Cadmium Plating and Screw-Locking Helicoils

[JCorsico]

Hi guys.

We have an aluminum part (7050) with several 10-32 tapped Helicoil holes. The Helicoils are the "screw locking" variety.

When we install the fastener, the fit is tight, as you would expect. But the Helicoil seems to strip the cadmium plating off the screw. Is this normal? See attached photo, where we ran a test screw into the part.

We removed the Helicoil and ran a tap into the hole, and the hole is the correct dimension (the tap didn't remove a meaningful amount of metal). We then reinstalled a new Helicoil, and tried a new screw, and still got the same result where the plating appears to be stripped...

Thank you!
Jon

 

[3DDave]

The interference fit on a Helicoil will definitely strip the rather soft cadmium plating. Most distorted thread lockers will. Nyloc is about the only thing I can think of that won't strip cadmium. Perhaps a thread locking compound would be useful?

 

[dik]

I specifically do not allow cadmuim plating on my projects. It is a dangerous material:

https://www.osha.gov/cadmium/health-effects

from another source:
"Cadmium is a highly toxic cumulative poison and an IARC Group 2A material (probable human carcinogen). Because it leaches easily, it is a frequent environmental contaminant from aircraft and engine washdowns. It is heavily regulated throughout the world, with a PEL of 0.1 mg/m3 in the US. Under the European rules ELV (End-of-life Vehicles), WEEE (Waste Electrical and Electronic Equipment) and RoHS (Restriction of Hazardous Substances), cadmium is restricted to no more than 0.01wt% of any vehicle and electronic material or coating, with exemptions for aircraft and military use. These exemptions are only temporary and are intended to be removed once alternatives become available. Under the new European REACH statute (Registration, Evaluation, Authorization and Restriction of Chemicals), cadmium plating is forbidden except for aircraft and some safety and electrical equipment. REACH has no exemption for military use, posing a problem for sustainment of military vehicles in Europe."


You should be investigating alternative coating systems. I think in Europe it is being phased out for everything except a few avionic parts.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ng2020]

Dik, Cd plate still used extensively in aerospace standard hardware (ms, Nas, an). Direct substitution for alternative platings is therefore difficult.

Similar problems with hexavalent chromium in finishes. Though there are more options there. Going off on a tangent now.

 

[JCorsico]

3DDave - thank you. We won't consider this a defect. But it does raise some corrosion questions... Hmmm. Need to think about those.

Dik - thank you, but not helpful! Cadmium is a standard finish in aerospace. Millions and millions and millions of fasteners are cad plated. We can't just switch to something else.

Happy Thanksgiving to my fellow Americans!

Jon

 

[dik]

Thanks, well aware of that... also for the military. It's on borrowed time and alternative coating systems have to be developed. Even in Europe the permitted use is conditional and limited and is being more restrictive.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ng2020]

Has anyone out there had success implementing a substitution for cd plated an/Nas/ms hardware?
Zinc-Nickel plating rings a bell, and I believe it's available for some standard hardware, bur I'm not sure of its suitability off the top of my head.

 

[Sparweb]

If you have a problem with cadmium plating, your best alternative it so specify stainless steel (A-286) or Titanium fasteners. First, make sure all of your company executives are happy to take a 10% pay cut to pay for the expensive hardware.

JCorsico,
I'm not concerned by what I see in your photo, though that's a bit more Cd removal than I usually see. If I was going to use a tap to test the condition, I'd be testing the screw with a female die. Compare the running torque on the Cd stripped end with the running torque on the fresh upper end. If they're mostly the same, then this is only cosmetic and continue as you were. If it goes from free running to hand-tight then I'd be careful never to re-use a screw after it's been removed from the helicoil (maybe your company policy already does that). If you need a screwdriver and a wrench to turn the screw through the die, then the screw you are using to test may be out of spec. It's happened, you never know.

Note that this idea ignores the cut thread / rolled thread difference in the screw you may actually be using so (a) expect some cut chips, and (b) discard the screw afterward!!

 

[JCorsico]

Got it. Thank you all. Very helpful!

 

[WKTaylor]

JC...

Color photos can be deceptive. What You may be seeing is 'burnishing' [rubbing-smooth] of the Cd plating [silvery] with post plating chromate treatment [golden-yellow] and a cetyl alcohol [soft thread lubricant, can make the threads appear to have a 'slick/white smudge']. On-the-other-hand, male-female thread matching, or miss-matching, [UNF VS UNJF, etc] is possible.

Please advise...

WHAT is the protruding head screw part number [exactly].

WHAT is the helical coil part number [exactly].

PER WHAT installation process [spec, handbook, manual, etc] are you installing the Helical coil [exactly]?

NOTE1. 'Helicoil' is a proprietary name for one manufacturer of helical coils [the one that got it's name associated with the family of coiled-thread inserts decades ago]. Helical coils have many/diverse specifications... and so do screws.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[JCorsico]

WHAT is the protruding head screw part number [exactly].

NAS603-18P

WHAT is the helical coil part number [exactly].

2TLF-3C-0380

PER WHAT installation process [spec, handbook, manual, etc] are you installing the Helical coil [exactly]?

we use a screwdriver...

 

[WKTaylor]

CS...

The NAS603 has a high enough tensile strength to minimize thread damage in a helical coil. The Cd plating thickness was poorly defined in the spec and base spec NASM7839... so it could have been on the 'high side', simply to ensure 100% thread root and head-shank fillet coverage.

JUST to be certain, RE Helical Coil...

KATO fastening systems 2TLF-3C-0380SF = Reference # : 1552TLF-3C-0380SF... Meets NAS1130-3L20 and MS21209F1-20 ... 10-32 Unified Fine [UNF] Locking Tangless Wire Thread Insert, 0.38in Length, 304 Stainless Steel, Plain Finish, Strip Feed Packaging???

Installation typical per NASM33537.

NOTE1. Use of a tangless helical coil [wire thread] insert 'pre-winder installation tool' is considered mandatory. It slightly tightens the coil [OD] and allows it to smoothly 'run' into the hole... then release-it as the tool is backed-out. The 'screwdriver blade' method is backyard mechanics... never for precision production. The screwdriver-blade edges can cause small nicks/score marks on the UNF thread side as the blade turns-in the untightened coil.

https://www.biscoind.com/manual-tools/kato?initialMap=b&initialQuery=kato&map=category-2,brand

NOTE2. Replacing Cd + CrO4 coatings on alloy steel fasteners is vwery twicky bwuisness... Cd + CrO4 coatings have many advantages for use on alloy steel fasteners... too many to discuss here. Replacement of Cd + CrO4 coatings with other coating-materials is a steep challenge that may never be overcome... except by opting for CRES, Titanium, Inconel, etc... where various coating systems simply have-to isolate the fastener from dissimilar structural alloys is necessary. Obviously Cd + CrO4 have high toxicity and 'enduring environmental persistence' that make them terrible for living creatures 'forever'. However for each of the alloys cited as alternates to alloy steel, other issue arise... sigh.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[JCorsico]

Thanks Wil. Very informative, as always.

And I answered your question wrong on Helicoil installation. We are indeed using a pre-winder tool to install the Helicoil. We are using a screw diver to install the screw. I don't think you could get the Helicoil into the hole without the pre-winder. We have both pneumatic and hand operated pre-winders.

The coils are not on a strip feed. They are loose. And they have a dry film lube coating.