Corrosion Protection--Introduction

Lee Erb, Chap 1000 Det 5, Arlington TX, Chap 34

Originally published January 1997

(This begins a series of articles on corrosion control of aluminum structures promised to us in the November 1996 "The Leading Edge.")

As I told our newsletter editor, "If my grandchildren are going to ride in your airplane, then you better build it so that there is no hidden corrosion." I kept telling him what to do (he did!). He then told me to either "put up or shut up" (I did!) and write what to do and how to do it. I am not about to shut up when it comes to my grandchildren (as for his own kid...).

Please note: As a designer my job is to specify what is to be done. As a builder or mechanic I have little experience in "how to." In later articles I will rely on co-workers to help me define "how to." Perhaps I can cover some subjects showing the differences and similarities between aircraft manufacturing and homebuilt "industries".

There are many types of corrosion. This article is strictly a brief introduction to the subject. To prevent the most common types of corrosion in aluminum structures (sheet metal and extrusions), I specify the following procedures during building:

  1. Inside structure not exposed to weather
    1. chemical conversion coating
    2. one coat of epoxy primer
    3. seal faying surfaces
    4. install fasteners "wet"
    5. (I personally like to add a coat of primer after assembly because of condensation)

      6.  
  2. Inside structure occasionally exposed to weather (baggage compartment or under fabric)
    1. chemical conversion coating
    2. two coats of epoxy primer
    3. seal faying surfaces
    4. install fasteners "wet"
    5. one or two coats of paint (two for looks)

      6.  
  3. External surfaces (not natural aluminum finish)
    1. chemical conversion coating
    2. two coats of epoxy primer
    3. seal faying surfaces
    4. install fasteners "wet"
    5. two or more coats of epoxy or polyurethane paints
One of the chemical conversion coatings is Alodine 1200 made by Amchem Products, Inc, Amble, PA. This is the recognizable name and Alodine 1201 (for brushing) is readily available from Aircraft Spruce and Specialty Company. Charlie Wagner has covered this quite thoroughly with Chapter 1000 (The Leading Edge, Oct-Nov 1996). The two things I wish to add is that I recommend brushing (as Aircraft Spruce mentions in their catalog) and be careful of the disposal of the rinse water. I will try to have more on both next month.

There have been many changes in primers since I learned to spray zinc chromate on everything. More on them in the future.

Sealing faying (adjoining) surfaces is the most important to prevent hidden corrosion. The idea of sealing faying surfaces is what the old ship builders did using pine tar between planks (faying surfaces) to prevent leaks.

As a sealant, the Navy primarily specified MIL-S-8802, but now there is a lower density AMS 3281, which the Air Force is now specifying. AMS 3281 has a limited shelf life, but is available in small cans (from where, I don't know yet). There are other specifications and some questions of availability which I will research.

Some people seal using only primer during and after assembly. Sealing with primer is usually not sufficient because this procedure tends to not fill voids, which allows moisture to collect. Spraying primer after assembly does help seal joint edges but not voids.

Sealing faying surfaces is as important between "same metal" surfaces as between dissimilar metals. Later I will present a horror story or two that should convince you to take the extra time to "seal faying surfaces".

Sealing faying surfaces takes care of electrolytic corrosion, of which salt water corrosion is the best known. It does not take care of intergranular, exfoliation, or stress corrosion. There are also concerns about electrical bonding too. More on them later.

There has always been plenty published on paints, but in the future (sometime) I will review those systems I have specified for specific applications.

A good practical book for maintenance is "Aircraft Corrosion Control," originally published by IAP, Inc. According to "Craig" at 1 (800) 443-9250 the book order number is now JS312630 from Jeppesen Sanderson, 55 Inverness Drive East, Inglewood, Colorado, 80112. It is also available from EAA (1-800-843-3612, stock number 21-37597, $10.95)

More Later.

(Frank Roncelli, Technical Counselor, EAA Chapter 49, was heard to say on a recent project tour that he once had the opportunity to open up a section of an old DC3/C-47 wing. Anywhere two pieces of aluminum touched, i.e. rivet joints, there was significant corrosion. Portions of skin panels that did not touch other structure were still shiny bright like new. Douglas may not have thought that these aircraft would continue to fly as long as they did, but I don't think we want to build our aircraft with a wartime mindset, i.e. the aircraft would be shot down within a month or two, so corrosion control was not important.--ed)
 

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Revised -- 21 September 1997