On Sat, Oct 17, 2020 at 01:54 AM, Dave Daniel wrote:
Those tend to be alkaline peroxide based cleaners. (Alkaline peroxides are good at dissolving microbes, and grease; but can/do attack some metals. Sometimes, passivators are used, to help protect the metal.)
I used a trisodium phosphate solution, which is an alkaline phosphate based cleaner. Trisodium phosphate is a very good degreaser; but, it can/will attack some unpassivated metals.
After some time, aluminum forms a very strong, but thin thin layer, of Aluminum oxide, that acts as passivation. On these very old filters (decades old?), the surface of the aluminum is bound to have oxidized.
I've cleaned up aluminum 500 series filters several times using trisodium phosphate. And, very dirty filters cleaned up so as they looked like new. However, milage may vary... and thus, caveat emptor.