To protect anodised aluminum surfaces, heat-resistant lacquers and varnish-resistant varnishes are used. The outer color of the metal is also retained.

Hi Morteza

I think silica sol gel is a good chose.

There are many additives used for this kind of coating depending on the corrosive electrolyte.  The precursor of this coatings can contain epoxy part to improve the adhesion and flexibility.

You can find many references for this kind of coating, I suggest to use 3-Glycidyloxypropyl trimethyloxysilane with 3-aminopropyl triethoxysilane.

Good luck.

Gobara

Hi Morteza,

the surface roughness is a very important parameter you must consider before to select a route.

If Ra is higher than 1 micron, I suggest you the approach recommended by Emil with varnishes, they are typically less transparent and thicker giving to the surface a less appealing aspect but they are certainly effective as barrier layer.

If Ra is a few hundreds of nanometers you can approach thinner layers deposited by Sol-gel or even PECVD (Plasma enhanced chemical vapor deposition) typically silica-based. They are more expensive, typically with thickness lower than 5 microns so that the barrier properties are effective only for low roughness surfaces but they gives a much more appealing surface quality being essentially invisible !

Ciao

Simone 

Thanks a lot for your useful ideas. I'd be more than grateful if you could share with me a sample formulation. 

P.S. We don't have any specified preference on the roughness; as long as it's transparent, applicable to anodised aluminium, and resistant to aggressive alkaline corrosive environment, it will meet our need.   

Morteza