Need a better mineral scale/corrosion cleaner

Hi:

I have to clean gold plated laser reflectors which are coated with mineral scale. I believe the scale is composed of copper and aluminum hydroxides and carbonates. This is based on the fact that the metals in the reflector assemblies where the corrosion occurs are aluminum and copper (and gold) in contact with each other and submerged in water. Pitting of the aluminim is clearly visible, suggesting that aluminum went into solution electrolytically and eventually precipitated out on the gold surfaces.

In the past I had tried "CLR" from the hardware store, as well as vinegar and various detergents, but these were very ineffective. The cleaning process mustn't scratch the gold, so it can't involve a lot of elbow grease.

I have been using 10% nitric acid since then, which is very effective. A few minutes of soaking in the acid, then the scale almost falls off. A little swabbing with cotton swabs while submerged under the acid gets the reflectors completely clean.

There are two problems with this method. First, it is more hazardous than I would like to be cleaning parts in a bath of 10% HNO3. Second, the acid attacks the copper substrates of the reflectors. Whatever boneheads plated these things did a rotten job, and the gold has little perforations. These allow acid to eat at the copper underneath, which results in bubbles in the plating and more surface area for future corrosion.

I would like to find a more mild chemical solution that can dissolve the scale or quickly loosen it from the gold surface without being so hazardous nor corrosive to the base metal of my parts.

I am wondering if some sort of EDTA solution might be workable. All I remember about EDTA is that it works differently at different pH's. What pH might be the right one for complexing Al and Cu out of their hydroxides/carbonates?

Perhaps just plain old HCl might not be so bad, at perhaps 10% or so.

Some other organic acids?

But if I could do without strong concentrations of mineral acid that would be best. Below 5% might be Ok. Acetic acid might be tolerable up to photo grade 28%. It only stinks, but can use a hood for this. I seem to recall though that it wasn't quite strong enough.

Suggestions appreciated.



--
Good day!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@xxxxxxxxxxxxxxx
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