Stripping gold from aluminum without damage to the aluminum

? I wonder why gold is coated on aluminum impeller! Are you sure it is gold and not TiN?

I would be mildly amazed if it were possible as both acids and bases attack aluminum. As said before, I bet that it is TiN and not gold.

A. The only chemical that does not harm Aluminium is Nitric acid. This acid will help you get rid of Gold from aluminium without damaging and you will also recover gold too.

A. Nitric does not remove thick gold, that takes cyanide or "aqua regia". Nitric will definitely attack high alloy aluminum like 2024 or 7075, with serious attack on weld joints.

A. The gold can be removed anodically in strong sulfuric acid with little or no damage to the aluminum. Some alloys will possibly be pitted, but I have never had any problems with the alloys I have worked with. The original US patent on this is #2185858 - search for this in Google Patents. Although Al is not mentioned in the patent, gold can be stripped from most metals successfully using this method. The only problems I have had were with high nickel or cobalt alloys.

I would dilute concentrated sulfuric acid with 5%, by volume, distilled water (add the acid to the water!). Let it cool before you use it. Concentrated sulfuric will work, but a little water speeds up the stripping. The patent uses weaker sulfuric but, in my experience, that is more likely to pit. Most any metal can be used as the cathode, but I prefer mild steel. Just about any kind of wire can be used to suspend the part.

Start with zero voltage/amperage and slowly crank it up. At first, the amps will be high and the voltage will be quite low. As the gold strips, the amps will decrease and the voltage will increase. Keep turning it up to maintain the amperage. When the voltage is somewhere between 6 and 8, don't make any further adjustments. At some point, when all the gold is stripped, the amperage will automatically go to zero - this will tell you when it is finished.

With too high an amperage, the solution will get quite hot. When adjusting the rectifier, I wouldn't allow the solution temperature to go above about 110F. The hotter the solution, the more likely the Al will pit. Just about any volt/amp combination will work, but I would suggest trying the parameters that I have suggested, to reduce the possibility of pitting. Watch the temperature!

The gold dissolves only momentarily and then is reduced to gold powder. All the gold can be collected by first allowing it to settle to the bottom of the tank.

A. Consider fine sand or glass bead blasting. Obviously this procedure will lead to an almost impossible recovery of the gold but it may salvage the impeller which seems to be of most importance.

 You're right, Guillermo. I didn't think about blasting. I once did some experiments with steel shot peening to strip the plating so that the parts could be replated. The peening action flattened and stretched the plating laterally, the bond was broken, and the plating just completely fell off. Worked very well on every type of plating that I tried. Zero attack on the parts. The gold might be recovered by separating the shot magnetically.