-- The Home Page of the Finishing Industry
HomeFAQsBooksHelpWantedsForum letter 38877
Serious Education ... plus the most fun you can have in metal finishing.

Some problems in alkaline zinc plating

A discussion started in 2006 and continuing through 2017 . . .


Q. I have problems with alkaline zinc plating because the performance of it's product(plated) is not bright, and yellow stains. About the clear coat/top coat's hardness is less, or easy to be broken and the colour is yellowish/yellow stain. Do you have any advise for me about it? Maybe the properly timed oven, or kind of clear coat, and chemicals/treatment used in plating? Thank you very much.


plating - Indonesia


A. Dear Ms. Anggraeni,

Have you checked with "Hull cell" the performance of the solution whether is it okay or not?

Your problem can be the additive system or lack of control of the additive and has other contamination like water hardness, do you use Demineralized water for the plating solution and might be rinsing before entering plating bath?

You did not tell properly what kind of Chromating that you are using now, either Hex-Blue or Trivalent Blue.
In my opinion, Trivalent Blue is more sensitive to be yellowish blue and non uniform for Alkaline Non-cyanide Zinc (I presumed) that has Iron contamination inside the solution.

Try to treat the solution by using Zinc dust treatment and consult with your local supplier.

Good luck.

Best regards,

Tony L. Chandra
- Jakarta, Indonesia


A. Dear Anggraeni,
You have not given any details about your process and other details about what kind of items you plate.
Kindly give details:

Process used: Alkaline Zinc, Cyanide or non cyanide:
Vat or Barrel plating:
Bath capacity:
Work being plated:
Metallurgy of the product plated :
amperage per sq. ft. applied:
Plating time:
Details of addition of all additives:
Details/intervals of regular maintenance carried out:
Details of regular treatment carried out:
Do you analyse your bath:
From your letter I think your bath is not properly treated or maintained. The additives and the constituents are imbalanced. Please give clear details to get clear suggestions.

t k mohan
T.K. Mohan
    plating process supplier 
Mumbai, India

Blue chromates work well, but green chromates turns blackish

March 26, 2012


ELECTROPLATING - Kolkata, West Bengal, INDIA

November 22, 2012

A. Hexavalent olive green passivation is dark green with blackish shade, which is okay. If you still want green, then change pH to higher side this will work.

Ramchandra Talwande
- India

Groove/gouge line caused by plating

July 14, 2013

Q. Deep like marks up to 4 micron observed on the non-cyanide alkaline Zn electroplated steel article. The article is a case hardened and tempered 16MnCr5 steel processed through sheet metal stamping route.
Plating is done by Rack method and there are no such marks observed on the article before plating. Hexavalent Cr passivation is done on the article.
Plating thickness is evidenced over the line mark and there is no discoloration observed.

marks on the non-cyanide alkaline Zn electroplated steel

Request the experts in guide me how this kind of deep mark is possible?

Srinivasan Rajagopalan
- Bangalore

July 17, 2013

A. Hi Srinivasan. I certainly don't claim to have seen everything, and I cheerfully welcome a contrary opinion . . .
But having never seen a problem like that resulting from a plating operation, I tend to disbelieve the evidence that the mark was not there before plating. I suppose it's possible that parts were racked in such a fashion that an interrupted contact could cause a bipolar effect that dissolved the component along this line . . . but again I have to doubt it.
Can you tell us anything about frequency, whether the line runs the same direction each time, etc.?

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

July 23, 2013

Q. Thank you for the feedback. The frequency of defect is 3 to 10 percent in every batch. The direction of loading is always in the same way. Find attached the jigging photograph.


Request your feedback.

Srinivasan Rajagopalan
- Bangalore

Brown spots develop in storage

June 12, 2013

Q. Zinc plating with surface discoloration after storing for a period of time. Hi, I am Peter, a plating chemical supplier. Recently my customer faced a big problem. He plated iron parts with non-cyanide alkaline zinc + iridescent passivation (no sealer). The parts originally show iridescent color after plating but would discolor and show brown spot when stored for a period of time (about 2~4 weeks). Not all of the parts discolor, but only a small portion of the parts. And it would happen randomly and continuously perplex him. He has plated the parts for 4 years and discolored in the last three months. Anyone have same experience and know what happened? Thank you!

surface discoloration on zinc plating

Peter Liao
Chemical supplier - Hsinchu, Taiwan

July 12, 2013

Q. Hi,
My site also facing the same problem too.

Jeremy Tan
- Klang, Malaysia

July 15, 2013

A. Hello Peter and Jeremy,

Please give some information:

Passivation (Chromate conversion coating): Hex chrome or trivalent chrome?

Bath: Jeremy, you have the same problem with an alkaline or acid zinc bath?

Thickness: Is it uniform? Is it above 5 microns? Are there scratches or chipping of the surface?

Storage conditions: Is it humid? The parts could be exposed to fogs or sprays?

Well, this is all I recall as important now. If you can give some more information, we may be able to help you both! Regards and good luck!

Daniel Montanes
- Canuelas, Buenos Aires, Argentina

July 17, 2013

Q. Hey Daniel,
Mine is alkaline zinc bath, trivalent yellow, thickness is 8-15 micron. Discoloration happens after 2-3 weeks later. After analysis chromiting bath: Fe 27 ppm, Zn 12-15 g/l and pH 1.6 -1.8! (new chromiting bath 3 months). Thanks.

Jeremy Tan
- Klang, Malaysia

July 24, 2013

A. Hi Jeremy,

Jeremy, the trivalent chromating bath generally works from 0 to 8-10 g/l zinc concentration. 15 g/l is very high, and the chromate must be very thin in this condition. I would suggest you to change, at least partially, the chromating bath.

Then, if this effect (discoloration) has always been there, I would see if the storage is done in humid conditions, if there could be some spray or fogs in the storage room or near the packaging, if the parts are exposed to light...

So, I hope we can help you too! Regards,

Daniel Montanes
- Canuelas, Buenos Aires, Argentina

August 5, 2013

Q. Trivalent iridescent chromate, zinc<9g/L
thickness : 5-15µm
storage condition: 25-33 degree, sometimes raining, but it has stored in same condition for 4 years.
I analyzed the part with EDS and found higher carbon and oxygen in the surface compared to normal part.
I also found they used water with unstable pH and flow out resin fragment randomly. This water is also used to make up passivation solution.
I have asked them to stop using the water recently.
Do you have any suggestions for this situation?
Thank you!

Peter liao
Chemical supplier - Hsinchu,Taiwan

August 8, 2013

A. Hello Peter,

First, if the storage hasn't changed, it isn't the cause... But you should keep the parts away from the rain!

On the EDS result, if has some more oxygen and carbon I cannot say that it is different, they are elements present in dust and in sample preparation could be some variations that could increase the carbon and oxygen proportion. If you have iron content, then you should worry!

I didn't understand the water situation, with the flow and resin. Could you explain what you mean? pH in the chromate process is very important, you should maintain the pH value in +/- 0.5 points from optimum pH value for a good corrosion resistance, and trivalent chromates need a good quality water for preparation. If you define your process with more detail we could help you more!

Good luck!

Daniel Montanes
- Cauelas, Buenos Aires, Argentina

August 19, 2013

Q. Hi Daniel

Thank you for your concern.I have checked the EDS of the discolored parts again and found no difference compared to normal parts. I use tap water now and still found discoloration happened randomly. The tap water pH around 6.5-7.5, current conductivity<10us/cm, TOC<2 59205ppm iC

Peter Liao
Chemical supplier - Hsinchu, Taiwan

August 21, 2013

A. Hi Peter,

Do you control the pH in your chromate conversion bath? It is very important to maintain this value stabilized. The nominal value (in standard chromates, you should ask your vendor in this case) is 2,0. Please check this up! If the chromate has 2 steps, check both steps have controlled pH.

I haven't understood the resin situation yet, what did you mean with "flow out resin fragment randomly"? The chromate has a resin part?

Good luck!

Daniel Montanes
- Canuelas, Buenos Aires, Argentina

August 28, 2013

Q. Hi,Daniel

Thank you! I have checked the pH and this was fine. My customer's pure water system contains resin and follow out. But now they have fixed it. Will discoloration come from zinc solution? I have tried every possible parameter after zinc solution tank and now I focus on zinc solution. My customer has tried other supplier's water purifier (to reduce Ca,Mg) and said it seems it can reduce discoloration. But the plating line has regularly added the conditioner (reduce Ca,Mg) already. I think this should not be the course, What do you think? Thank you!

Peter Liao
Chemical supplier - Hsinchu, Taiwan

August 29, 2013

A. Hello Peter!

Ca and Mg are a problem in some chromate formulations and are a problem in zinc electroplating bath always. For alkaline baths you have some conditioners that improve bath performance with the presence of Ca and Mg by getting them out the way, but you should try to avoid to replenish your bath volume with this "hard" water.

You should try preparing your chromate with another water (not hard, maybe demineralised) and see if you have some change in your process. If this improves your result of "discolored" parts, you have some problem there. If there is no change, then you have some problem elsewhere...

Hope you can solve this issue! Keep us informed and maybe we together can sort it out :)


Daniel Montanes
- Canuelas, Buenos Aires, Argentina

September 6, 2013

Q. Hi Daniel:

Recently,my customer find a way to detect the discoloration in two days and no more have to wait for two weeks.They put plated parts into high temp/moisture oven and enhance the aging. So, we found the discoloration always happens on Monday. They will maintain the plating line on sunday. The maintenance include change rinse water and wash the tank. Beside,the zinc solution also be pending during maintenance. Will the pending cause any organic crack or metal be absorbed on anode? I think it would be something wrong during the maintenance or pending zinc solution. Please give me some suggestions.Thank you!

Peter Liao
Chemical supplier - Hsinchu, Taiwan

September 10, 2013

A. Hi Peter,

That the zinc plating solution is still and not working during maintenance is not a problem. Is this what you refer as "pending"?

If they always do maintenance on Sunday and you have problems with the parts plated on Monday, there must be a connection between the maintenance routine or the Monday startup and this. I can't tell what could be wrong in this, but if you know what is the routine for maintenance, startup and normal work, the differences between them are possible causes...

Hope you can solve this! If you do, please share what you found with the community :)


Daniel Montanes
- Canuelas, Buenos Aires, Argentina

September 18, 2013

Q. Dear Sir,

We have alkaline zinc plating tank, and we use the HNO3 for yellow dip approx 2-3 ml/ liter , and also we use the H2SO4 for color maintaining. Now it's improved.

We are facing a problem in cleaning now; we are doing pretreatment (cleaning) process in HCl , but component shine is not coming, so please give us the cheap and good suggestion so that we can get shining on component.


Sanjeev Yadav
- Delhi INDIA

September 27, 2013

A. Hello Sanjeev,

That your parts shine, or your parts brightness, could depend on how you clean your parts or how you plate your parts... If you have a cleaning problem, you should see if you are loading fully degreased parts, that your HCl is in correct concentration, if you use or not a wetting agent (if you don't, you should, I think it is very helpful, and cheap), if you use or not an electrolytic degreaser...

If you answer most of these questions with "yes", then you must tell us more about your process in order to help you. If not, you should see if you can make that happen.

We (at least I) can't say anything else to help you if you don't specify more what is the defect, what are you doing, and in which conditions.

Hope this helps! Best regards,

Daniel Montanes
- Canuelas, Buenos Aires, Argentina

December 29, 2013

Q. Dear sir,

1. Why the discolouration comes at the olive green zinc plating?

2. Why the white passivation comes at the metal bonded area at green zinc plating?

3. Salt spray requirement for green plating?

4. What is difference between alkaline plating and zinc plating?

rubber to metal bonded - Tamil nadu, Madurai

December 2013

A. Hi Kumar. Please ask as many questions as you like, but please provide details on each of them rather than presenting the readers with a flash-card quiz; it's considered best netiquette to use more words in your question than you want people to spend on their response :-)

1). You have given us no data about what you are doing, from which people might be able to tell you what you are doing wrong (or what you mean by discoloration).
2). I don't think you mean "white passivation" ... maybe you mean "white rust"? White rust is corrosion of the zinc plating.
3). The customer often specifies the required salt spray hours, but 96 to 192 hours to white rust could be pretty customary.
4). Zinc plating can be done from cyanide-based zinc baths, alkaline non-cyanide zinc baths, and acid zinc plating baths. Cyanide has the disadvantage of being highly toxic, and is rapidly falling out of favor around the world. Today's alkaline non-cyanide baths can replace the cyanide processes. Acid zinc baths give a brighter plate, but usually have poorer throwing power so are less capable of achieving relatively even plating thickness.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Iridescent passivation peels off of alkaline zinc plating

February 23, 2014

Q. I am facing problem of passivation at customer's end.
The process is as below:
Base : Fe
Plating : Non-Cyanide alkaline Zinc
Passivation : Iridescent hex-chrome.

Problem : Passivation Adhesion problem. The plated parts after passivation shows slip out of passivation meaning improper adhesion of passivation.
Could anybody explain the reason for this and remedy for it ?

Yogesh Kulkarni
- PUNE,Maharashtra,India

September 2, 2015

I have a component in diameter 8.
Material: Mild Steel
Total length of component is around 800 mm

My customer is asking plating thickness more than 15 Micron

My question is
1. Whether it is possible to give coating thickness 18 micron?
2. What will be effect of cost?

Pramod shah
United Pressings - Pune, Maharashtra, India

September 2015

A. Hi Pramod. Yes, even 25 microns is still a "standards-based" thickness for "very severe" exposure.

Nobody can quote you the cost of someone else's services, of course. But you should have little difficulty finding a plating shop willing to do 15-18 micron plating for twice the cost of 8 micron plating, It will take twice as long, and will half their production rate, but they will see materials savings in the other processes, and probably labor savings, if they half their production rate. Good luck.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Yellowish zinc deposit after alkaline zinc plating

February 10, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We use alkaline non-cyanide zinc plating with 800 liters. The bath is working good but lately the plated barrel shows yellowish or golden deposit after plating instead of bright silver color. Please tell me the problem.

Eskedar bekele
plating bath supervisor - Addis Ababa Ethiopia

February 23, 2016


popat patel
Popat Patel
    plating shop
Roseville, Michigan

December 31, 2016

Q. If the temperature of the alkaline Zinc bath is increased up to 35 °C then what will be the effect on the plating and the reactions going on in the plating tank??

Veerendra kumar
- delhi, India

January 2017

A. Hi Veerendra. Alkaline zinc plating temperatures can range from room temperature up to about 48 °C. As a very general rule, plating speed increases at higher temperatures, but so does brightener consumption. What temperature does the process supplier suggest? Why do you want to exceed it? Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

January 9, 2017

Hi Veerendra,

Alkaline zinc temperature range 25 °C to 35 °C , Optimum temp 26 °C. You can run with higher temperature but it burns brightener and other components and increases plating speed.

Howard Finishing LLC - Roseville, Michigan

January 18, 2017

Q. Hello sir, I face a problem that diameter of component becomes less after plating. What is the reason? Please help. Material is 16mncr5

Sidharth kamboj
- Karnal,Haryana,India

January 2017

A. Hi Sidharth. That strikes me as exceptionally unlikely. To begin solving the problem you would need to measure the diameter after each step in the plating process and determine in which tank it happens. If you discover a reduction in diameter after rechecking, please tell us the starting diameter, ending diameter, and which tank it happened in (soak cleaner, electrocleaner, acid activator, alkaline zinc plating tank, chromate conversion coating tank, etc.) Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Burning in Alkaline Non Cyanide Zinc plating

March 22, 2017

Q. We have been seeing problems where the edges of the parts end up with a serious amount of high current density burning. We have added low current formula and brightener to keep these concentrations up. I've sent samples of the plating bath to our chemical manufacturer and they come back in balance. We still see this issue and it's causing issues with powder coat paint applications after the plating process.

Paul King
QC for Manufacturing - Lynchburg, Virginia, USA

March 2017

A. Hi Paul. Obviously there is a current density beyond which burning in the high current density edges is unavoidable, and the only resolution is to plate at lower current density. I'm not sure how to say exactly where that point is except from vendor suggestions, reports of successful and unsuccessful current density, and previous experience.

It might vaguely be possible to increase the current density by shielding those edges, but only if it's a long run of identical parts.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

March 24, 2017

"...sent samples of our plating bath to our chemical manufacturer and they come back in balance" ?!
Did he run a Hull Cell Panel? The solution might titrate "in balance" and be loaded with dragged in HCl which has converted to NaCl and will surely chew up the HCD edges.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como

March 24, 2017

thumbs up signThank you for the responses. Yes, we did a hull cell. We did that first and it came up with the parts looking good. Of course the hull cell was a flat panel and didn't have any variance in the plating because of this. Thanks for the thought there though.

What we did and will continue to do is add a small daily quantity of a LCD booster. And change the concentration of our brightener feeder pump. Both of these additions have resulted in more uniform finish across the surface of all the parts and specifically in the HCD areas on the bigger parts. When the brightness gets too high, I'll cut both back and see how long it takes to plate out.

Thank you both for you contribution.

Paul King
plating - Lynchburg, Virginia, USA

March 25, 2017

A. Hi Paul, I would suggest a brightener feeder system based on your rectifier amp hours, especially on tanks larger than 500 gal. Adding a small amount, say every 500 to 1,000 amp hours, based on the Tech Data sheet would maintain an even level. Takes the human error out of it ... well, not completely.

jim conner
Jim Conner
Anoplex - Dallas, Texas USA

Shiny edges and dull centers on zinc plated parts

April 19, 2017

Q. Hello

I work in a metal finishing shop and we have been having issues with our zinc plating on some parts. The biggest issue is the finish is not uniform and we are getting shiny edges and dull centers, especially on larger parts. I have heard this referred to as "TV screen effect" but hours of googling have not given me any results.

38877-7a  38877-7b

Our technicians are reasonably certain that the problem lies in our zinc solution and not anywhere else. Our zinc phosphate and even our yellow zinc do not have these issues. We have been in business for over 20 years and never had an issue like this.

I believe our problem started because we have recently moved our shop across town. This meant pumping out our zinc solution into totes and and then back into our zinc tank. (3000 gallon tank). This is what leads me to believe there is some contaminant in our solution. (I assume organic contaminants, possibly oil). We never had issues like this at the old shop and the only change we have made is our location. This is the first year that we have not froze out the carbonates but I don't think that is our issue.

As per our chemical supplier, we have added 5 gallons of 7.4% sodium hypochlorite. Plating seems to be improving but we are still seeing the same issues.

One of our technicians thinks it could be a bad batch of carrier or brightener but the boss has ruled that out.

I imagine adding more sodium hypochlorite would help? 5 gallons into a 3000 gallon tank doesn't seem like enough to me. I've also heard EDTA or activated carbon or Rochelle salts but I have no experience with those.

Any ideas or suggestions would be very much appreciated! Thanks! I will try to attach some pictures of our problem.

Josh Ziemba
Plating shop employee - Ottawa, Ontario, Canada

April 2017

A. Hi Josh. I am not a zinc plater, so we will will have to wait for someone who is to offer their best guess at the problem. But what I do know is that you don't start by adding sodium hypochlorite or EDTA or Rochelle salts to a 3000 gallon tank ... you add stuff to your 267 ml Hull Cell and see what helps and hurts. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

April 20, 2017

A. What are your numbers? (Zn, NaOH) And, what's your Hull panel look like? Might be additives.

I don't think bleach will help, either.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

April 28, 2017

A. Hi !! Josh,
Add 0.5gm/ltr Potassium Permanganate (KMnO4). Mix well. Settle the solution for overnight. Impurities will settle at the bottom of tank. Next morning separate clear solution from sludge. Discard the sludge and use clear solution for plating. I think your problem will get solved with this process.

Amol Chitte
- Pune, India

ADD a Q or A to THIS thread START a NEW THREADView This Week's HOT TOPICS

Disclaimer: It is not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2017     -    Privacy Policy
How Google uses data when you visit this site.