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Cadmium Plating Copper

Cadmium Plating Copper

Cadmium Plating Copper

(OP)
I'm looking at a drawing that calls out Cadmium plating per QQ-P-416 on some copper parts and wondering if it is really necessary.
The parts are electrical busbars. They will be installed in an enclosed space, not directly exposed to the elements, but moisture could ingress into the area.
Yes, this is being installed on an aircraft. It will connect the HF radio antenna to its transmitter. All components are mounted internally, not externally.
Fasteners used to secure terminal lugs onto the busbars are steel with cad plating themselves, including the washers and the nuts.
I have reviewed the current AMS standard and I do not find any prohibition against using it on copper, but I am wondering... why bother?
I can wave my hand and state that I am ensuring protection against corrosion, but is it really likely?
I will be digging deeper into the standard practices for these materials based on the aircraft OEM specifications - but I think I am unlikely to find what I need from that source.

Please remember: we're not all rednecks!
www.sparweb.ca

RE: Cadmium Plating Copper

You need to take this up with your customer.

RE: Cadmium Plating Copper

(OP)
In this case, I am the customer and the specifier.
This is a legacy drawing (almost 20 years old) on a product of which we are the owner, but we haven't supported in a long time, but is still on the books.
Our customers operate aircraft that are >40 years old. When they need replacements, we fabricate replacement parts.
All grey-beard knowledge on the matter has long since vanished.

Please remember: we're not all rednecks!
www.sparweb.ca

RE: Cadmium Plating Copper

Cadmium is metallica non gratia these days.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Cadmium Plating Copper

(OP)
I've had second thoughts about it too. Nobody has answered "what the heck is wrong with tin-plated copper" or even bare copper for that matter.
I'm directing my group to figure out how much protection is really needed, and if so, plan a re-design that allows for plating or some other less toxic coating. Also interested in finding out if common epoxy primer will stick to it well enough to substitute. Then at we wouldn't even have to send it away to a special process, and just finish it in-house.

RE: Cadmium Plating Copper

FYI random thoughts ONLY...

Busbar copper alloy ASTM B152 or ASTM B187?

NAS44 copper busbars [copper alloy per ASTM B152] may be 'bare' or may be cadmium plated [bare-silvery Cd, no post-plating treatment].

Electrical grade [high purity] copper alloys tend to designed for very low resistance across bonds/grounds. Cadmium plating helps maintain a constant surface condition on the copper over-very-long-times.

Tin is a common protective coating for copper connectors and woven-wire jumpers... but not busbars [tin whiskers?].

Be wary of bare copper [even high purity] in a seacoast environment. Copper eventually tarnishes in moist air [brown appearance]... which can eventually develop the green corrosion patina.

Alternates to plating can be 'blackening' or epoxy primer over bare/etched copper... except at connector/contact points. Use electrical grade silicone RTV over the contact areas [washers-screws-nuts-connector-tabs, etc] help to minimize corrosion potential, long-term [DO NOT use RTV with acetic acid 'smell' on etch-cleaned copper].

ARP6903 GUIDE FOR ACHIEVING PLATING/FINISH COMPATIBILITY WITH CONNECTORS AND ACCESSORIES USED IN ELECTRICAL WIRING INTERCONNECT SYSTEMS (EWIS)

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]

RE: Cadmium Plating Copper

(OP)
Thanks Wil,
My own research has led me toward tin plating instead of Cadmium.
Not as far across the galvanic compatibility chart as Cad, but honestly not much of anything really is.
There will also be a re-selection of the fasteners that connect them to the aluminum structure because cad-plated steel washers are complimentary to both the steel-Al contact and the steel-copper contact.
Lastly, sealant in a "blob" over the fastener stack (on both sides) is standard practice for the aircraft type, which gives me a set procedure that defines the sealant and its application.
None of this stuff was given due consideration on the old drawing, sadly. No "grey beard" involvement as I implied in my first posting.

It did not occur to me that the SAE would weigh in on the topic, but, gee, look at that, an ARP for exactly this situation. If I hadn't found everything I need already published in AC 43.13-1B, then I'd consider shelling out for the SAE doc.

My manufacturing people are keen to see this changed, too. They're not only displeased with the Cadmium spec, but it's also a rare process to find a vendor in Canada that can do it. The parts would be out for processing for a long lead-time, too. Other directions of my investigation have led me to a guess as to how the OEM came up with this Cad plating practice, but that was the 1980's. Times have changed.

RE: Cadmium Plating Copper

Why not solder dip (lead/tin) vs. pure tin plating. The lead content should suppress tin whisker formation, and the resulting coating would be about as corrosion resistant as a tin plating, or silver plate.

RE: Cadmium Plating Copper

SW...

IF tin were suitable for a busbar conductor I wonder why it wasn't in NAS44...?? On the other-hand copper alloy terminals are often tin-plated. SOMETIMES there is a need to avoid similar metallic finishes on mating parts to prevent a 'fusion-between' or other adverse reaction of the surface-finishes [whiskers?]. This is known to happen between high-temp silver finishes... only one of the [2] thread surface may have silver plating... typically nut-threads. Unsure...

NOTE1.
In-lieu of cadmium plating on structural parts... 'we' are using zinc-nickel alloy plating [AMS2417] with CrO3 post-plating treatments [more-benign than CrO4] for parts MF C/LA-steel, CRES, HRA's, copper alloys, etc [NOT titanium] where practical. For high strength steel, Low HE zinc-nickel alloy plating [MIL-DTL-32648] is essential. For parts that can stand a high-temp bake... the aluminized ceramic finish [NAS4006 typical] is highly suitable/benign for many fastener alloys [NOT steel] in parts best adapted for exterior 'paint finishes' such as bolts, screws, washers, bushing OD's [not ID's], etc... but NAS4006 coating has high electrical resistance and takes a specially trained technician to apply.

Copper is mostly a no-brainer for plating. Hmmmmm...
I wonder if ONLY a flash-nickel would also work.
Perhaps IVD Aluminum + MIL-DTL-5541 Class 1A or 3 conversion coating would work fairly well.

NOTE2.
Make sure Your busbar(s) is(are) carefully smoothed/deburred. Rough surfaces and sharp edges tend to prevent maximum/tight surface contact and are harder for any-type protective finish to cover smoothly/evenly.

Also... these might provide useful references for electrical/electronic materials, thus...
MIL-STD-1547 ELECTRONIC PARTS, MATERIALS, AND PROCESSES FOR SPACE AND LAUNCH VEHICLES
MIL-STD-1631 PROCEDURE FOR SELECTION OF ELECTRONIC AND ELECTRICAL PARTS AND MATERIALS DURING DESIGN OF MILITARY ITEMS
MIL-STD-11991 GENERAL STANDARD FOR PARTS, MATERIALS, AND PROCESSES [EE/EL]
ARINC 670 GUIDANCE FOR MATERIALS, PROCESSES, AND PARTS EQUIVALENCIES
AIR4129 METALLIC WHISKERS
AIR5444 INVESTIGATION OF WHISKER FORMATION ON TIN PLATED CONDUCTORS

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]

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