What's So Special About Cadmium Plating?

A. Leaving out radiation control, the features that make cadmium attractive are: good corrosion resistance, solderability, cathodic protection of steel, galvanic compatibility with aluminum, excellent lubricity, freedom from stick-slip when torquing, malleability, economy, and no gummy and voluminous corrosion products.

However, it is unusual to plate nickel and then cadmium on a part. When it is done it is usually for the purpose of making a diffused nickel-cadmium coating which is a rather different thing than cadmium plating. Nickel plating straight to gold plating is a more common finish, but I wonder if the cadmium layer is intended to sacrificially protect the nickel or aluminum, or maybe discourage mold. It's pretty important to try to find out why it's there before deciding it isn't needed :-)

A. Cadmium plating is wonderful. I want all landing gear on every plane I ride to NOT have a substitute for cadmium, NiCd, chromate conversion coating, etc.

I still don't get why we have to eliminate cad and chrome, and I am a card carrying tree hugger. (Keep hard chrome on those chain saw blades too!) All we have to do to waste treat the effluent from the plating shops, and we are done.

A. Here's why NASA doesn't like Cadmium plating:

1. Cadmium is known to sublimate in a hard vacuum environment (especially at temperatures above 75° C). The sublimation products, which are conductive, can redeposit resulting in short circuits. The sublimation products may also interfere with sensitive optics.
2. Cadmium is a toxic material that should not be used in manned space flight applications
3. Cadmium is subject to the spontaneous growth of Cadmium whiskers. The propensity of Cadmium to grow whiskers appears to be lower than that of zinc and especially tin. Cadmium whiskers (like tin whiskers) grow spontaneously and are capable of causing electrical failures ranging from parametric deviations to sustained plasma arcing that can result in catastrophic short circuits. See prohibition against pure tin plating for additional insight regarding the risks of metal whiskers.

I also think that when it's exposed to certain types of radiation it becomes extremely toxic, emitting a deadly gas. I heard long ago that a nuclear devices' potential for rendering an area uninhabitable can be enhanced by covering the device with cadmium. Items sold to the government are prohibited from having cadmium plated parts for this reason.

---

2003

Thanks, Shawn. Good points, all . . . except your last sentence, which isn't true. There are countless military and aerospace parts that must be cadmium plated per military spec. In fact, if you research the scandal which resulted in passage of the Fastener Quality Act, I think you'll find that the heart of it was that the military felt lives were endangered because some plating shops substituted zinc and zinc alloy plated fasteners where cadmium was specified.

Cadmium is certainly unsuitable for many applications, and it is a cumulative poison we are all working to get out of the environment -- but we can't substitute anything for cadmium without a design review of each individual component. Rather, we need to re-examine, for that particular component, how important good corrosion resistance, solderability, cathodic protection of steel, galvanic compatibility with aluminum, lubricity, malleability, freedom from stick-slip when torquing, and freedom from gummy and voluminous corrosion products is. Nothing has all of the properties of cadmium (if it did, it would be cadmium of course), so we need to figure out which are vital and which are less consequential or of no importance for the particular part.

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

A. Hi, Jim. The majority of cadmium plated work receives a yellow chromate, which wouldn't be mistaken for tin. Letters 31938 and 38853 suggest spot tests for cadmium. Good luck.

Regards,

A. Hi, Jim. Although there may be a few such suppliers, your local metal finishing chemical distributor will be familiar with all of them, and is always your best best for small quantities because they stock locally and are often able to split orders. If your company buys no plating chemicals yet, so you are unfamiliar with this, look in the yellow pages of your town or the nearest city for a local plating supplies distributor. Good luck.

Regards,

A. Hi, Palavesam. Please see my response to Susan (the 2nd entry on this thread). Your customer may decide to change the spec for the coating after considering the toxicity of cadmium, and you are encouraged to ask him to review the situation; but zinc is not a proper substitute for cadmium in some cases because of lower malleability, incompatibility with aluminum, stick-slip problems on fasteners, and gummy corrosion products. Good luck.

Regards,

A. Hi, Jeff. Auto manufacturers do not use cadmium plating anymore and you probably should not either. A zinc alloy like tin-zinc or zinc-nickel may be appropriate but, as mentioned, you need to look at which properties of cadmium you need and to select a replacement coating that offers those particular properties. You can't find one single plating that will replace all cadmium applications. Nothing has all the properties of cadmium. If something had all the properties of something else, it would be that other thing :-)

At one time cadmium was very cost competitive with zinc. The only thing to say about the cost of cadmium now is that it is on an upward trajectory as it gets harder to find. Good luck.

Regards,

A. Hi, John. I don't think that's exactly what I said, but cadmium offers sacrificial protection to nickel just as it does to steel, so I don't see a galvanic issue. I am not familiar with the connectors in question, but sometimes the cadmium and olive drab is required for camouflage color (and perhaps for fungus resistance? -- I'm not too familiar with that aspect).

Electroless nickel is an extremely corrosion resistant, highly conductive, barrier layer finish. To plate onto aluminum requires either cyanide copper or alkaline electroless nickel after the zincate, so it's a question of choosing between the two than choosing to do electroless nickel or not do it.

Regards,

A. Hi, Sam. I would expect the cadmium plating to vaporize from the heat of the laser, if you are actually digging into the metal rather than simply blackening it with Cermark or something like that. So I would not suggest this operation unless there is fume removal capacity in the system.

If good provision is made for fume removal, though, I wouldn't think it would be a big deal though. Cadmium is a powerful biocumulative poison, but it's not plutonium, and in the old days muffler repair shops would burn off cadmium plated bolts and brackets, probably releasing several times as much fumes, without formal fume extraction systems.

Regards,

A. Cadmium is a ductile, very malleable metal. The metal is soft enough to be cut with a knife, and it tarnishes in air. It melts at 321 °C and soluble in acids but not in alkalis. It is similar in many respects to zinc including Galvanic Corrosion protection capability but it forms more complex compounds.
High exposures Cadmium occurs with people who are exposed to fumes and when people breathe in cadmium fumes it can severely damage the lungs. Physical contact to cadmium is harmless. Still cadmium is one of the metals on the ''black-list" which should theoretically mean "zero-option". However, attempts in various countries - notably Sweden - to ban its use have been unsuccessful since it has been recognized that for certain applications there is no immediate substitute. Products and components of products used in aerospace, mining, "offshore" and nuclear fields where lubricity, ductility and resistance to hydrogen embrittlement is sought.

Whilst allowing the use of cadmium to continue for applications where there is until now no proven substitute available, still regulatory authorities are vigorously pursuing stringent targets on discharge limits.

In many cases this is surely causing platers to think carefully about the financial consequences of continuing to offer cadmium since new investment in effluent plant will be inevitable. Cadmium would be a good choice of coating material for parts in contact with aluminum. choice of cadmium plated fasteners for assembly of aluminum airframes in the aerospace industry.
Cadmium does have some specific application and capabilities but where alternate material can be used it is better to avoid its use.

Vijay

A. Hi, Pete.

There was a time when lead was used in paint & gasoline, and Mercurochrome & mercury thermometers & mercury treated hats were a commonplace, and cadmium was plated onto miscellaneous auto items, and hexavalent chromate conversion coatings of gold/yellow/green were fairly widely used. But those uses are all history because of toxicity.

Cadmium is plated only by a few plating shops today, and basically only on some exotic legacy aerospace applications; and even this will be short lived. Regular blood tests and other medical monitoring are required. Similarly, hexavalent chromate conversion coatings are a rarity (especially after Erin Brockovich [affil link to Amazon]) and European RoHS requirements, with almost all shops having converted to proprietary trivalent chromium conversion coatings.

The cathode is the component that you plate onto, the anode is the chunk or slab of metal you are dissolving in order to plate out. Electroplating is a reduction process, and you will find it troublesome to electroplate from an oxidizing acid like nitric acid. Although it is easy to do electroplating as a simple science lesson (see our FAQ: "How Electroplating Works"), robust electroplating on rusted components is not a simple process. However, Eastwood offers a tin-zinc plating kit.

They also offer a paint designed to imitate the color of cadmium, Eastwood "golden cad" [affil links]. This may give you an acceptable iridescent yellow color. Good luck.

Regards,

Q. Hi Ted,
Thanks for the quick response, but I was hoping for it to be a bit more positive.... I'm and old school guy, but as an EE, I had a staff of ME's and Chem E's to work out the plating issues (we never did Cd or Chromate Conversion coatings). Back in the days, we used dilution to prepare the laboratory's effluent in preparation for discharge from the facility. I'm aware of industrial risks, as I've worked with 50kV, megawatts of RF energy and even machining/sanding Beryllium metal in the past. I survived all that, with (and without) the help of OSHA so I figure I can handle the Cad -- yes, I did read the MSDS!
I'm aware of the Eastwood offerings, but the guys that buy and trade my kits know me to be a real nit-picker and I'd like to say that the bits were prepared to the original factory conditions, and not just painted to look like it. Cad plating plus the Eastwood overspray would be OK.
I was hoping that the Cd in the bottle of Grainger solution could be used to plate-out onto my parts, which have been sandblasted to bare metal (steel). Also have "stuff" to clean the parts with prior to plating.
Bottom line is the material I have just will not do the job--correct?
Again, thank you for the info and recommendations,
Pete
BTW, you have a GREAT website!

A. Hi, again.

The test solution which you mentioned, besides being based on nitric acid (which oughtn't be electrolized because that generates NOx gasses), is too weak for utility by a factor of a hundred. Commercial plating solutions based on cadmium cyanide are available from any plating supplies distributor (as are some cyanide-free processes), but even with your wall full of experience, they may be unwilling to ship to you for fear of liability--you can ask. Established plating shops are subject to so many EPA and OSHA regulations that a vendor can be confident that the buyer has had yearly Hazmat training, wears proper PPE, has an emergency preparedness plan, etc. I know that you only want to buy a very small quantity; I'm just explaining why it may be hard to find a vendor, despite them being in every large city.

In truth, if I were a chemical salesperson, I would not sell cadmium to a hobbyist, not only because of potential accident liability but because cadmium is a bioaccumulative poison that the Air Force is spending millions of dollars annually trying to remove from the remaining life-critical applications, and authenticity of restoration just isn't a life-critical application. But not everyone is as concerned; I believe you can still find this poison on some brake linings :-)

A large library will have electroplating texts with a chapter on cadmium plating, explaining the interrelationships of the variables; access to Mil specs for cadmium plating is free (https://assist.daps.dla.mil/quicksearch/). If you are far more patient than me, you can probably find operational parameters on the net on this site or others, a nickel and dime at a time. Thanks for the kind assessment, and good luck.

Regards,

----
Ed. note 5/23/13: Make that quicksearch.dla.mil/ now

A. Hi, Edward.

Just about anything is possible, but it's probably a bit difficult to remove the chromate and activate the cadmium plating without destroying it. If you don't need the cadmium, and if you have a willing shop, you can just pass the hardware through a standard pretreatment sequence and it will remove the chromate and cadmium. Good luck.

Regards,

A. Hi David.

I can't imagine any realistic basis for this worry.

Regards,

A. You mean : "if cadmium plating BATH is exposed to acid, it will release cyanide gas" I guess.

Best Regards ;)

Thanks Ted and Francois. You confirmed what I thought, that acid exposure to plated parts is not an issue. Francois, thanks for your comments on acid exposure to the bath. Since this is finished part that is plated, does not look like any issue.

A. Hi,

It's the protection from corrosion. If you heat treat cadmium and nickel together you have the the best corrosion protection.

Regards

A. Hi Bob. I don't think a whole book was ever written about cadmium plating and, since its usage has been steadily declining for 50 years, I don't see any prospect that one will ever be written. One suggestion I can make is Metal Finishing Information Service in England (http://finpubs.com). They have abstracted all of the plating journals for several decades, and offer the abstracts as a CD-ROM, or online, or they will do the search for you -- whatever is your preference. I've used them many times over the years, and find it easy and inexpensive.

Regards,

A. Hi Tim. This thread lists the many reasons that things may be cadmium plated, and the special properties that cadmium offers. It also explains that only very rarely are ALL of those properties required. If all are required, there obviously is no substitute.

If only some of the properties of cadmium are required, there is a substitute -- but which substitute that will be will depend on which of the properties are important. The most common substitute is a zinc alloy plating like zinc-nickel, zinc-cobalt, or tin-zinc. But again, these do not have all of the properties of cadmium plating and it is dangerous to guess which properties were important to the designer. Only the design authority can answer your question about a substitute. Sorry.

Regards,

A. Cad is very soft compared to steel and is hard compared to lead or solder.
Yes, it can be damaged when the threads are damaged. If it chipped off, then the adhesion was not excellent. It could have passed QC testing and still chipped.

A. Hi Bill,

Cadmium is very soft, that is what makes it good on threads!

You have two choices for repair, either strip and re-plate or, if the damage isn't too bad touch up cadmium (MIL-STD-865 is a good place to start for that process)

A. Hi Akshay. I think you should investigate whether tin-zinc alloy plating will meet your needs. But if good compatibilitiy is actually required you are limited to cadmium plating or plating/coating with aluminum.

Regards,

A. Hi Neeta. I think your syllogism is valid :-)

Regards,

A. Hi Dave. No question that it can be done. But few plating shops have the chemistry and capability to do plating on aluminum, and few shops do cadmium plating anymore. Finding a shop that offers cadmium plating on aluminum might be a chore, and Google ain't what it used to be (it used to display only pages which contained every word and phrases you entered, so 'cadmium plating on aluminum' would do the trick, it's harder these days).

Regards,

A. Hi Todd. The plating shop does all necessary cleaning and acid activation (rust removal) before the cadmium plating step, and chromate conversion coating afterwards. You probably need to specify the thickness of plating and the color of the chromate. Good luck

Regards,