Re: Need a better mineral scale/corrosion cleaner
- From: "Ron Jones" <ron@xxxxxxxxxxxxxxx>
- Date: Mon, 20 Feb 2006 19:51:49 -0000
Chris Carlen wrote:
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.
Acetic too week.
Try citric - cheap, easily obtainable, no smell, low pH, low tox (and really
great for cleaning dishwashers....)
HCl - better than nitric, should not attack the copper, anything less than
28%, or so, doesn't fume.
Dilute sulphuric might work, but a pain to dilute if you can only get the
98%.
--
Ron Jones
Process Safety & Development, Alfa Aesar Avocado Lancaster UK
Don't repeat history, see unreported near misses in chemical lab/plant
at http://www.crhf.org.uk
Only two things are certain: The universe and human stupidity; and I'm
not certain about the universe. ~ Albert Einstein
.
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