Trivalent Chrome Plating Q&A's, Problems & Solutions

A. SANTOSH,

DO YOU HAVE WHITE PATCHES AFTER TRIVALENT CHROME AND WIPE OFF, NO PLATING PROBLEM YOU HAVE PROBLEM IN YOUR RINSE TANK.
CLEAN & WIPE WALL OF RINSE TANK.

? Myself, and Popat and the other readers are happy to hear that your problem has been resolved! However, this is a Mutual help forum, not a help Santosh forum :-)

If "some other factors" were involved, we readers would all appreciate if you would take the time to tell us what those other factors were. Thanks!

Luck & Regards,

A. Hi Juan.

All of the major suppliers offer trivalent chromium plating including Atotech and Macdermid and we can't publicly debate which proprietary process is best (huh? why?). But Chevy Truck vs. Ford Truck, Mac vs. PC, iPhone vs. Android style debates usually get silly quickly.

Many plating shop managers believe that the supplier's chemistry is not nearly as important when making a choice between proprietaries as the expertise & dedication of the supplier's local tech service person, and my observation has been that they often switch suppliers when that serviceperson switches employers :-)

Luck & Regards,

A. Trivalent chrome is susceptible to salt attack.

We use trivalent for our decorative chrome and it bears up well with plenty if nickel underneath it. But we won't do marine pieces and have seen many failures where trivalent has been exposed to north eastern road salts.

Trivalent will not tolerate the wear of hex chrome either.

So it's horses for courses.

If your work pieces are decorative, trivalent is fine. If it is subject to salts or wear, use hex.

A. Trivalent chromium is good as a decorative finish but not very good as an engineering coating. It is slightly darker and grayer than hexavalent chromium and its colour is more varied. This can present problems when trying to assemble multiple parts into one assembly. The technology for trivalent plating is continuously advancing, so it would be best to talk to reputable suppliers about what you need and what they can supply. When it first became available it was quite difficult to use and at least one process required a divided cell with a membrane separating the anode and cathode to prevent the anode oxidising the Cr(III) to Cr(VI). However, things have improved since then! As far as I am aware, there is no barrel plating trivalent system commercially available, but if a supplier has now got a reliable one, no doubt I will be corrected. At the end of the day, you have to decide what you what and whether a supplier can meet your needs - its all "horses for courses". Trevor Crichton R&D practical scientist Chesham, Bucks, UK A. We have been using trivalent chrome to plate Class 8 truck bumpers and grille surrounds for approximately 10 years. We have never had any problems with chipping or any particular abrasion problems. The classic problems you might experience with trivalent chrome as opposed to hex is that trivalent typically has a different color compared to hex and it is more susceptible to metallic contamination. Depending on your rinsing, metallic contamination can be considerable. Speaking from personal experience, nickel and iron contamination can be very detrimental and cause big problems with color. Most suppliers recommend a small ion exchange system to remove metallics. If you have superior rinsing, it may not be required. Trivalent tends to scratch and abrade more easily than hex so care must be taken in not being too aggressive when cleaning the finished parts. Some people will use abrasive waxes on hex chrome or even chrome buff. You should avoid the use of them when plating with Tri. Conversely, trivalent doesn't have the problems of burning, whitewash and toxicity problems that hex has. You don't have to worry about a Chromic Acid/Sulfuric Acid ratio. The chemical constituents are much more expensive with tri than hex. But, the overall reduction in wastewater costs, regulatory compliance and health and safety cost avoidance may help make up the difference. Hope this helps! Daryl Spindler, CEF decorative nickel-chrome plating - Greenbrier, Tennessee

? Just to clarify before we get lost in the woods: You are talking about black chromium plating, not zinc plated parts with a chromate conversion coating, Joel?

A. To my knowledge there are no generic trivalent chromium plating processes, only proprietary ones. My understanding is that one type uses semi-permeable membranes around the anode compartment that do not let the hexavalent ions reach the work. Another type uses anodes of a special material that supposedly prevents the formation of hexavalent ions.

I can't be of much help except to say that I think you need to learn from the supplier, depending on which type of system you are using, what protocol to use to deal with formation of hexavalent chromium.

A. The simplest way of stopping Cr(III) being oxidised to Cr(VI) is to use a divided cell, so the chromium ions don't get the chance to see the anode.

A. A divided cell is one where two electrolytes are kept apart by a membrane or sintered barrier. The electric current can pass across the barrier, but large metal ions cannot. I have successfully used a porous clay flowerpot as a cell divide; one electrode (anode) was inside the pot with an electrolyte (10% sulfuric caid) and the other electrode (cathode) was outside the pot in another container. The workpiece was in the outer container and an electric current was passed between the two electrodes. The voltage drop was higher than one would normally expect when plating, but the process worked and I didn't get Cr(III) oxidising to Cr(VI).
As far as the other question is concerned, trivalent chromium is very susceptible to contamination of any type, so Cr (VI) could well cause these problems. However, faults with trivalent chromium are also dependent on the bath makeup, so I would check with your supply house when trying to trace faults.

A. I can only answer on one of the commercially available TriChrome processes. Hexavalent chromium is a contaminate in trivalent chromium plating and will cause significant problems. Unless your solution is out of balance you should not be forming hexavalent chromium at the anodes.

A. Hi, Deepak. There are no U.S. federal laws governing this, although there might be local restrictions in areas like San Francisco Bay.

European RoHS, WEE and other standards drove a transition from hexavalent to trivalent chromate conversion coatings on parts plated with zinc and zinc alloy. This has to do with the unacceptability of leaving hexavalent chromate conversion coatings on the surface of parts. But it does not dictate whether hexavalent or trivalent processes be used for the process as long as there is no hexavalent chromate on the parts.

Chrome plating baths are different than chromate conversion coating baths; they deposit metallic chrome onto articles and leave neither trivalent nor hexavalent chrome on the parts. So the question of hex vs. tri has little to do with chrome plated parts, only with chromate conversion coatings.

To my knowledge, most automotive chrome plating specs still require hexavalent chrome plating. Good luck!

Regards,

A. Hi, YF.

Asif Nurie makes the point in thread 34810 that we really shouldn't judge today's best trivalent chrome plating processes by the weaknesses of the processes of 25 years ago. Perhaps you should contact a few suppliers for their take on whether whiter trivalent chromium plating is available today. Good luck.

Regards,

A. It is always advisable to use passivate after Trichrome process. They help both in discoloration (aging as per what you said), as well as CASS performance is increased substantially.

A. It is an electro-chromate with a small amount of hexavalent chromium which accelerates the aging of the chromium deposit by increasing the amount of chromium oxide on the surface.
This is an electrodeposition in which Tin/lead (4-7%) anodes are used with 0.5 A/dm2 current for typically a minute.

A. Hi Shivakumar. We'll try to advise Mayank of your question, although his posting is from almost 2 years ago, so the contact info we have may not work.
But I think you need to be very explicit re. exactly what do you mean by "Is the concern of replacing Hex Cr to Tri Cr achieved?". Are you asking whether the process generates RoHS compliant parts, or whether the process is safer for workers than hex chrome plating, or whether it complies with some local ordinance in Mumbai, etc.?
The general question of whether it produces RoHS acceptable parts greatly interests me! I know chrome plated parts are RoHS-compatible whether plated in hex or tri baths because they both produce metallic chrome, not hex or tri chrome; whereas hex-based conversion coating chemistry generates non-RoHS parts. Indeed it's not clear to me either whether what Mayank described leaves hex chrome on the parts :-)

Regards,

A. Hi Ted long time. I received your email. It's good using forum as Q&A. Yes it defies the purpose of being 100 % hex chrome free, and yes hex chrome is electro deposited on top of trichrome deposition. But with hex chrome nature as excellent rudimentary passivate (can be used as passivate over copper, giving gloss to brass and silver). We don't have any useful chemistry yet to take over amazing properties of hex chrome completely. But it's only 2-3% w/v of hex chrome used in this bath, which is 100X drop to our conventional system. On top of it if we use hex chrome and trichrome regularly for next 20 years even the so called high cost (bath maintenance and additions) of trichrome is at par with hex chrome bath. So it does solve the purpose to very great extent.

Hope I was not off point.