How to avoid aluminum -> hydroxide

[shackney]

I have a product with some small aluminum components - specifically a gauge with aluminum pointer and adjustment mechanism. The gauge is fluid filled, and recently we have been required to take the fluid fill from glycerine to a mix of glycerine and water.

Over time, some of the pointers are getting a cottony-looking contamination, which I first thought to be a mould or fungus. Turns out that it is made up of aluminum and oxygen (Doh) -- aluminum hydroxide, I suppose.

I don't want to replace the aluminum with other materials if I can avoid it. It would be difficult to achieve the cost/performance combination in any other way.

Coatings will be problematic. The adjustment mechanism includes friction fit and tiny gears which are likely to abrade/wear and expose the aluminum again. This is not to say impossible. Just tricky.

Mechanical loads on the aluminum part of the gauge are neglible. The problem is aesthetic only. The hydroxide ends up suspended in the fill fluid, looking like dirt or fungus. The oxide that I suppose was originally on the aluminum surface was never a problem, because no one could really see it. I am dubious that a sacrificial anode would work, because the corrosion products would still be liable to migrate around, looking like a contaminant as well.

There are other materials which will inevitably be exposed to the fill fluid. These will be any of brass or other copper alloys, stainless, silver solder, perhaps soft solder.

Any suggestions?

shackney

 

[unclesyd]

Why the water? Is pure glycerine too viscous? If water is present you will need and corrosion inhibitor for your solution to protect the aluminum. There several available that want color your solution. They should work at a level that want affect your viscosity.

 

[shackney]

Thanks.

The water is required because the freezing point of a glycerin water mix is lower than that of either pure glycerin or pure water. (Go figure.) So we have a 2:1 mix.

Can you suggest a corrosion inhibitor for this application? I am off to search the web for such, but if you have recommendations this would be appreciated.

Thanks again,
shackney

 

[unclesyd]

It may be a simple as adding a metasilicate. I'll will check my files and see if i can find a supplier that's still in business.

 

[unclesyd]

Here is one place to try.

http://www.metalprotection.com/index.html

There used to be an addiative available in a auto parts store to add to Aluminum Radiators. I was told it worked very well.

 

[shackney]

Thanks for the help. This hasn't been a life-threatening problem, but it sure has been a nagging pain in the neck, and if this solves it....

sh

 

[TVP]

Another option to consider would be to anodize the aluminum surface. This oxidation treatment forms a corrosion-resistant and wear-resistant layer which can be subsequently colored (or not). Perform a search of this site and read-up on the expert information provided by member kenvlach.

 

[stevenal]

Not really my field of expertise, but have you considered antifreeze? Comes complete with corrsion inhibitors and works with aluminum engine blocks. Get the pink stuff, mix to the freeze protection needed, and you're good to 150000 miles of pointer travel.

 

[kclim]

Quote:

"The water is required because the freezing point of a glycerin water mix is lower than that of either pure glycerin or pure water. (Go figure.) "

FYI, this is to do with colligative properties of solutions - tough to explain but I'll give it a shot.

When you have solute molecules in a solvent (glycerine in water or vice versa), it increases the entropy (disorder)of the solution relative to the solid phase (glycerine and water are partitioned/separate when frozen).

There is a general tendency towards disorder (decrease in free energy) in any process. Thus the solution phase becomes more favourable than the solid phase, relative to a pure liquid.

This is manifested as a lowering of the freezing point (the solution phase is favoured over a wider range of temperatures to the solid/frozen phase).

Pure water or pure glycerine have lower entropy than their mixtures, and do not exhibit freezing point depression. This is why the lowest freezing point comes somewhere in between.