Some problems in alkaline zinc plating

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,

A. Hi Kumar.

1). You have given us no data about what you're 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.

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 good netiquette to use more words in your question than you want people to spend on their response :-)

Regards,

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.

Regards,

A. AFTER ALKALINE ZINC PARTS COMES OUT YELLOW, CHECK THE NITRIC DIP CONCENTRATION OR MAKE UP NEW; AFTER THAT NO MORE YELLOW PARTS COMES OUT.

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.

Regards,

A. 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.

A. When increasing temperature on an Alkaline Non Cyanide Zinc line the formation of carbonates speeds up which are detrimental to the bath once significant build up happens.

Brightener consumption increases.

The above two will increase running and maintenance costs.

I find there is a point in temperature where the bath performs adequate to the point of "diminishing returns", meaning increasing temperature further doesn't give much benefits in terms of the quality of finish.

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.

Regards,

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.

Regards,

"...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.

Thank 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.

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.

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.

Regards,

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.

A. Hi !! Josh,
Add 0.5 gm/ltr potassium permanganate ⇦this on eBay or Amazon [affil links] (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.

A. Hi Saurabh. Unfortunately, I don't think people can even visualize the defect you are experiencing, let alone guess what might cause it. Please send pictures of the defect to mooney@finishing.com for posting here, and tell us what you see in your Hull Cell testing. Thanks!

Regards,

A. Hi Ram. Using wire for racking can be a problem, and is usually okay only for certain types of parts. These pins should probably be either barrel plated or use custom racks which do not leave rack marks in unacceptable areas. Good luck.

Regards,

A. Hello Kartik,
My advice would be to employ any of the following types of agitation in your rinse(s).
1) Work bar agitation in the direction best suited to remove the entrapped solution.
2) Solution agitation
3) Air agitation
4) Work bar vibration.
If you don't use work bars in the rinses and it's manual dipping, you could employ #s 2 & 3. Good Luck!

A. OOZING OUT CAUSTIC.
INCREASE WATER RINSE IF POSSIBLE AIR AGITATION IN WATER RINSE.
AVOID HEXAVALENT PASSIVATION, GO FOR TRIVALENT.
BAKE COMPONENT AFTER PASSIVATION, SO TRAPPED CHEMICAL GETS DRIED.

? Hi Arvind. The picture seems to be of a stained part hanging on a plating rack but, sorry, I do not understand the shinier units I see in back of it, nor do I understand your text. Hopefully another reader familiar with this type of part will catch on better. Apologies :-(

This is a public forum and questions may sometimes remain unanswered for weeks or months; please contact a plating consultant for urgent needs. Good luck.

Regards,

A. Dear Arvind,

Please improve the welding between the two plates.
Avoid the Gap. Your problem is not from plating but from a badly made assembly.

The uneven gap between the two plates entraps the solution and causes bleed out of plating solution later.

A. Hi Bartek. Looking at the graphics it looks like the parts are pitted. Perhaps the replated parts are more prone to white rust because they are pitted from their two trips through the acid pickling? But if you search the site for "11SMnPb30" you will find some suggestions regarding the pickling and plating of this leaded steel. Good luck.

Regards,

A. Extending on Ted. Lead on the surface forms and insoluble salt in most "pickling solutions" like HCl, and sulfuric acid. Make up a bucket of 30%/vol fluoboric acid which does not form an insoluble salt, try it, come back, let us know.

A. Hi Kevin. Can you send a photo of the "melted" rack & parts to mooney@finishing.com for posting here so we can have a better understanding of the situation?

Leon and Robert probably each have more practical experience in this than I, and plating can definitely "burn"; nevertheless, I am reminded of the several 'parlor trick' examples of how difficult, if not impossible it is, to even burn paper or cloth when it's wet. The idea that you could actually melt a steel part or a stainless steel or copper rack through solution-borne plating current while it is immersed in a vat of watery alkaline zinc plating solution just does not compute for me :-)

Underwater welding can be done but it's not trivial, and it's not the watery solution conducting the current through an ionic path, it's metal to metal. I'd suspect the parts actually hitting the anodes, if anything.

Luck & Regards,

Gentlemen,

Thank you for sharing with me your ideas. I will be checking those items you pointed out.

A. Hi S. Logu,
The info you provided is very helpful, but it may not be enough. A photo would help, plus, what does a Hull Cell panel show? Does the same whiteness show up on the LCD or HCD end of the Hull Cell panel? Thanks.

Luck & Regards,