WE HAVE RECENTLY CONVERTED OUR ZINC BATHS TO ACID CHLORIDE AND WERE HAVING A PROBLEM WITH CHROMATE ADHESION. WE NOTICED THIS ON PARTS THAT GO TO SILKSCREEN. WE USE A CHROMATE AT A pH OF 1.6-1.8 AND CONCENTRATION OF .75 TO 1.0 %. WE HAVE TRIED PREDIPS PROPRIETARY AND GENERIC TO NO REAL SUCCESS. WOULD APPRECIATE ANY ADVICE.KEN CLAY
My experience with the same problem was traced to excessive brightener and the breakdown products of the brighteners. as the bath ages, carbon treatment is required. Brightener is corrected. After this is done a few times, really bright work takes excessive brightener and then it will not take chromate. The vendor, a major existing company never provided a suitable answer. we eventually had to dump and make up new. Bug hell out of the vendor for a cure would be my suggestion.James Watts
- Navarre, Florida
The process of acidic zinc is not new so there are solutions to this problem. Usually suppliers have special chromates to acidic zinc and even special ones to chromates that come prior to painting. Ask your supplier.
Make sure that prior to chromate you have a dip of 0.5% nitric acid.
Besides, overdosage of brighteners can cause adhesion problems in the chromate.
Do you pay attention to the iron concentration in your zinc solution? If it is too high, it may also affect adhesion. You have to get rid of your divalent iron in your solution every now and then.
chemical process supplier
Sara's comment on iron reminded me to ask how do you add peroxide. I know that there are some not so new acid zinc platers who add entirely too much, too often, and not at the correct time. Double check your technical bulletin. Since the peroxide reacts with the organic addition agents as well as any iron, it is easy to add too much, causing early contamination by breakdown products.
Falls Township, Pennsylvania
Try adding 0.25 to 0.5 oz/gal boric acid to the chromate. This gets the chromate to bite rather than polish. Chloride zincs are notorious for this.
Some chemical suppliers have chloride based yellow chromates. The Kenlevel systems by MacDermid Inc., is one such line. I have been involved in two situations where both silk screening and painting were involved. First, if there is no specification involved, look for low temperature cure inks and paints.
Secondly, although chloride zincs are really not designed to run at low brightener levels, it helps. The less amount of organics that are included in the deposit from the brightener system, the better. Organics like to expand and vaporize when heat is applied.
Finally, there is an old Northern Telecom plating spec, dating back to the 60's, that quiet clearly spells out the use of a cyanide based zinc system.
Believe me, it still works today.Ray Delorey
- Industrial Processing
I recommend to you, add to your chromate sodium acetate (acetato de sodio en español) and to dry with hot air your pieces, also, don't make acceptance probes until pass 24 hours.ING. E. RENATO VILLASE--OR MEDOZA
maquilas "save" - Mexico
You have received a lot of good advice. One thing I'd like to add is that an 1% add of carrier/starter (the grain refiner portion) can do wonders to improve chromate adhesion. Feel free to call to discuss the situation further.Richard Painter
chemical supplier - Cleveland, Ohio
Before you spend a lot of time searching out pre-dips and changing chromate, carbon treating, etc... Carefully check your post chromate procedure. Any hot water dips should be no more than 110 degrees F. Dryers should not exceed 140 Deg F. Your problem could be that simple. Don't trust the gages...check temperatures manually with thermometers.Mike McDonald
mack products - Jefferson, Wisconsin
I had the same experience. This phenomenon is generally caused by:
- metallic (Cu, Ni, and Fe) or organic contamination of plating bath
- low pH value of chromate solution
- high concentration of chromate
- long chromating time
- insufficient surface cleaning prior to chromating
- build-up of trivalent chromium ion and zinc ion in chromating solution
- slow drying rate and insufficient dry, or inadequate chromating solution
Find the corresponding causes, then solve the problem easily.Ling Hao
- Grand Rapids, Michigan
But there are some chromating problems specific to acid zinc solutions, which may require chromate formulations specifically for these baths.
Falls Township, Pennsylvania
I "second source" my plating needs to an outside party and we have had an increasingly high rate of yellow chromate (applied after clear zinc) that either rubs off or comes off when applying adhesive tape and abruptly "ripping" the tape off. I have asked my plating supplier to provide me with a root-cause corrective action response, however, I quite frankly am not "up to speed" enough on this subject to intelligently ask the "right" questions of him. It would be appreciated if any one who has experienced this problem or is educated on this subject could guide me in the right direction. Randy FlowersRandy Flowers
Advanced Metal Works, Inc.
You are up to speed if you know that the chromate should not rub or peel off. Try rejecting the lot and see if your vendor doesn't return good parts. The right question is specifying the peel test in your purchase order as a requirement, and let your vendor decide if they can supply the chromate as you need it (along with any other requirements, such as minimum thickness, adhesion of deposit etc.).
In most cases you will never find the root cause, as the problem will disappear when the vendor goes out on the floor to find out what happened. This happens when you reject the work and it ends up back on the loading dock of the vendor.
If pressed for a root cause, you will get some answer back for your paperwork, but I don't believe that this kind of activity improves the breed. Specifying what you need is what is good for your parts and for the finishing industry.
Falls Township, Pennsylvania
November 5, 2012
Please guide us for the standard method of checking hexavalent yellow chrome adhesion over zinc plating.
- Pune, Maharashtra, India
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