RV-8F

N772RW

Building, testing and operating an experimental aircraft

Priming and Corrosion Control

04 Jan 2009

Update: (January 2010) See also the entry titled "Surface Prep and Priming" for a step-by-step description of the priming process we ended up using on the RV-8 project. Continue reading below for notes on why we chose the method we did.

This post is a series of random notes on priming the interior of aircraft structures to prevent corrosion. Its purpose is to document my early research findings on the subject. As I move from the planning phase into the actual build, I’ll put together a follow-up entry on my chosen priming method and results.

Primed Parts Primed Parts

According to Wanttaja, “Every item made from 2024 or 7075 alloy should be cleaned, etched, alodined, and primed before assembly.”

You could probably condense the first three processes and just call them “Surface Preparation.”

Some Initial Thoughts

There seems to be a sort of continuum that defines the “RV Internal Priming Theory” spectrum, which runs from “No Primer Required” on the one end, to “Scotchbrite - Alumiprep - Alodine - AKZO” on the other. If it can be reasonably argued that the AKZO, etc. method offers the most thorough protection, and if a case can also be made for doing no priming whatsoever without the airframe turning to dust in your hands, then it’s safe to say that several adequate “middle roads” exist.

I’m finding several builders whose process is similar to this:

I think that’s what I’m going to do. But just for kicks, let’s go over the range of possibilities.

Cleaning

Cleaning means the removal of surface grime and corrosion. Exposed aluminum surfaces quickly form an oxidation layer, which, ironically, helps avoid further oxidation, but it needs to be removed before primer is applied. To break the corrosion and lift surface grime, try a mildly abrasive household cleaner (Dawn?) and Scotch-brite pads. When finished, wipe it down with MEK, naptha or acetone to get rid of fingerprints.

Some enviro-friendly solutions for degreasing include using Citra-Safe/d-Limonene instead of MEK or acetone.

As a straight solvent, d-Limonene can replace a wide variety of products, including mineral spirits, methyl ethyl ketone, acetone, toluene.

Etching

Primer doesn’t like to stick to the smooth, shiny aluminum surface, so we etch the components by hand, with Scotchbrite, or with a weak acid solution called an etchant.

Scotchbrite Pads

Aluminum oxide, silicon dioxide, and silicon carbide are classed as ceramics. Although one aerospace manufacturer specifies aluminum oxide pads for cleaning before chemical film treatment, all three ceramics are okay for aluminum.

You can buy Scotch-Brite 7447 Hand Pads from Cleaveland Tool. Stewart Systems EkoEtch is also a commonly-used etchant.

Chemical Conversion Coatings (Alodining)

Alodining chemically changes the outer layer of aluminum to make it more corrosion-resistant. The end result is a golden-brown tint to the aluminum.

Although I believe the best corrosion control is produced by starting with a chemical conversion coating, very good corrosion control can be had by using two coats of corrosion inhibiting primer in place of the chem film. – Lee H. Erb, EAA Chap 1000 Det 5, Arlington TX; EAA Chap 34, June 1997

The simplest way to prep the aluminum is to wet abrade with Scotchbrite and water, followed by solvent cleaning with acetone, or MEK, or toluene. The alternative is alumiprep 33 acid plus alodine 1201 conversion coating. This is NOT preferred because of the hazardous chromates in alodine. In either case, before priming, it is essential to remove the thin oxide layer that forms on aluminum. Scotchbrite or acid etch will accomplish this. – John Griffin, Sales Manager Akzo Nobel Aerospace Coatings

That’s all I need to hear to skip the alodine process. My new process will be to wipe the parts down with acetone or laquer thinner to remove the oils, then alumiprep, and finally prime. That seems to be the least hazardous method to using AKZO. – Mike Bullock

PreKote

This cool product might just take the place of the first three steps altogether, and is non-toxic to boot.

PreKote can be used on a wide range of surfaces including aluminum, steel, magnesium, titanium, CRES, galvanized surfaces, composite and plastic surfaces. Consistent, high performance results are achieved with a variety of paints and primers.

PreKote is non-toxic, non-corrosive, non-flammable, CFC free, ODS free, odor free, and readily biodegradable upon disposal. PreKote eliminates the forced air containment suits required for traditional chromated conversion coatings. Simple rain gear and goggles are all that is needed.

Priming

A note on priming from Van’s RV-8 Preview Plans, Section 5A:

Generally, it is accepted that two-part epoxy primers provide the best corrosion resistance. However, they are expensive, toxic, heavy and dry slowly all of which makes them problematical for the home builder. If you can tolerate those issues and want your RV to be in good shape when your grandchildren inherit it, they may be “best” for you.

SEM Self-Etching Primer SEM Self-Etching Primer

Self-Etching Aerosol Primers

SEM Self-Etching Primer

Randy Lervold’s notes on interior priming for his RV-3B:

In deciding on the SEM for interior use I did consider the more thorough processes such as acid etch, alodine, and epoxy primer. In the end however, I decided that the materials listed above represent the best balance between the time investment and the protection they afford. I did informal testing with most of the commonly available self-etching rattle cans and found the SEM to be at least as durable as any of them, but left a nicer finish. [emphasis mine]

Brad Oliver’s thoughts on epoxy vs. self-etching primer for his RV-7:

I hate mixing and spraying this epoxy crap! I’ve been using the rattle-can self-etching stuff for smaller parts recently, and it is sooo much easier. No mixing, no fancy equipment, no waiting hours or days for it to cure. If I had to do it over again, I would have used the rattle-can stuff from the beginning. [emphasis mine]

Sealing the Faying Surfaces

The “fayed surfaces of similar metals” are joints where two parts contact each other, creating an environment for corrosion. Seams and joints that possess fayed surfaces of similar metals must be protected, at a minimum, by application of primer coating to each surface.

On installing rivets “wet”: Just before riveting two pieces together, brush (spray or lightly wipe) the faying surfaces with primer, then install the rivets “wet.” If the rivets are inside and not exposed to weather, the “wet” can be primer. If one end of the rivet will be in the “weather,” then use sealant. Use a “Q” tip to put in a lot of primer just before placing the rivet in the hole. If you are using sealant, experiment a bit so that you don’t get too much squeeze-out.

Why the rivets should be installed wet: The deburring [process] leaves a small chamfer which may or may not be filled with rivet when it is installed. Installing “wet” pretty well ensures that there will be no void for electrolyte (water) entrapment. – EAA 1000

References

  1. AvWeb.com Corrosion Control
  2. Muroc EAA 1000 Corrosion Control Articles
  3. Aviation Maintenance Technician Handbook, Chapter 6
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