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topic 10821

Cadmium free fasteners


 

Hi,

Eliminating Cadmium: We are going to stop using Cadmium plated alloy-steel fasteners due to environmental reason. At the moment we consider one of the options below:

a. Replacing the Cadmium plated alloy steel with A286 screws We would use those screws mainly in combination with passivated 303 CRES helical coil insert to fasten Aluminum parts. Do you have any idea what are the chances for galling phenomenon for this combination? Would Black-Oxidation treatment helps to prevent galling? What other finishing options are relevant? Other possible problem is galling to nuts made from A286, is there proven solution to this problem?

b. Coating alloy steel screws Do you have any idea whether the aircraft industry uses Zinc coated fastener? Would IVD Al. be a reasonable solution for this case? I read in several sources that this coating lacks the required lubricity and has a high coefficient of friction needed for screws. Can you enlarge on that?

I have recently heard of a several Zinc alloy (Zinc-Nickel, Zinc-Iron Ö) coating that might fit. Do you have any information on that?

I heard that a ìCathophoraseî (I am not sure about the spelling) coating is being used in fasteners for the car industry. Do you have any information on that?

Sincerely,

Ami Givol
- Neher, Israel


 

If I am reading this right you are thinking of stainless steel to stainless steel contact with the fastner and the heli coil.

There are problems with the stainless to stainless conatct.

The normal way around this would be to plate 5 microns of silver in the thread of the screw.

It would be easier to plate the screw all over but the you might find problems from silver aluminium contact.

The problem with cadmium replacement for fasteners is that cadmium is about the best material due to its low volume of corrosion product and dry film lube properties.

Dry film lube? - a thin coat may help.

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England


 

In critical applications, where safety and reliability concerns are paramount, changing engineering specifications requires a thorough search of past problems which caused the original callout of cadmium. It is a complicated system, with an added requirement for the ability to remove and replace the fastener more than once, as in repairs or rework.

Zinc is an obvious choice for noncritical applications, and I listened to a talk at an AESF meeting in New York City, where, years ago, IBM Corporation, for environmental reasons, switched all cadmium plated fasteners to zinc plating in one fell swoop, and handled problems as they came up. It turned out that only a few parts out of thousands of assemblies needed an adjustment from zinc plating to solve various problems.

Your case does not sound like it is amenable to just switching over for environmental reasons.

The zinc alloys do offer promise, what have your vendors said about their alloy for this substitution of cadmium?

tom pullizzi portrait
Tom Pullizzi
Falls Township, Pennsylvania



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