Hcl dissolves gold plating

[kjavanb123]

This has never happened. I have this boards




As soon as I put in hcl, gold plating is gone immediately after few seconds plates back black powder.




Any ideas??

 

[Lino1406]

No. It is coated by galvanic effect unless if H2O2 is present

 

[anachronism]

It's not dissolved the gold, the gold is covered in Tin now.

 

[kurtak]

What Lino & Jon said :!:

Kurt

 

[kjavanb123]

Thanks all for your comments. Here is an update on this.

I leached 950 grams of similarly gold plated boards in 1500 ml hcl at room temperature and got tthe most ICs and SMDs and I noticed a dark powder settled at the bottom.


Please note this is from only leach in Hcl, no oxidizer was added at any point.

So I collected all the black and some gray powder and added to a hot Hcl solution to that added few drops of nitric acid and got the following


That in AR


And this is what result indicates from stannous chloride testing solution, positive for gold


And boards look clean no sign of gold or blac plating


So based on this observation, two differently plated boards were leached in room temprature in Hcl and both resulted in gold plating dissolving very similar to one can see during a cyanide leach.

But it plates back on the nickel layer as black powder whic settles at the bottom.

I am going to simmer the residue in filter paper in AR to make sure all the gold is dissolved and refine the gold from SMB addition tommorow.

Best
KJ

 

[anachronism]

It's not similar to a cyanide leach.

What does your chemistry tell you about the different active ingredients here?

 

[Lino1406]

That's "a new kind" of hydrochloric acid which does the job of HCl + oxidant without being HCl + oxidant. Mendeleev we need help

 

[niks neims]

I leached 950 grams of similarly gold plated boards in 1500 ml hcl at room temperature

For how long did you keep them there?

I seem to recall to have read it here that ANY solution with chlorine in it has the potential to put gold in to solution, it only needs some kind of oxidizer to briefly steal an outer shell electron from gold atom for chlorine to be able to snatch it up? In AR the nitric acid plays the role of oxidizer, in AP it's H₂0₂, but I think I read it here that plain old oxygen from air can saturate solution of HCl to a point where it is able to dissolve gold? and since KJ's plating is most likely thin ENIG,it should not take much...

I could be wrong, of course, but this is an interesting topic for me, as I often have wondered wouldn't AP eat away some of the gold from fingers, for an example... also it seems that a couple of people use HCl to depopulate boards or clean them from Sn... I would really like someone smarter than me to clarify - Is gold, left for a time in HCl safe or can it steal some? After all, we all do "harold's washes".....

then again, on a secound thought, a solution with both tin chloride and gold chloride present would immediately drop out gold as a colloid...

 

[anachronism]

Good points Nick

Funnily enough copper in the presence of HCl can dissolve gold. Admittedly it's hardly a fast and vigorous reaction and in the case of AP the base metals still precipitates out the gold as powder which is why the solution doesn't remain pregnant.

If you study redox reactions there's a lot of good info to be had.

As a case in point- when cleaning dirty gold in an HCl/water mix with absolutely no Nitric present this is the reason why some gold is often taken up into solution.

Hope that helps.

Jon

 

[kjavanb123]

It's not similar to a cyanide leach.

What does your chemistry tell you about the different active ingredients here?

I know it is not similar to cyanide leach, what I meant was as soon as Hcl was added the golden layer of plating dissolved just like it does when leaching with cyanide and oxidizer.

Main ingerident is Hcl, there are tin solders coper nickel as the base of plating, again no oxidizer was used.

 

[kjavanb123]

I also tried Hcl leach of another set of boards for more than 12 hours, and this time parts of gold plating were dissolved and percipitated as black powder (much smaller amount)

There were tiny specs of gold foil suspending in the solution, but most of gold plating were intact.

I will study the boards to confirm your points

 

[kjavanb123]

I leached 950 grams of similarly gold plated boards in 1500 ml hcl at room temperature

For how long did you keep them there?

I seem to recall to have read it here that ANY solution with chlorine in it has the potential to put gold in to solution, it only needs some kind of oxidizer to briefly steal an outer shell electron from gold atom for chlorine to be able to snatch it up? In AR the nitric acid plays the role of oxidizer, in AP it's H₂0₂, but I think I read it here that plain old oxygen from air can saturate solution of HCl to a point where it is able to dissolve gold? and since KJ's plating is most likely thin ENIG,it should not take much...

I could be wrong, of course, but this is an interesting topic for me, as I often have wondered wouldn't AP eat away some of the gold from fingers, for an example... also it seems that a couple of people use HCl to depopulate boards or clean them from Sn... I would really like someone smarter than me to clarify - Is gold, left for a time in HCl safe or can it steal some? After all, we all do "harold's washes".....

then again, on a secound thought, a solution with both tin chloride and gold chloride present would immediately drop out gold as a colloid...

Hi Nick

It was in Hcl for 12 hours with no agitation

 

[kurtak]

Ok - its time to put this discussion to rest :!:

When using HCl to dissolve tin as a method of depopulating boards the HCl does NOT dissolve the gold plating (unless an oxidizer is present) not even ENIG plating

as Lino said - "No. It is coated by galvanic effect" --- or as Jon said - "the gold is covered in Tin now"

:arrow: https://en.wikipedia.org/wiki/Galvanic_corrosion

I just got done depopulating some lap top RAM which I then (after depopulating in HCl) cut the fingers off

In the picture you will see that on the left side of the fingers it "appears" the gold is gone there by exposing the nickel layer under the gold - however - it just appears to be that way --- a drop of nitric on the right side of the fingers "instantly" removed the VERY thin galvanic tin lay "on top" of the gold plating --- the gold plating did not dissolve - its still there - it just got coated with a thin layer of tin as a result of galvanic effect wherein the HCl was the electrolyte & the circuit boards (with there different metals) acted as both anode & cathode

Kurt

 

[kurtak]

Thanks all for your comments. Here is an update on this.

I leached 950 grams of similarly gold plated boards in 1500 ml hcl at room temperature and got tthe most ICs and SMDs and I noticed a dark powder settled at the bottom.
IMG_1145.JPG

Please note this is from only leach in Hcl, no oxidizer was added at any point.

So I collected all the black and some gray powder and added to a hot Hcl solution to that added few drops of nitric acid and got the following
IMG_1146.JPG

That in AR
IMG_1147.JPG

And this is what result indicates from stannous chloride testing solution, positive for gold
IMG_1150.JPG

And boards look clean no sign of gold or blac plating
IMG_1152.JPG

So based on this observation, two differently plated boards were leached in room temprature in Hcl and both resulted in gold plating dissolving very similar to one can see during a cyanide leach.

But it plates back on the nickel layer as black powder whic settles at the bottom.

I am going to simmer the residue in filter paper in AR to make sure all the gold is dissolved and refine the gold from SMB addition tommorow.

Best
KJ

Now then - to further explain what is happening here Kevin

The gold you are getting from the "black powder" after using HCl to depopulate your boards is NOT the result of gold dissolving & then cementing back out during the process of using HCl to depopulate the boards

Rather - the gold is coming from the solder its self

the solder used on circuit boards is an alloy of tin & antimony with the antimony making up about (+/-) 5% of the alloy

Antimony does not dissolve in HCl - so when you dissolve the solder - the tin dissolves & the antimony remains behind as a "black powder"

The reason there is gold in this black powder when you treat this black powder with AR is because the gold - from the gold plating on the board gets dissolved into the solder when the SMDs are soldered to the board

In other words - during the soldering process - the solder acts like a "solvent" for the gold plating on the board thereby "alloying" the gold into the solder

So when you dissolve the solder (tin) from the board with HCl - the black powder you get is a combination of the antimony & the gold that has "become" alloyed with the tin during the soldering process

This was discussed in detail here in Tzoax's thread - read the whole discussion

:arrow: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=33&t=22951&hilit=solder&start=90#p267815

Kurt

 

[kjavanb123]

Thanks kurtak and everyone else.

 

[anachronism]

It's not similar to a cyanide leach.

What does your chemistry tell you about the different active ingredients here?

I know it is not similar to cyanide leach, what I meant was as soon as Hcl was added the golden layer of plating dissolved just like it does when leaching with cyanide and oxidizer.

Main ingerident is Hcl, there are tin solders coper nickel as the base of plating, again no oxidizer was used.

Did you not read our comments- the gold was NOT dissolving "like it does when leaching" - tin was dissolving and plating OVER the gold? 8) 8) 8)

 

[Shark]

Kurt, thank you for that clear explanation. I have seen the same thing in the past and was fairly confused by it back then. As I learned more about it I knew what was going on but lacked the understanding or the ability to word it properly. Sometimes my thick head needs it put in grade school words to fully grasp the understanding even when I know what is going on.

 

[anachronism]

Shark I had exactly the same thing when cleaning some pinned processors when I started out. I was convinced that I had dissolved the gold, so I do understand where the Kevin's perception came from. 8)

I don't think anyone needs to be a qualified chemist to do this, the only proviso being that a base level chemistry really helps. Certainly understanding the interactions between acids and metals plays a big part in understanding why certain things happen. 8) 8)

Jon

 

[UncleBenBen]

I don't think anyone needs to be a qualified chemist to do this, the only proviso being that a base level chemistry really helps. Certainly understanding the interactions between acids and metals plays a big part in understanding why certain things happen.

Jon

Absolutely. Especially when things "don't go exactly as planned." :wink:

I think this has turned into a pretty good thread for explaining to our newer members why gold sometimes doesn't go exactly where we have learned it should go. I know I had some struggles with that for a while. Special thanks to Kurt for laying it all out in a manner that is easy to understand and for saying it like it is.

 

[Lino1406]

HCl did not dissolve, it separated the gold. The way it works is this 1. HCl + oxidant, dissolves a little gold 2.The wound formed, gives HCl + oxidant access to the base metal. Nature works in stages: until all the base metal is dissolved, no further gold will dissolve (note: if the base metal is copper or nickel it also needs an oxidant to dissolve)