Black spots in zinc plating

A. Hi Pravin. I don't know the phrase "pill up issue", but I think you are saying that the plating peels? It could be that the parts were simply not clean enough, or it could be that the plating is so highly stressed that it peels off over time.

I would suggest that you observe the plating line in operation and see if you can detect any sign of water-break before plating, and that you also scrub a few parts with a scrub brush ⇦ this on eBay or Amazon [affil links] and a water solution of powdered pumice ⇦ this on eBay or Amazon [affil links] before putting them into the cleaning & plating sequence to see if these things indicate that cleaning may be the problem.

And I would suggest that you plate some Hull Cell panels or other test parts, and bend them to look for excessive stress in the plating. There is also a test device called "stress-tabs" which look like hull cell panels coated with maskant on one side and serrated like a comb where the serrated areas curl up to indicate tensile or compressive stress at different current densities.

Unfortunately you haven't yet told us much about your process and there are almost countless possible causes for black spots.

Luck & Regards,

A. Hi Ansan
Giving "some reasons for black spots" in acid zinc barrel plating is what the 50+ postings on this thread have been about for 20+ years. Readers would like to help you with your specific problem, but we would need data that distinguishes your problem from those other problems in order to move forward.

Yet we don't even have photos of those "like black spots" and "like stained in parts"; we don't know if this is a new startup which has never plated successfully, or a place where things went south when they switched to your chemistry, or a shop which has been using your chemistry successfully for years and suddenly encountered this problem; we don't know if it's a few dozen parts per month or every part in every barrel.

Probably the only thing we can offer without data is that if the black spots are in a pattern which looks like the pattern of the barrel holes, then it is 'barrel printout', probably caused by excessive iron in the bath.

But if you can give us some photos & data we'd be willing to try. Thanks.
Luck & Regards,


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

A. Hi Ansan,
The first picture and the detailed view of it may well be a "black spot" ... but the 3rd and 4th pictures don't show "spots" they show very severe rusting with no plating at all in those rusted areas.

Unfortunately, figuring out whether the plating peeled off before the rusting occurred or whether the plating was there but got consumed on the way to rusting may not be easy to determine.

Still the answer is that this plating was highly unsatisfactory and it must be determined why. Some guesses to be further investigated include poor rinsing after plating or chromating, being packaged damp rather than dry, corrosive packaging materials, or inadequate cleaning & pickling.

Luck & Regards,


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

A. Paul,

Problems such as you describe can come from within or from the outside. From within, is usually coming from porosity, and shows itself through the plated deposit. You did not say if parts had a chromate passivate or not. "Spotting In," results from storing the finished plated parts in an area where the air is laden with corrosive particles. This may happen right in the plating shop, in transit, or while in your storage facility. A good test would be to take one or more parts and segregate them. Seal them off and put them into another location. If they develop the same spots, look into a "Spotting Out," condition. Meaning: Reactive trace chemicals may be seeping through the pores to the surface and showing themselves as black spots.

Lastly, check out the plating bath. Examine bath to see if there is any kind of contamination present, either organic or inorganic. Under the correct condition, iron may show itself as black spots on a Zinc deposit from certain types of plating baths. Try to duplicate your results in a laboratory setting in a plating cell or Hull Cell.

A. Your black spots tell me that perhaps the monsoon has set in Karachi already. The increased Humidity in the Air aggravates this problem which is not noticeable when humidity is less than 50 %.

I can bet that your parts are blue passivated and the best thing would be to switch to yellow chromate which protects the parts from humidity better . You will also notice that the CR sheet parts will have a better fate than the HR sheet parts which are more porous and if this is correct then try only to use CR grade material. Also run a plating thickness check on the spotted out parts as well as make your plater change his post plating rinses and post passivation rinses more frequently. Regards,

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

Regards,

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.

If 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!

A. If you're running a Chloride Type Zn bath the black spots may be coming from a hi concentration of copper in your bath, compounded with hi organics in either Zn or Chromate rinses and as the chromate hardens or cures chromate is not adhering

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".

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.

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 .

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.

A. Hi, Raja

We need to do a hundred things right to get satisfactory plating, and just one thing wrong to get unsatisfactory plating, as Praveen and Asif and others have noted :-(

So it is often not possible to discern which of those hundred 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,

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.

Thanks for getting right to the heart of the matter, Mr. Vahaanwala, which I completely missed: you can't really do proper barrel zinc plating in 15 minutes, it takes somewhat longer :-)

Luck & Regards,

? What are you plating and what are you plating with? More detail of what you are doing will certainly help get a response

A. Hi. It looks to me like zinc plating with clear chromate.

The discoloration may be lack of plating (and thus lack of chromating), and it is at an inside corner where we would expect it. But also possible is water damage (lack of drying), which again would be most likely at an inside corner. This is not acceptable aesthetically and will probably rust quickly. Either way it is unsatisfactory, although different steps are needed to correct it.

The smaller dots could be improper cleaning or acid dipping, which will be a big problem, or they could be fretting, which would be a smaller problem. Are the parts rack plated or barrel plated, and (as Trevor asks) what kind of plating is it? Thanks.

Regards,

A. Hi, Mr. Yuu.

The black marks could be scuffing/fretting if the parts are barrel plated, and this might not be a serious problem. But if the parts are rack plated, it is hard to envision what could cause the black dots except for serious problems.

Regards,

A. Check plating thickness near the spots. If the thickness is poor you either have a thew problem or cleaning problem. If your have this problem with both barrel and rack I'd lean toward cleaning. Try hand cleaning some parts wipe them really good. Then rack them up and see if you still get the spots. If this fixes the spot you then know it's a cleaning problem.

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,

A. Hi, Vikram. Thread 30399 is about such problems in alkaline zinc plating, and Asif Nurie offers some good possibilities. 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, such as hull cell panels showing burning in HCD area?

Regards,

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,

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.

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,

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,

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,

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.

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,

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.

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,

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 DE-PLATE AND FOLLOW YOUR REGULAR PROCESS.