Oh Boy, Where to Begin…?
This is one topic in the “Never Ending Debates” category. On the web you see all sorts of opinions and conjecture. Well Dan and I are engineers and we like data. Before we get too far, Jeff Shultz has an excellent webpage on this topic, you can read it here and choose “Corrosion Protection” on the left menu.
Oh yeah, here is a fun read, FAA AC 43-4A, FAA Corrosion Control For Large Aircraft.
Here is a summary article from Avweb, Corrosion Control
Some basics about aluminum corrosion that I have learned from the book “Aluminum: Technology, Applications, and Environment. A Profile of a Modern Metal” 6th Edition, by Dietrich G, Altenpohl.
Metals occur in nature in their most chemically stable state. For aluminum it is a hydrated oxide form called bauxite. The bonds between the metal and the oxygen are very srtong. Give the right conditions in an electrolytic cell with enough energy, the aluminum can be seperated from the oxygen. Now the aluminum is at an artificial higher energy state. Corrosion will tend to return the aluminum back to the stable aluminum oxide form. So we need to create a shell around the metal to prevent corrosion reactions
Oxide Layer
Alumium will react with oxygen in the air to form a thin oxide layer on the surface, this creates a natural shell around the metal. This transparent layer helps in protecting the aluminum from water and chemicals that could cause corrosion. Essentially the oxygen creates the oxide layer which then block further oxygen from reacting with the aluminum. Great stuff!! To contrast, with iron the oxididation process does not create a barrier. The iron oxide, rust, will eventually flake away exposing the iron underneath to the oxygen. So the rust will continue into the part.
Many people think that aluminum cannot corrode due to this oxide layer. But it is not perfect, this thin layer has “pores and weak points” that can allow external liquids and gasses to get to the aluminum. From what I read, if corrosion starts at a pore in the oxide layer, frequently the corrosion percitpates will “fill up the pore” and prevent/block the external gas or liquid from further corrosion. Some call this self healing corrosion pitts. (Some peole may have seen small bits of white powder on sheets of alumiunum. These are probably the precipitates from corrosion pitts. Form what I understand the powder can be cleaned away, the pitts sanded out, and a new oxide layer will form in that area.) Now with a very aggressive environment the corrosion percipitates may not be able to block the external environment. What would start the corrosion through the pores? One example is an industrial atmosphere where chemicals might condense from the air onto the aluminum and sit there for a while.
So this oxide layer is pretty great for corrosion protection. For example, 2024T3 Alclad sheet metal is 2024 alloy aluminum with a thin layer of high purity aluminum bonded on both sides. The pure aluminum creates that oxide layer and protects the 2024 alloy underneath. The “cladding” thickness is 5% for sheets under .062″ and 2.5% on sheets .062 and over. So for .032″ thick alclad 2024-T3 the aluminum cladding on each side is .0016″ (or one and half thousands).
Types Of Aluminum Corrosion
Chemical Attack – The oxide layer is resistant to solutions with a pH values between 5 and 8. There are certain acids than can eat right through the layer. (This is how one can “etch” the aluminum, more on this later)
Localized Pitting – We touched on this a little before
Intergranular Corrosion – Usually a result of poor manufacturing processes
Exfoliation – Corrosion parallel to the surface
Stress Corrosion Cracking
Galvanic Corrosion – When aluminum and another metal contact in a conducting fluid, corrosion can occur.
What Can Be Done, Barrier Coatings
Anodizing – A process unique to aluminum where the metal is a anode in an acidic solution. A current is applied and an artificial oxide layer 250-3000 times thicker than a natural layer is formed. The part is immersed in boiling water to seal the film. There are many different types of anodizing due to the acidic solution used. The trouble with anodizing is that is will reduce the fatigue life of the part and this is critical for aircraft parts. Some say anodizing results in a brittle coating chemically bonded to a ductile core. If so, a crack in the brittle coating may result in a stress concentration on the core. Anodizing structual parts is out (at least for homebuilding).
Conversion Coating – This process replaces the natural oxide layer with a chemically reacted coating of aluminum oxide and chromic oxide. They provide excellent corrosion resistance and are an excellent binding layer for coatings, like primers or paints. The traditional conversion coating used contains hexavalent chromium (Cr6+). This is a known carcinogen!! Nasty stuff!! Of course the military has been using it for years and you can still but it at auto body supply stores. Common trade names are Alodine 1200 or Iridite. For our airplane parts the additional work includes: Personal Protection, Part Preparation (cleaning & etching), and Responsible disposal of this Hazardous Material.
Along comes RoHS, which is Reduction (or Restriction) of Hazardous Substances. This EU directive restricts/reduces the use of six nasty materials and hexavalent chromium happens to be one of them. So the industry and the military are hard at work inventing new polymeric coatings to replace the chromium ones. Some military technical papers indicate the performance of the new coatings isn’t as good as the chromium but they are much easier/safer to apply. Some trade names include Pre-Kote (available at Spruce), Iridite NCP, and Alodine 5200(?).
Tim’s assumption: Conversion coatings may not be very tough to resist scratches an wear.
Primer Coatings – The most common is a two part expoxy primer. These primer will seal the part and provide a tough shell. There are lots of vendors from PPG, Sherwin Williams… A lot of these products some from the autobody repair industry, although a few are aircraft specific. The challenges include: part preperation, two part mixing batch mode, need to toss if you have extra, personal protection, & clean up. For Sonex builders, we need to think about the weight the epoxy primer may add.
more to come…
6061 & 2024 Alloys
Now the Sonex is made with 6061-T6 aluminum, Alcoa Data Sheet. The chemical composition contains lower amount of copper than 2024. Copper is one of the culprits in corrosion reactions. However, 6061 has a lower ultimate strength than 2024. So we have a trade off, lower ultimate strength but better corrosion resistance.
Alloy Ultimate Tensile Yield Tensile Corrosion Resistance
Strength (ksi) Strength (ksi) Rating
6061-T6 38 35 A-B
2024-T3 62 45 D
2024-T3 Alclad 63 42
According to the Aluminum Association, a Corrosion Resistance Rating of A-B is “used in industrial & seacoast atmospheric conditions without protection.” While in the Alcoa 2024-T3 Alclad link says the 2000 alloy series (2024 included) “are susceptible to atmospheric corrosion especially in industrial or seacoast atmospheres. These alloys should be protected…when exposed to these conditions. Alcladding … provides high resistance to atmospheric corrosion.” Notice the ratings say “industrial & seacoast atmospheres”, these environments would have slat and other chemicals in the air that could react with the aluminum.
Bare 6061
So it appears the 6061 composition and the oxide layer this material has great corrosion resistance. BUT, as we learned earlier, it can still corrode. Given the right conditions and the chemicals corrosion can occur.
From what I understand Sonex has used bare 6061 for all of the company planes, and they do not require or encourage priming.
Two Different Opinions:
1) It’s 6061, don’t worry about it. The plane will outlast you as far as corrosion. Save the weight
2) I’m building a airplane, a bit of extra work would buy a lot of corrosion protection.
Each builder will have to figure out what they are comfortable with. One pertinent opinion I heard was to do some corrosion protection in areas that you cannot inspect, like inside wings and other structures that are closed up.
Priming Options:
Listed in increasing order of protection (in Tim’s opinion)
A) Nothing
B) Mating Surfaces with Rattle Can Self Etch Primer
C) All Interior Surfaces with Rattle Can Self Etch Primer
D) All Interior Surfaces with Rattle Can Self Etch Primer, Top Coat With Rattle Can Primer
E) All Interior surfaces with 2 part Wash Primer (self etch)
F) Alodine only (Not sure how this compares with self etch for abrasion/scratching)
G) Alodine , Top Coat With Rattle Can Primer
H) Alodine , Top Coat With Rattle Can Primer, Rattle Can Paint/Clearcoat Cockpit
I) Alodine , Spray Gun 2 Part Epoxy Primer (ULTIMATE)
J) Alodine , Spray Gun 2 Part Epoxy Primer, Paint/Clearcoat Cockpit (ULTIMATE, but also heaviest)
*** There are water based products from http://www.stewartsystems.aero/ that can be substitued for some of the above ***
*** There is a newer coating called PreKote by Pantheon Enterprises. Here is a Technical Data Sheet PDF about it. I think this what the mililtary has switched to. ***
Factors in evaluating coatings:
1) Abrasion / Scratch resistance
2) Fluid resistance – Fuel, other chemicals
3) UV resistance – mostly for cockpit area.
4) Adhesion to substrate and for coats on top
Process Steps:
Self Etch
Scuff parts with Scotchbrite
Clean the parts (de-grease / Dawn detergent / water should sheet)
Rinse Well
Wipe with a solvent (Lacquer Thinner,…) allow to dry
Spray with self etch primer
Alodine
Scuff parts with Scotchbrite
MEK or Acetone
Clean the parts (de-grease / Dawn detergent / water should sheet)
Rinse Well
Alodine (also called conversion coat)
Rinse Well
Spray with top coat
Different Products There are a lot of opinions and new options being used, for example this is a video of what the Zenith Aircraft company uses
Confused yet? Please remember this is just one guys thoughts on primers, use at your own risk….
What are we doing? (March 2011)
Well Dan an I have finally started assembling some parts and right now we are just spraying with a self etch primer. We bought a bunch to test but started using Mar-Hyde and SEM. It looks good so far. We started with an aerosol can but we will buy a gallon and use a decent spray gun.
I want to compare the two, so we might run a crude test of some 6061 coupons in a salt water jar. We had actually started constructing our own Salt Fog Chamber to mimic the ASTM test. After doing a lot of reading from the DoD the test may take months and then it is still a subjective comparison and only a relative comparison between the primers
We think that self etch priming and then epoxy primer on top might to a bit heavy for the Sonex (remember 640 lbs). We’ll update this page if we try something else.