Corrosion protection/Primer
Aluminum
The most important factor in using aluminum is to understand its corrosion mechanism. Aluminum reacts with the atmosphere
(oxygen) and turns into aluminum oxide. This is a white powdery appearing substance that most resembles white mildew. As the
layer of aluminum oxide on the surface of some aluminum thickens, the reaction rate slows and eventually stops. The oxide
layer is usually described as protective. The oxide layer will always grow in thickness until oxygen is no longer able to
penetrate it to react with the underlying aluminum. The reactivity of the aluminum or aluminum alloy, and the conductivity
of the environment, determines the rate and thickness of the oxide layer. Pure aluminum is not very reactive and so its oxide
layer is very thin, 2024 is very reactive aluminum and its oxide layer will be much thicker. A humid and/or salty environment
(near an ocean for instance) is more “conductive” and the oxide layer on aluminum in this environment will be
thicker than would occur on aluminum in the hot dry climate of Arizona. It is important to understand that this oxide layer
starts to grow whenever the situation allows it, i.e. right after you remove the oxide layer, say in preparation for paint,
as the oxide layer begins growing immediately. This means you must not let prepared aluminum sit for very long before you
paint it, overnight at the most, or you run the risk of the oxide layer re-growing enough to inhibit the adhesion of the paint.
Corrosion
Corrosion fixatives
Alodine and anodizing are simply techniques to harden the oxide layer to make it more durable. If you run a fingernail
across raw unfinished aluminum you will have scratched through the oxide layer, even if you can’t see the oxide layer,
oxygen will have better access to the aluminum under the scratch and more aluminum will react with the oxygen and turn into
aluminum oxide. The point is the oxide layer, as a loose powder, is easily penetrated. Alodining puts chromate and xxxxxx
in solution with the aluminum and creates an aluminum xxxxxxx compound which is attached to the aluminum rather than sitting
on top of the aluminum like the oxide layer. You can scratch through an alodine layer but it is harder to do so and it is
very resistant to being rubbed off. So the oxide layer is stable, and non-reactive (inhibits oxygen transfer) despite its
being a very thin layer. Since this aluminum-xxxxxxxxxx-oxide layer is an organic compound (is it ???????) it breaks down
after some years, 10-20 is typically referenced a the life of an alodined surface. Anodizing however, add more steps
to create a harder surface than the alodined surface. This is a very hard surface, more akin to alumina (an extremely
hard ceramic made from aluminum-oxide) than aluminum. The coloring that usually accompanies anodizing is simply dye. Some
of the anodized techniques result in a porous surface is very receptive to picking up dye particles (interstitially) which
are then trapped after the layer is hardened.
Anodized layers to not allow oxygen to permeate through them and they are not conductive and so they are perhaps the ultimate
aluminum anti-corrosive measure.
Prep
You aren’t scratching the surface to create tooth for the primer or paint, you are mostly trying to remove the oxide
layer so the paint can attach to the raw aluminum rather than the oxide layer. The roughening of the surface does add tooth
to the surface that increases the paint adhesion but this is just a side benefit. Paint will not stick to aluminum with an
oxide layer no matter how rough the surface is. The effect is similar to trying to stick tape to a dusty surface, the tape
ends up stuck to the dust and not the surface.
The water break test is the gold standard for this. Details of this technique can be found elsewhere. Chemical etching
is another effective way to remove the oxide layer using an acid. All of the acid must be neutralized with water after etching.
Self-etching primers have some of the chemical etching acid mixed into the paint. The aluminum must still be clean for self-etchers
to perform well as the weak acid used in the primer cannot penetrate oils and other contaminants. I find an extensive solvent
wipdown followed with a soap and water scrub results in a beautiful shiney surface that self-etching primers stick to very
well. The soap and water is required and must come after the solvent cleaning. Soap is “basic” the chemical opposite
of an “acid” also know an a hydroxide (OH). Rinsing your aluminum in a basic solution creates aluminum hydroxides
on the surface which increases the reaction with the primer.
Primers
Primer is good. Properly applied primer is better. Properly applied primer under paint is best.
Primers really stick to an Alodine surfaces. This is very reliable, and the only way to go if you are using a regular primer
(not self-etching). You cannot use regular primer on bare aluminum. It won’t stick for very long, if at all. You can
use self-etching primer on properly cleaned aluminum (see above for technique) however, you must get the self-etching for
aluminum and not the version of self-etching primer made for steel. The steel self-etching primer does not etch aluminum very
well at all.
The Sherwin Williams self-etching wash primer that VANS uses on quick builds is not really a good choice for homebuilders.
A wash primer is only an adhesion layer and provides little corrosion protection and it is not very durable. VANS uses it
to prevent surface corrosion from forming during the voyage in a shipping container from overseas. If they didn’t, the
salt air would cause a visible white oxide layer in a very patchy way on the surfaces both inside and out. The customer would
not react well and this is really not the best state for an airplane wing or fusalage to start life. I doubt airplane design
engineers, given a choice, would specify a salt air soak as part of the manufacturing process for an airplane. The Sherwin-Williams
product protects the surface far more than is needed for the ocean voyage so you get some residual protection, however, it
does not approach the protection of non-wash type primers, alodining, and/or paint over primer. Wash primers are also supposed
to be top-coated with paint or regular primer withing days of applying the wash primer. If the topcoat is applied months later,
then less than optimal adhesion of the topcoat is expected. I am not saying that Sherwin-Williams self-etching wash primer
is no-good, I am just pointing out its limitations. I use the stuff myself quit a bit because it is EASY to use and it protects
ok . So for an enclosed space that won’t see much, if any moisture and if a little corrosion developed it would compromise
the structural integrity of the aircraft, then shoot some S-H wash primer.
Alclad
Alclad, the aluminum used in most of an RV, is not magical pixy dust. It is a composite made with a core of 2024 aluminum
that has been coated with a thin layer (<.001 inch) of pure aluminum. 2024 is very strong, very light, and very reactive.
Pure aluminum 1000 is very weak, very light, and NOT very reactive. However, the pure aluminum will still have an oxide layer
as all aluminum must, and this oxide layer can be eaten through in a corrosive environment (humid salty sea air). Scratches
will usually penetrate the pure aluminum layer. Many have come to believe that the pure aluminum coating is just as effective
a properly primed surface (primer over alodine) and this is not true. The alclad will corrode and can corrode badly, primer
cannot corrode unless the primer is compromised. I am not saying that the inside of an airplane must be primed, or that alclad
is no good, I am just detailing the actual behavior of aluminum and of coated aluminum. I personally focused on overlaps,
and primed the lapping surfaces such as a wing rib flange to wing skin joint. These lap joints trap moisture and dirt and
create the ideal conditions for an electrolytic corrosion cell and are where most/all corrosion that is enough to be damaging
occurs. As long as I was spraying primer on the flange surfaces I usually found it easy to wave the gun around a little more
and put a light layer of primer on any adjacent surfaces.
Bending Aluminum
Aluminum is soft in comparison to most metals. It yields in strain and exhibits hysterisis below the yield point. The very
non-linear stress strain curve of aluminum reflects this behavior. What this means to you is that if you bend aluminum you
stretched it, and the material became thinner in the stretched region. The thinning is where the extra material “created”
comes from. The picture attached demonstrates where this becomes a problem. Since a spar is structural bending damage is especially
bad and such parts must be discarded rather than repaired. Take heart, as all or part of this part can often be used later
in the construction of an aircraft, with the damage area removed. Mounts for ELT’s, antennas, auto-pilot servos, etc…
Damage on the edge of a sheet like is shown here xxxxxxxxxxxxxxxxxxx are another example of where the stretching must be considered.
Attempting to bend this back to straight will not work. The extra material must be dealt with. A large dent can be turned
into smaller dents - think about how you dimpled the flanges of ribs to “shorten” the flange, this would be similar.
The downside of any further manipulation of this damaged area is that you will be work-hardening the material and a crack
will almost certainly develop in this damaged area. Leave the dent and let the painter use filler to make the outside surface
smooth and cosmetic.
