[R-390] Why polished radios?

[Curt Nixon]

My experience  (in offshore yachts) has been that unless you are ready 
to do a proper acid etch/zinc chromate and epoxy primer with some sort 
of 2 part epoxy or urethane top coat, it is better to leave the clean 
aluminum bare.  It is its exposure to oxygen that creates the micro 
oxide protective coating...just like stainless steel.  Take away the 
oxygen..as in rattle can krylon or similar, and you can create oxygen 
depeted areas that can pit and corrode very quickly..even in a mild 
environment.

Also, stainless fasteners into aluminum with no galvanic barrier results 
in white powder destructive corrosion of the aluminum--in a marine 
environment.

Not sure if Alodine  (TM) m is exactly zinc chromate or not.  It is 
still sold by the aircraft maintenance folks like Aircraft Spruce Co..  
The nice thing about Alodine over anodize is that Alodine can be done 
over older Alodine.  Anodizing can't really do that because of the 
insulation layer. 

Anodizing is ALL clear..the color comes from dyeing the bare aluminum 
before the anodize process.

This is what I remember  +/-  the old age frequency drift ;)

Curt
KU8L


Tisha Hayes wrote:
> I think that with aluminum the passivation layer reforms in a matter of
> seconds. If it is not exposed to a corrosive atmosphere or an electro
> galvanic reaction it does not go any further.
>
> I use a metal polish with a cloth wheel to clean up things like IF cans in
> an SP-600 or brass gears for drivetrains and it contains some sort of
> protective layer like a wax. I have never thought of doing it to an entire
> chassis and would think that taking down all of the anti-corrosive coating
> on an R-390 would be quite an exercise.
>
> Is the yellowish color a chromate coating? I would not want Erin Brockovich
> to show up at my door because I used hevavalent chromium to restore a finish
> and dumped the residue down my septic system. For a shiny radio you may be
> able to use a lacquer varnish to lock in the pretty look or if you really
> wanted to get into it you could completely strip the chassis, take out all
> of the components, tube sockets and steel parts and anodize the entire
> thing. You can even do anodization in colors (Caswell kits). It would take
> some pretty serious solvents, strippers, acids, etc.. to get down to bare
> metal. I would probably go with the anodized gold color <s>.
>
> Maybe we can simulate the yellow chromate color by putting the radio in the
> same room as a dozen smokers for a year <j/k>.
>
> I have seen some industrial equipment in shiny aluminum cabinets put in
> places where they were eaten up in weeks. At water plants where there are
> high levels of chlorine, fluorine or ozone gas or in salt-water environments
> like on the pier at Pensacola NAS. It is amazing how quickly ozone eats
> stuff up and leaves nothing but a black sticky residue behind.
>
> Anodization is actually an insulating layer and I could see problems where
> tube socket screws need to get to chassis ground. Then again, slight
> differences in alloys can also cause dissimilar levels of anodization and
> you could end up with a very grainy, porous finish on external surface areas
> and almost no coating inside of the little cavities inside of something like
> the bottom of an IF deck.
>
> I DESPISE the cadmium coated screws in a receiver like the SP-600. Almost
> every one of those things looks awful. I went through a small fortune in
> stainless steel hardware to replace those things.  Pulling stuff apart to do
> a re-cap; old screw into the trash can, dig out a new shiny philips head
> screw, washer and nut in it's place. It is amazing how many intermittent
> problems on a receiver can be caused by loose hardware.
>
>