details which your shop probably hasn't shared with you yet; unfortunately, it's hard to realistically suggest from a buyer's description of a problem exactly what the buyer should tell their plating shop to do next.

Regards,

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



May 28, 2018

Q. We have been observing an issue in Acid Zinc Barrel Plated parts with trivalent clear passivation since last 2 years. Plating specification is 5.0 micron thickness minimum and corrosion resistance up to 75 hours.

During the receipt of material from plating, we check the parts for plating thickness, chrome thickness in XRF and check the parts visually by 100% to ensure no black patches/black spots.

We provide 24 hrs minimum for curing of plating before handling the parts for inspection and packing.

During packing of material, we do not find any black spots on the parts. The parts are packed in polythene bag with silica gel and put inside a carton box and sealed.

These parts are transported by Road in a truck for a duration of 5 days. When customer receives the material, we could see more than 50% parts appear with black spots. Requesting your help to come out from this issue.

zinc plating with fretting corrosion a   zinc plating with fretting corrosion b   zinc plating with fretting corrosion c

Thanks & Regards,

Dinesh Babu
Supplier Quality Engineer -- Chennai, Tamilnadu, India
^


May 2018

A. Hi Dinesh. Five days on possibly rough roads sounds like -- and this looks to me like -- what is sometimes called "fretting corrosion".

You say the parts are packed in a polythene bag, but I'm betting it's not one part to a bag, and that five days of jouncing and vibrating against each other is rubbing away the chromate, irritating and hardening the surface of the zinc, and causing it to oxidize. I suggest leaving one of the carton on the dock instead of loading it on the truck, and seeing what the parts look like after that 5-day period.

If the carton left on the loading dock shows no black spots, my bet is that fewer parts per bag, some bubble wrap in the cartons, or some other practical measures that one of your people thinks up will easily solve this problem.

Regards,

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



July 28, 2018

Q. Hi everyone,

We are automobile parts manufacturer,
Our supplier does acid zinc plating, about 13 to 15 microns on tube and brackets (10 mm dia. hollow tube). Recently we are having black spot issue on tube. Our supplier says it's due to tube material problem. After keeping it store for a few days we observed white salty surface which has black spots. Please suggest us the solution.

zinc plating: black spots or missing the plating?

Mahesh addi
- Bangalore. Karnataka. India
^


August 1, 2018

A. Hi Mahesh,

To me, that looks like missing plating, not just black spots. This looks very much like a quality issue with the plating, rather than anything to do with the base material.

Have a look at other parts in the batch, is there any blistering on the surface? This would be an indication of poor preparation prior to plating.

Brian Terry
Aerospace - Yeovil, Somerset, UK
^


August 2018

A. Hi. I agree with Brian that this looks more like missing plating than black spots on the plating. Four quick questions:
1. Are the pieces soldered or welded together before plating or plated separately?
2. The plating on the piece with the screw holes looks better; is it?
3. I assume the parts are steel, not brass or copper?
4. They are bent before plating, not after?

Regards,

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


April 15, 2021

A. Hi Mahesh,

I saw black spot on part -- looks like to me real heavy smut that your cleaning process cannot remove. Then you plate over this parts causing black spots or plating layer may peel.
Try some part sand blasting, then run.

popat patel
Popatbhai B. Patel
electroplating consultant - Roseville, Michigan
^



December 1, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have 400 lit barrel plating, all parameters within control -- even still I am getting black spot on M12 bolt if voltage is kept above 5. If I am keep voltage less than 5, nut thread is not getting proper coating.

Bala subramani
Employer - CHENNAI/Tamilnadu/India
^


December 2018

A. Hi Bala. Unfortunately there are all kinds of 'black spots', often with differing causes. If you can forward pics of the black spots we might have a better idea; also acid, alkaline, or cyanide plating?

But if you can plate a Hull Cell panel and show us or tell us what you get, it should be easier to figure this out. Thanks.

Regards,

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

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