Copper turned silver-color with crystalized soldered joints. How did this happen?

Please remove if this question if not allowed on this sub-exchange. I can post elsewhere if suggested.


I was working on a small project requiring cleaning and polishing copper pipe, but found sparkling solder joints and silver-like finish instead.


I attempted to clean copper pipe with white vinegar + salt mixture. I let the copper sit for around 30 minutes in a bucket of this solution. At this point I'm aware of the copper oxide appearing on the surface. Pictured below.


After drying off with a towel I attempted to shine the copper. I applied [Copper and Brass cleaner](https://rads.stackoverflow.com/amzn/click/com/B0083PEZ00) which contains quartz, citric acid, ammonium chloride, and magnesium aluminum silicate. To my surprise, 2 changes happened to the copper/solder:


1. The copper has a shining silver-like appearance.
2. If you look closely, the joints which were soldered with [lead-free silver solder](https://www.homedepot.com/p/Oatey-Safe-Flo-8-oz-Lead-Free-Silver-Solder-Wire-290242/100342559) are sparkling in the light (very apparent), almost as if they contained tiny crystals.


It's important to note that the sparking soldered joints only appeared *after* using the copper and brass cleaner. They're still dull after only removing them from the white vinegar + salt solution.


How in the world did this reaction happen? Is it possible to remove the silver finish?


* I'm aware this home-depot bought copper may have small impurities of tin, iron, zinc, or nickel, but doubt that makes a difference.


Before putting the copper in a white vinegar + salt solution:


[![enter image description here](https://i.stack.imgur.com/Y0eZO.jpg)](https://i.stack.imgur.com/Y0eZO.jpg)


30 minutes after drying the copper pipes from the solution:


[![enter image description here](https://i.stack.imgur.com/cimXd.jpg)](https://i.stack.imgur.com/cimXd.jpg)


After polishing with the cleaner:


[![enter image description here](https://i.stack.imgur.com/5f2KC.jpg)](https://i.stack.imgur.com/5f2KC.jpg)




The copper pipe system is a large copper cathode (the copper tubing) with a small area of exposed silver solder (\*) anode comprised mostly of tin. When it dissolves in the citric acid plus ammonium chloride, apparently enough silver becomes soluble to migrate over to some other copper surface and plate out - silver dendrites are a common result in concentrated silver salt solutions, but in the OP's picture, they are so tiny as to just look crystalline. Here is a picture of large silver dendrites forming on copper wire. Ref 1.


[![enter image description here](https://i.stack.imgur.com/ii4Xd.jpg)](https://i.stack.imgur.com/ii4Xd.jpg)


The culprit, if there can be said to be one, is the silver solder. It sounds like an upscale binder, but may have limitations. Of course, the copper and brass cleaner is part of the problem, by providing the corrosive potential.


Apparently the entire copper surface is now plated with a thin silver film that would develop dendrites if there were more silver.


Is it possible to remove the silver finish? It is probably so thin that the best way would be strictly mechanical: rubbing with abrasive powder like silica, or sandblasting with sodium bicarbonate, without chemical corrosion.


* Oatey Flo 8 oz. Lead-Free Silver Solder is good for soldering copper pipe and fittings together. It is safe to use in potable water applications. This silver solder has a wide melting range of 415° to 455°F. It can be used for plumbing, HVAC, fire sprinkler and other systems.
• 95.5% tin, 3% copper, 1% bismuth and less than 2% silver alloy
• 420°F - 460°F melting range
• Meets ASTM standard B-32 and the requirements of the safe drinking water act
• Formulated for plumbing/potable water line applications


Ref 1. <https://www.cheminst.ca/magazine/article/chemisteam-2021-silver-crystal-fern/>


 2

The Oatey solder listed in the question contains no silver, the guy writing the ad thought "silver solder" was a good sounding name. Silver solders melt above 1150 F, more accurately called silver braze. Copper pipe, tube, and fittings are commercially pure copper[ ASTM 42 is between 99.4 and 99.9 % minimum copper per Copper Development Association data book, 1973 ( these specs do not change much with time). You are using a high tin solder containing bismuth and other elements. I can not explain how a cleaning solution could have coated the part with solder. The silver colored structure looks like it is covered with solder as if it had been dipped in a molten bath like galvanizing or heated in a furnace. Furnace solder or braze of a smallish complicated structure as shown would not be unusual.


 2

The anode here is the tin (which is subject to corrosion, aka, dissolving) and the very noble silver is the cathode (see this table of [relative anodic behavior posted here](https://www.zygology.com/cms/upload_area/pdf/Zyg-Anodic-Index.pdf)). One could also argue at some point the copper becomes the cathode.


In any event, the tin goes into solution whereupon the more chemically active copper displaces it, resulting in a tin plating effect on the copper surface.


The ammonium chloride can act here in both the roles of an electrolyte, the electrochemistry part, and as a complexing agent for the tin (and copper) as part of the standard chemistry.


Note: If it is determined that the plating metal is, indeed, silver metal, then one must account for the creation of, say, aqueous $\ce{Ag(NH3)2Cl}$. This would likely involve the presence of solvated electrons (from the galvanic chemistry) interacting with oxygen exposure in the presence of the $\ce{NH4Cl}$.


Kind of an incidental, but interesting and possibly advanced example of mixed chemistry ([as occurs with copper](https://iopscience.iop.org/article/10.1149/1.2780873) and is noted [to occur with silver here](https://www.sciencedirect.com/science/article/abs/pii/0022508882901564)).


[EDIT] At home one can test for silver metal with pure household disinfecting bleach (see the discussion here on the so-called ["Bleach Test"](https://sdbullion.com/blog/how-to-test-silver)).


 1