Black spots on zinc plated parts with trivalent chromate

A. Hi Raghava. This thread is just one of more than half a dozen , each with dozens of entries focused on the subject of black spots on zinc plating. You're welcome to search the site and study them, but if you would like your specific problem addressed please tell us whether you're doing acid, cyanide, or alkaline non-cyanide plating; rack or barrel plating; and what material the parts are made of so we can get beyond the cliches that the plating either is not thick enough/good enough or the parts were packed before they were truly dry :-)

Photos e-mailed to mooney@finishing.com for posting here will probably help.

Luck & Regards,

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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!

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 ⇦liquid caustic soda in bulk on Amazon [affil link] 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?

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

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,

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.

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,

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

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,

Wonderful response, KV, and the levity is appreciated. Thanks!

Regards,

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

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,

Thanks for sharing!

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.

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.

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.

Regards,

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.

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A. 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!

Regards,

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.

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

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.

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 ⇦in bulk on eBay or Amazon [affil links] (5% HC2H3O2) daily.
Hope this was helpful.

Regards,

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.

Regards,

A. BLACK SPOTS (BARREL MARKS) ARE FROM IRON BUILD UP IN ZINC BATH. CHECK FOR PARTS INSIDE (AT BOTTOM )OF ZINC TANK.