Review of EAA Tape on Corrosion


 

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.

   

Images on this website are either Copyright Zenith Aircraft Company and used by permission or are copyright Gary Liming