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Why use cadmium plated fasteners in the aeronautical field

 


Q. As the father of a final year mechanical engineering student, who has a thesis to write on Cadmium plated fasteners, can you please explain why Cadmium plated fasteners are used on aluminium alloy body panels in the aeronautical field? What are the advantages and disadvantages of Cadmium plating in such applications.? .

Harold Michael George
- Selby, North Yorkshire, England


 

A. The most significant disadvantage is environmental, Harold: cadmium is a toxic, cumulative poison like mercury and lead. There have been intensive efforts to reduce cadmium plating for a very long time now; the easiest switch-overs came decades ago, and the toughest ones aren't completed yet, and may not be for some years to come.

Asking for the advantage of cadmium is much like asking why wood is better than brick, or steel is better than glass, or rice is better than wheat. It's no one single thing, it's a combination of properties that tend to make it more fit for a particular application than other materials. And just like brick is not wood, no other material is cadmium, so no other material has ALL of the properties of cadmium.

The useful properties of cadmium as a fastener in aircraft manufacture include: corrosion resistance, sacrificial protection, freedom from gummy or bulky corrosion products, galvanic compatibility with aluminum, excellent lubricity and freedom from stick-slip for consistent torquing, softness and malleability, non-galling, the ability to accept chromate conversion post-treatments, and the ability to be deposited in forms that will not trap hydrogen so baking can relieve the potential of hydrogen embrittlement. There are dozens of substitutes that offer some but not all of these properties. A coating that comes close for aircraft fasteners is Ivadizing (vacuum deposition of very pure aluminum); but depending on what's considered most important, I'm sure some people would challenge the claim that Ivadizing comes "close", while others would say it not only comes close, it's better :-)

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

A. Cadmium plating is an excellent corrosion protector and has been in military and aviation specs for many years. These are controlled by Government bodies and consequently they do not like change! No-one is prepared to take the plunge and come up with a good alternative, just in case it goes wrong and lots or people get killed. In real terms, it is debatable if there is anything better. You may well hear more about this one!

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


 

A. Hello Harold!

Cadmium is used to coat fasteners for the simple reason that it works, and quite well. Cadmium is relatively easy to electroplate onto high strength steel, and for those steels whose strength is too high to tolerate the hydrogen embrittlement from electroplating, it can be vapor deposited ("vac cad" is the jargon). The cadmium does two things promoting corrosion resistance: first, it is a sacrificial coating for the steel, and readily takes a chromate conversion coating which improves the corrosion resistance many fold. Second, it's galvanic characteristics are compatible with aluminum. That means it won't form a battery enhancing the corrosion, a common problem when dissimilar metals are in intimate contact in the presence of an electrolyte (water).

Hope this helps!

Lee Gearhart
metallurgist
East Aurora, New York



A. Cadmium has a few of very nice engineering characteristics. As a coating it has excellent corrosion resistant properties. Because of its nobility, it is sacrificial to most metals (ferrous). When it does start to corrode, the byproduct is a white corrosion which is relatively lubricious. It's corrosion actually makes assembly, or more importantly, disassembly, easier.

Tim Martin
plating shop - Springfield, Massachusetts



June 13, 2010

Q. I am glad I found these discussions on cadmium plating! After the recent scares from China I found I was not as well educated as I should have been. As I began researching building my own airplane I was at first horrified that all of the fasteners were plated with this toxic substance, so I started searching and found this forum. The airplane I'm going to build uses aluminum skin and formers but steel blind rivets to hold it all together. So now I know the cadmium plating is there to keep the two different metals from corroding and weakening which would result in me being dead pretty darn quick! I'll take my chances with the cadmium plating! OK, so my questions; 1.) When handling plated materials, I should wash my hands? 2.) What should I wash them in and with? 3.) When blind rivets pop they release a good deal of energy, should I wear a mask to keep from breathing any cadmium dust released? 4.) Am I taking this too seriously? I am looking at potentially 3 years of working with this stuff before I can paint it and reduce exposure. Thank you in advance for your insight.

John Bryant
- Marietta, Georgia, USA


June 14, 2010

A. Hi, John. Indeed it is very difficult for us in the finishing industry to not be cynical after our decades of hard work on cadmium reduction, and so many small businesses closed down from environmental pressures -- only to see Wal Mart selling jewelry to kids with up to 70 percent cadmium, followed by MacDonalds giving away children's drinking glasses with cadmium paint. And all this jewelry and these glasses, with hundreds of times higher concentrations of cadmium than plating shops struggle with, is just dumped in the general trash rather than requiring a secure land fill, as is required for plater's much more dilute cadmium waste. And it seems EPA could hardly be bothered with the Walmart and MacDonalds fiascos.

Still, cadmium is a bio-accumulative toxin. So, yes, I'd wash my hands frequently if I were you, with regular hand soap. And I'd probably wear a painter's mask when pop riveting -- although these measures would be more as protection against the hexavalent chromate conversion coating on the fasteners than the cadmium plating. Good luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


March 21, 2012

! I'm puzzled about a couple of things in the last response above, hope this topic is still of interest. The question was about steel pop rivet mandrels being discarded while pulling them in an aluminum aircraft structure. There is no cadmium anywhere except a very few plated nuts and bolts which are elsewhere. How can this result in any cadmium exposure? Is there in fact any significant human exposure resulting from handling cadmium plated nuts and bolts that have been barrel plated in a shop following good procedures?

Likewise, where does the hexavalent chromium come into the picture? I have not seen any kit aircraft parts using Alodine type conversion coating for a long time, it's mostly alclad aluminum or corrosion resistant alloys like 6061.

David Josephson
- Santa Cruz, California USA


April 1, 2012

A. Hi, David.

John implies that the rivets are cadmium plated. He also says they are "steel", and I can't see how you would put an unplated steel fastener (whether a rivet or a screw) in an aluminum aircraft skin -- so I am assuming that the rivets are cadmium plated, as he claims.

Cadmium plated hardware almost always receives a chromate conversion coating very similar to the "Alodine" coating for aluminum that you are familiar with.

I personally don't worry much about casual and occasional handling of cadmium plated hardware, but I don't think you will find authoritative studies on the subject, only educated guesses. But if California demands warning labels on nearly everything, I can't claim they're wrong -- I'm no epidemiologist. If you do find such studies, please let us know.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


April 2, 2012

A. This could easily drift away from finishing issues to aircraft structural issues, so I will try to focus my answer only on the materials/finishing ones.

I am not aware of any aircraft rivets for light aircraft that are cadmium plated. Solid aircraft rivets are almost always 1100 or 2117 aluminum in light aircraft construction, and may have a chromate conversion coating or be anodized. The coating is often damaged in the process of bucking the rivet, but some protection is provided. Common tubular pulled rivets ("pop" or more advanced kinds) are mostly aluminum, sometimes monel, stainless, copper, or (usually zinc plated) steel. The mandrel that's pulled through the rivet to form the "shop head" breaks off inside. Sometimes the retained mandrel forms part of the shear strength of the installed rivet while more conservative designers assume that the mandrel will fall out and do their testing accordingly. The finishing concern here is about corrosion over time of the (usually steel) mandrel, so some builders fill the open hole with something to be sure it's sealed by layers of primer and paint. When a steel (even monel or stainless) rivet is needed for strength in an aluminum part, sometimes the rivet is installed "wet" with a coating of zinc chromate paste to reduce the galvanic corrosion that can happen.

Common aircraft fasteners that *are* cadmium plated are all the screws and bolts made of steel, which may be bright cad plate (Cad I) or might have a chromate conversion coating (Cad II) on top of that. There is a page at OSHA www.osha.gov/SLTC/cadmium/ that refers mostly to hazards associated with cadmium plating chemicals and dust of the metal. The only aircraft-related link I found was www.osha.gov/dts/hib/hib_data/hib19890221.html which refers to an airline shop where workers were grinding cadmium-plated landing gear parts without ventilation. The OSHA limit is 0.5 mg per cubic meter, and if my arithmetic is right based on 0.0003" plating thickness and 8.65 g/cc for Cd, you would need to take all the plating off a typical large bolt head and make it into dust, for every cubic meter of air in your shop to reach that level.

After some years alleged Cad I plating sometimes exhibits dusty white corrosion. It's the wrong color to be cadmium oxide, my guess was always that the parts were actually zinc plated. What else could it be? Could this be toxic?

David Josephson
- Santa Cruz, California USA



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