Electrical contacts

[Reggy]

I bought some “silver” electrical contacts. These are fairly large rectangular and square pieces.

I weighed them out and calculated the amount of dilute nitric needed to reduce the lot. I placed some (3 or 4 pieces) in 50/50 nitric/distilled my water solution (580ccs each) and heated the bath. I only add a few at a time to prevent an over run reaction. I got an almost instantaneous reaction, as expected, however it lasted a few minutes. Then the reaction completely stopped! Leaving large intact pieces.

So to me it sounds like there was a silver coating on some other metal. Any clues?

Most contacts are made of:
Gold
Silver
Nickel
Palladium
Platinum
Copper
Graphite
Copper/chromium
Silver tungsten

Out of that list, The only things I can think of are chromium or platinum. Gold would be observable, nickel palladium and copper would have dissolved, tungsten would have continued to react but slowly and graphite is brittle. These are definitely heavier than graphite and cut like metal. The pieces are non-ferrous. How can I tell what they are?

If they are platinum, why would they silver plate them? Obviously if they are platinum I would like to recover.

however if they are chromium IDK if there is value there. If they are copper chromium it would seem the reaction would have continued.

Any ideas?

 

[GoIdman]

Can it be that your solution has been saturated and cannot dissolve any more metal? Usually the contacts are silver or silver alloys. Try to add a little bit more acid and see if the reaction reignites.

 

[nickvc]

I would also suggest you heat your solution and do not stop adding dilute nitric if the reaction is ongoing , you can always add more contacts if you overshoot the nitric to consume it.

 

[kurtak]

I got an almost instantaneous reaction, as expected, however it lasted a few minutes. Then the reaction completely stopped! Leaving large intact pieces.

Without pictures of the points it's hard to say - but - based on this description ------

These are fairly large rectangular and square pieces.

Per the bold print - I am willing to bet that they are the tungsten/silver type points & the reason I say that ---------

tungsten would have continued to react but slowly

Tungsten does NOT react with nitric (or most any other "common" acid)

First you need to understand how the tungsten/silver contacts are made

The tungsten & silver are not actually alloyed & that is because of the EXTREMELY HIGH melt temp of tungsten so the silver & tungsten can not be melted together to make an actual alloy

So instead what they do to make these contacts is what is called sintering - they mix tungsten & silver powder together - they then take that mixed powder & put it in a press that provides VERY HIGH pressure as well as HEAT squeezing the metals together into a very tightly bonded solid chuck of metal

So the silver particles (powder) are VERY tightly compacted in a matrix (rather then an actual alloy) with the VERY tightly compacted tungsten particles

So - because nitric does NOT react with the tungsten this is what happens

When you first put the points in the 50/50 nitric/water the nitric reacts with the silver particles exposed at the surface of the points - but - because the nitric does not react with the tungsten you soon get a surface layer of undissolved tungsten & the nitric can not get past that surface layer of undissolved tungsten which then prevents the nitric from dissolving silver particles deeper in the tungsten/silver matrix

That is why at first you had a reaction - but then it stopped

So - in order to get all the silver leached out of that matrix you have to literally BOIL the points in the nitric/water

The BOILING action of the acid/water provides a kinetic energy that allow the acid to penetrate the undissolved tungsten & get to the silver deeper in the points thereby leaching the silver "out" of the tungsten matrix

Depending on the size of the points & or the size of the batch it can take as much as 2 even 3 days of BOILING the points in the acid to get all the silver leached out of the tungsten matrix

Generally speaking they make these points in 2 types ------

1) 30% silver sintered with 70% tungsten (most common)

2) 40% silver sintered with 60% tungsten (less common)

The way to tell when you have all the silver leached out of the tungsten matrix is to try to break them in half after they have been boiling for quit awhile

If they don't break in half at all there is still a lot of silver in them

If they break in half somewhat but not real easy - look at the center of the break & if you see any silvery color in the center you have most of the silver leached out but not all of it

If they break very easy & they are black all the way to the center you have the vast majority (if not all) the silver leached out --- there should be NO silvery color at the center

Bottom line with these type points is they are relatively low yield (only 30% & at best 40% silver) & take a lot of time literally BOILING them in nitric to get all the silver leached out of the tungsten

Make sure to keep your water up to "at least" the 50/50 nitric/water ratio --- in other words due to all the boiling you have to account for water evaporation & if the nitric gets to be more then 50% it becomes less effective at dissolving the silver

Kurt

 

[kurtak]

Most contacts are made of:

Gold - NEVER "made" of gold but sometimes gold "plated" &/or topped with a very thin gold cap

Nickel - NEVER "made" of nickel but may have "some" (very little) nickel alloyed with them

Palladium - rarely if ever found as large points - they will mostly be found as "very" small points found in the OLD telecom rely switching gear

Platinum - rarely (if ever) found anywhere outside of the soviet (Russian) block nations

Silver tungsten - already discussed

They also make points that are a silver/molybdenum alloy- they are next to impossible to dissolve with nitric - MUCH worse then tungsten/silver point - they are NOT common but do exist - I have only seen them one time & after boiling them in nitric for a week only dissolved about 10% of them

Kurt

 

[anarxi]

The density of the material can be determined by hydrostatic weighing.
Tungsten is as heavy as gold.
Almost 2 times heavier than silver....
You can check it by pressing, tungsten is very hard, there will be no marks left on it from side cutters.

 

[anarxi]

They also make points that are a silver/molybdenum alloy- they are next to impossible to dissolve with nitric - MUCH worse then tungsten/silver point - they are NOT common but do exist - I have only seen them one time & after boiling them in nitric for a week only dissolved about 10% of them

Kurt

molybdenum is better than tungstenit is much more expensive

 

[anarxi]

By the way, tungsten is magnetic...
contact detail from an ancient vibration transducer after nitric acid and a burner...

 

[Reggy]

Without pictures of the points it's hard to say - but - based on this description ------


Per the bold print - I am willing to bet that they are the tungsten/silver type points & the reason I say that ---------


Tungsten does NOT react with nitric (or most any other "common" acid)

First you need to understand how the tungsten/silver contacts are made

The tungsten & silver are not actually alloyed & that is because of the EXTREMELY HIGH melt temp of tungsten so the silver & tungsten can not be melted together to make an actual alloy

So instead what they do to make these contacts is what is called sintering - they mix tungsten & silver powder together - they then take that mixed powder & put it in a press that provides VERY HIGH pressure as well as HEAT squeezing the metals together into a very tightly bonded solid chuck of metal

So the silver particles (powder) are VERY tightly compacted in a matrix (rather then an actual alloy) with the VERY tightly compacted tungsten particles

So - because nitric does NOT react with the tungsten this is what happens

When you first put the points in the 50/50 nitric/water the nitric reacts with the silver particles exposed at the surface of the points - but - because the nitric does not react with the tungsten you soon get a surface layer of undissolved tungsten & the nitric can not get past that surface layer of undissolved tungsten which then prevents the nitric from dissolving silver particles deeper in the tungsten/silver matrix

That is why at first you had a reaction - but then it stopped

So - in order to get all the silver leached out of that matrix you have to literally BOIL the points in the nitric/water

The BOILING action of the acid/water provides a kinetic energy that allow the acid to penetrate the undissolved tungsten & get to the silver deeper in the points thereby leaching the silver "out" of the tungsten matrix

Depending on the size of the points & or the size of the batch it can take as much as 2 even 3 days of BOILING the points in the acid to get all the silver leached out of the tungsten matrix

Generally speaking they make these points in 2 types ------

1) 30% silver sintered with 70% tungsten (most common)

2) 40% silver sintered with 60% tungsten (less common)

The way to tell when you have all the silver leached out of the tungsten matrix is to try to break them in half after they have been boiling for quit awhile

If they don't break in half at all there is still a lot of silver in them

If they break in half somewhat but not real easy - look at the center of the break & if you see any silvery color in the center you have most of the silver leached out but not all of it

If they break very easy & they are black all the way to the center you have the vast majority (if not all) the silver leached out --- there should be NO silvery color at the center

Bottom line with these type points is they are relatively low yield (only 30% & at best 40% silver) & take a lot of time literally BOILING them in nitric to get all the silver leached out of the tungsten

Make sure to keep your water up to "at least" the 50/50 nitric/water ratio --- in other words due to all the boiling you have to account for water evaporation & if the nitric gets to be more then 50% it becomes less effective at dissolving the silver

Kurt

 

[Reggy]

These are not magnetic.

The acid is not spent I added 1/4 gram of known 99.9% silver and it took right off.

I added pictures

By gold I meant gold plated

They are heavy

Wire cutters leave about the same mark as on pure silver same thickness see pic

 

[Reggy]

molybdenum is better than tungstenit is much more expensive

Well if these are molybdenum I have 2 lbs what’s that worth

 

[anarxi]

I could sell it at home for $30.

 

[anarxi]

There are also often contacts: a mixture of graphite and silver.
they are quite fragilethe amount of silver can reach up to 90%

 

[ssabovic]

hi ,
tungsten is not magnetic , alloys of tungsten may be like densalloy which have nickel in it.
regards

 

[Reggy]

hi ,
tungsten is not magnetic , alloys of tungsten may be like densalloy which have nickel in it.
regards

That’s what I thought, just the other person said they were and I wanted to make sure

 

[Alondro]

Gold - NEVER "made" of gold but sometimes gold "plated" &/or topped with a very thin gold cap

Nickel - NEVER "made" of nickel but may have "some" (very little) nickel alloyed with them

Palladium - rarely if ever found as large points - they will mostly be found as "very" small points found in the OLD telecom rely switching gear

Platinum - rarely (if ever) found anywhere outside of the soviet (Russian) block nations

Silver tungsten - already discussed

They also make points that are a silver/molybdenum alloy- they are next to impossible to dissolve with nitric - MUCH worse then tungsten/silver point - they are NOT common but do exist - I have only seen them one time & after boiling them in nitric for a week only dissolved about 10% of them

Kurt

I have found a few extremely high-end contacts in relay switches that were solid gold. They were very flat and very small. If they're the 'button' type, then they're always plated. But if you have certain critical industry boards from just the right era, the little flat contacts are solid gold. I've found them in high-end telecom boards (which also had all the PROM leg slots entirely gold-plated).

Those relay switches are very small, the boxes barely 1cm wide, and are kind of a pain to dismantle. I still have half a jar of them, because they take SO LONG to get apart. Each one had 6 tiny gold contacts inside.

 

[lazersteve]

I've seen Au/Pt alloy "contacts". I quoted contacts because the contact is nothing more than a 0.5mm diameter wire approximately 1.5 mm long from what I've seen. I'm of the opinion that there is very little incentive for a manufacturer to use solid Au or even Au alloys for contacts without very stringent operational criteria. Perhaps Avionics or Aerospace applications and even then the contacts will be very small. I refer to these more accurately as "points" to better describe their physical size.

Steve

 

[Golddigger76]

How large are these contacts ?
I can't tell the size by looking at the pictures you posted.

Also did the person who you bought them from tell you what the contacts came out of ?

 

[Reggy]

How large are these contacts ?
I can't tell the size by looking at the pictures you posted.

Also did the person who you bought them from tell you what the contacts came out of ?

1 1/2to 2 “ long 3/8-1/2” wide and no

 

[Reggy]

View attachment 62019View attachment 62019

Did the pictures help?