Video Review of “CORROSION”, Part of EAA’s
“Build Your Own Plane” series
This video is basically a lecture by Geo Hindall,
and although it is geared toward sailplanes, he makes it clear it
applies to all aircraft. In
addition to the lecture, it has lots of great film clips of seaplanes
taking off and landing. It
is divided into 6 sections: Causes
of corrosion, Inspecting for corrosion, Corrosion prone areas, Types
of Corrosion, Maintenance and Protection, and Float Plane Storage.
Geo starts off recommending the FAA’s Advisory
Circular AC43-4A (published in ’91) and which had to his knowledge,
the only color photos in any AC.
A.
Causes of Corrosion
A cursory explanation of
galvanic ion exchange is presented, with emphasis on the fact that
corrosion occurs on practically all metals.
He shows examples on aluminum, steel (both tube steel and
stainless – stainless steel is merely stain and corrosion resistant,
not corrosion proof), magnesium, and copper.
B. Inspection
Other than the great
shots of seaplanes in action, this is the one area were the video
helps in that you can see the different kinds of corrosion in action
on the metal, paint, etc. He
emphasized the need to build inspection ports, to stay ahead of the
corrosion before you have to replace whole sections, and using mirrors
and flashlights, etc. in the ports.
C.
Corrosion Prone Areas
The areas of greatest
risk are:
1.
Exhaust gas trail – as the exhaust leaves the plane, it forms
a trail over the surface of the plane that elevates temperature and
provides corrosive gases that do a number on paint and edges. Regularly inspecting this exhaust trail is critical.
2.
Battery Compartment – when charging or when a rogue
alternator overcharges, the battery can emit corrosive gases or liquid
that causes rapid damage. Often,
a battery compartment will be vented under the fuselage in the same
stream as the exhaust, giving a double threat.
Check the battery vent trail.
3.
Spot welds – in some construction, spot welds are used and
are susceptible to corrosion, often under the paint.
4.
Magnesium – magnesium castings or other parts are very prone
to corrosion due to the galvanic value of the metal as well as it’s
porosity. Check all mag
parts.
5.
Wheels – water can often stand or be trapped in wheels.
Checking them regularly might rescue them from early
replacement.
6.
Cables – regular steel or, less often, stainless steel cables
are susceptible as well. One
test is to sharply bend the cable 90 degrees.
IF it springs back, its ok, if it stays bent, time to replace
it.
7.
Engine compartment – any areas that are exposed to high temps
or subjected to exhaust gases are suspect and need close inspection.
D.
Types of Corrosion
Surface texture that
looks like dirt is how it begins, but when you clean it, you start to
see the pitting beneath. Over
time, the pitting gets more severe and emerges from the other side of
sheet metal, or penetrates the grain of thicker pieces.
Localized pits can join together to cause whole sections of
uncorroded sections to fail. Cracks
can develop from edges or along extrusions.
Corrosion can get sever under a coat of paint due to poor
adhesion of primer, usually starting around rivet holes or other
edges.
E.
Protection
To treat a corroded
part, take the surface down to bare shiny metal with 360-400 grit
sandpaper (wet or dry) and a brass (not steel) brush.
Use a cleaner for aluminum, like Dupont’s two part 225S and
226S cleaner. Then, use
AlumaDyne (a phosphoric acid etch) to prep the surface for proper
adhesion. He says you can
use Zinc Chromate spray for spot repairs (with the etch) but for
larger pieces he recommends something else. (Personal note from reviewer – I saw zinc chromate being
applied at a rudder workshop directly onto new aluminum without the
etch, and indeed saw the “beading up” of the chromate on the
aluminum, like water on a waxed surface.
We initially thought the aluminum might be oily, so we tried to
clean the surface with some lacquer thinner (about all we had) but
this only helped, but didn’t stop, the surface tension problems.)
For new aluminum pieces
he recommends an Epoxy etch – he showed Dupont Corlar 824 and 826
epoxy to be used after the etch step.
He also highly recommended using the lightest color on the
parts to make inspection for cracks, etc. much easier, especially in
interior areas that are illuminated by flashlight.
For new steel pieces,
like tubing construction, he recommended Variprime self etching
primer.
For lubrication
and corrosion protection of metals in linkages, etc. he recommended
ACF 50, and said everyone should have a can of LPS3 spray.
He also said that good ole auto transmission fluid is one of
the best corrosion inhibitors around, and he sprays the inside of
seaplanes with a air sprayer. Also,
linseed oil in the steel tubes helps them quite a bit.
F.
Float Plane
Storage
This is something he
defined as a condition where the plane is not being used for more than
1 week. He recommended
cleaning it thoroughly, opening up all hatch ports and letting them
drain, and leaving them open to dry.
He also recommended inspecting the aircraft for place where
water is likely to stand, and drilling pin holes for them to drain,
and then making sure that they remain clear to drain. |