[Antennas] Re: Cleaning Aluminum

2002年10月31日 02:34:03 -0500

Kees, K5BCQ, asked:
> What's the best way to clean up aluminum elements, refurbish
> traps, and fastner coating to prevent corrosion. I assume
> going to stainless hardware is a good idea since most
> manufacturers offer that. Does anyone coat the aluminum to
> retard future corrosion ?

Several people replied with the suggestion to use a solution
of lye. As a chemical engineer, I prefer the term caustic
soda while a chemist would say sodium hydroxide. Since I am a
ChE/EE I will go with caustic soda!
This will work if you follow the instructions in many ARRL
Handbooks about obtaining a satin finish on aluminum panels.
Just remember that the reaction is dissolving the aluminum and
producing hydrogen gas in the process. Not only is the
solution extremely dangerous for skin or eye contact, the gas
released is extremely flammable. If you must do this, work
outdoors wearing gloves and safety glasses.
If the aluminum surface has deep pits and severe corrosion
already, you may find you have to dissolve quite a bit of the
aluminum to get a smooth finish. Also the caustic may not
attack all varieties of corrosion deposits on the aluminum
surface such as imbedded rust.
An alternate approach is to use some of the commercial
aluminum cleaning/brightening agents such as Duro (Loctite,
Permatex) Aluminum Jelly�. A number of companies make similar
products. These are essentially a solution of phosphoric acid
with just a tiny amount of sulfuric or hydrofluoric acid and a
collection of thickening agents to keep the material clinging
to a surface. Again they dissolve a little of the aluminum
and produce hydrogen, but these will generally do a better job
on stubborn corrosion. Gloves, safety glasses, and working
outdoors still holds true.
It is important to rinse the aluminum thoroughly after either
of these treatments. The disposal of either solution is not a
major environmental problem unless you have to dispose of
large amounts. With either solution, you may see black spots
remaining on the aluminum if the alloy contained much copper
(added to increase hardness).
Both treatments leave a somewhat porous oxidized aluminum
surface, although the phosphoric acid treatment leaves a
thinner but tougher oxide layer. To make good electrical
contact to the surface of aluminum parts treated these ways,
you should still use an abrasive in the contact area.
And this brings up an alternative way of cleaning the
aluminum. In many cases you will find a Scothbrite� pad
faster and easier to use to clean the entire piece. At home
repair and paint stores, they sell different varieties for
coarse, medium, and fine grit. The ones sold in the grocery
stores usually work fine and are cheaper.
To protect the aluminum after cleaning is the most difficult
problem. Clear Krylon� acrylic spray works extremely well if
the surface was brushed with a Scotchbrite pad. Other acrylic
paints work well too and they generally have good ultraviolet
resistance for outdoor applications. Phosphoric acid cleaning
leaves a moderately resistant surface, but better protection
is provided by a hexavalent chromium treatment. Commercially
this is known as alodine or iridite treatment. It leaves the
surface with a greenish yellow brown finish with the exact
color depending on its thickness and the alloy involved. It
is commonly found on military and aviation aluminum parts.
Unfortunately, it is exceptionally toxic, carcinogenic, and is
an extreme problem for waste disposal. Some aircraft supply
companies (Aircraft Spruce is one) carry the solution, but I
strongly recommend against using it.
The ultimate protection is generally to have the piece
anodized. This is an electrolytic surface oxidation process
that produces a thick layer of aluminum oxide on the surface
of the metal. This layer is porous and can be dyed. A final
step in the treatment is to heat the aluminum in boiling water
or other solutions which convert the thick porous oxide to a
different crystalline form. The result is that the oxide
layer swells, sealing the pores (which are what absorbs the
dyes), creating a very hard, abrasion resistant finish.
When reassembling the aluminum pieces, remember to clean the
areas of electric contact. Noalox/Penetrox or other similar
compounds help improve electrical contact between aluminum
parts, but the parts should still be still be abraded first to
remove thick surface oxides.
Hardware used around aluminum can cause corrosion problems if
the wrong metals are used. Heavily zinc-coated fasteners are
best from a corrosion standpoint. Zinc and cadmium plated
parts are also suitable but remember the plating is thin and
the parts will have to be replaced periodically. Stainless
steel parts that have not been passivated are useful but
aluminum will still corrode with them. However they are
especially useful when a copper item must be attached to
aluminum. Use a stainless washer between the aluminum and the
copper item. Another approach is to coat the copper with a
heavy coating of tin/lead solder in the area where it must be
joined to the aluminum.
Unless you live in an area with lots of coal burning power
plants and have very acidic rain, you really may not need any
additional surface coating on the aluminum. However, it is
still best to spray all mechanical connections with something
like clear Krylon after things are tightened. This is
especially true of electrical connections.
All of my mailing list subscriptions are in digest form so I
normally do not get in on discussions until after the digest
is sent. In this case, Paul, W8JN, sent me a personal note
asking me to join in!
I always recommend avoiding anything containing significant
amounts of hydrofluoric acid as Mike, AC5P, suggested.
Caustic or acidic HVAC coil cleaners have gone out of favor
because they dissolve too much of the aluminum in the cleaning
process. The modern cleaners are much more mild, attacking
mainly dirt. They also contain biocides to inhibit
mold/mildew/algae growth. The older acidic cleaners contained
as much as 20 percent hydrofluoric acid.
Believe me, you do NOT want to mess with hydrofluoric acid.
It causes extremely painful burns, and if your skin absorbs
enough of it, you die - medical treatment only treats the
pain. If you do not get a lethal dose, the burns are quite
painful and they do not heal for months (and sometimes they
never heal properly). I am amazed that this stuff is still
allowed to be sold except to a skilled licensed user.
To summarize, my preferred methods of cleaning aluminum are
mechanical (Scothbrite pad), phosphoric acid treatment,
caustic treatment. To prevent future corrosion I suggest
anodizing (if possible and you can afford it), paint, or
nothing.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]