Caustic soda, Lye, sodium hydroxide, NaOH, is a very strong base the opposite of acid, it will not dissolve gold, some base metals can react with it to form oxides and hydroxides of these metals, or the metals may break down like solder.
Caustic soda can react with oils, and other organic material, like oils in your skin or hands making soap out of your skin, this is why when getting it on your hands they feel very slippery, and come so clean, dissolving your skin making soap, in the old days when it took many months at sea to travel, if someone died on ship they could dump the mans body over board into the sea, to prevent disease on the ship, but if they wished to keep the man and bury him on land, they would pack his body in a wooden barrel of caustic soda, the caustic soda would make soap out of his flesh, they could dump this soap overboard and take his bones to land for burial, caustic soda made from leaching wood ash, and animal fat (hog fat) was how soap was made it makes a very good soap when just slightly caustic, I have used wood ash caustic soda when making leather out of animal hides, the hide soaked in caustic soda make's the hair and fur very easy to remove from the raw hide, it also breaks down the fat and scarf skin making scraping the raw hide clean an easier chore.
The caustic soda can get under the solder mask (green coating) on circuit boards and loosen it.
The gold discolor you see is most likely just base metal oxides, or some other material, from the circuit board coating the gold not dissolving the gold, I suppose with very thin plating the base metal under the gold could oxidize and discolor the metal and make the gold look different, or even dislodge some gold as fine powder into the solution, but the gold would not dissolve into solution.
For a metal to dissolve into solution it must lose electrons, sodium hydroxide cannot take an electron from gold, the gold is too un-reactive.
Sodium hydroxide (caustic soda) if you get it in your eye, will easily dissolve, or cook the eye, and blind you, eye protection is a must.
Caustic soda NaOH the opposite of an acid a very strong base, which can burn your skin making it into soap by dissolving the oil out of it, if mixed with a strong acid like HCl in correct proportions until the solution becomes neutral pH7 will make sodium chloride NaCl (what we eat as table salt).
NaOH + HCl --> NaCl + H2O
Since the gold will not dissolve but some of it may loosen as fine powder, I would stir the boards in solution and rinse them off with a spray bottle back into the caustic solution, let your caustic solution settle well, decant the solution (use or save this solution to treat your acidic waste with), now carefully adding acid to the remaining powers and materials in the bottom of the bucket neutralizing the powders to salt pH 7, let settle decant salt water, then use boiling water to dissolve salt, and any water soluble oxides, or chlorides (like lead chloride soluble in hot water, not very soluble in cold water), this would leave some of the less reactive metals, any gold or silver that may have dislodged mixed with solder mask flakes or other trash, these can be incinerated, cooled, boiled in HCl, to help remove tin and other soluble chloride metals from these powders, boiling hot water washes will help to clean these up further removing lead chloride, and other base metals, letting the solution settle well before decanting but keeping solution hot (silver chloride is not soluble in hot water, but is fluffy and easily floats around, so keeping the water wash as hot as possible and letting silver settle before decanting the lead chloride), if washes were colored strongly I would repeat acid and water washes, this should leave you with any of the more valuable powders if they were dislodged from the original material.
Some times there are ways to make your own acid or chemical you need, if you cannot get it, from chemicals you can get.
Just a few examples:
HCl acid can be made from salt and sulfuric acid.
Nitric acid can be made from sulfuric acid and nitrate fertilizer or stump remover.
Ferrous sulfate (copperas) to precipitate or test for gold can be made from Iron and sulfuric acid.
Poor mans aqua regia made from HCl and nitrate fertilizer.
Ammonium chloride can be from ammonia and HCl.
Actually sulfuric acid can be made from rocks :lol: , but most countries use batteries and sell sulfuric acid.
The work around methods are very useful and can sometimes be preferred for some materials, like the copper II chloride leach for copper, or the HCl sodium hypochlorite to dissolve gold.
