Corrosion Control - Sealing Faying Surfaces

Lee H. Erb, EAA Chap 1000 Det 5, Arlington TX; EAA Chap 34

Originally published June 1997

Why?

The idea of sealing faying surfaces is the same as the old wood ships using pine tar between planks (mating surfaces) so that the were no leaks. Also it kept the water from being trapped between structure and rotting (corroding) the wood at the faying (contacting) surfaces.

Years ago at the beginning of adhesive bonded metal helicopter blades there were a number of blades that became unbonded during service use. This was due to the adhesive wicking (ever so slightly) water though the bond line. The water then caused corrosion of the metal underneath the adhesive. The metal essentially became powder at the bond line. Obviously additional research was quickly conducted into other adhesives and surface corrosion preventative coatings which eventually greatly mitigated the problem.

The Navy has always (at least during my professional career) required sealing of faying surfaces on their aircraft. Bare 2024 aluminum can corrode in hours. Think of the Navy trying to keep their planes from corroding on carriers in salt air.

The Air Force, desiring longer operational lives, now requires improved sealing of faying surfaces, bushings and bolts on all new designs using metal.

This article covers the sealing of faying surfaces only. You should have already read the article on chemical conversion coatings (which I sometimes call "chem film" (either as a noun or as a verb)) (Corrosion Control--Chemical Conversion Coatings on Aluminum Alloys). There are two parts to "sealing" faying surfaces: (1) the surfaces in contact and (2) the edges where an electrolyte (water) could enter.

It may appear that I use "primers" and "sealant" interchangeably. When I use primer, I mean primer. Whenever I use the word "sealant" or "sealer" I mean MIL-S-8802 or similar.

MIL-F-7179 Requirements

This MIL Spec, Finishes, Coatings, and Sealants for the Protection of Aerospace Weapons Systems is the spec for the what and how of corrosion control. It covers the primary metals and features of aircraft, missiles, and missile sites. It is used here as the basis for discussing aluminum structures as it can apply to homebuilt aluminum aircraft faying surfaces.

Fayed Surfaces of Similar metals - Seams and joints that possess fayed surfaces of similar metals shall be protected, at a minimum, by application of primer coating to each surface.

In addition to primer coating, all faying surfaces that have an exterior edge shall have the faying surfaces, seams, and edges sealed with a sealant other than primer.

A minimum gap of .020 inch is required for exterior surface butt joints of similar metals.

Fayed Surfaces of Dissimilar metals - Fayed surfaces of dissimilar metals shall have a primer coat and be sealed with a sealant.

Exterior surface butt joints of dissimilar metals shall have a gap of .060 to .120 inch. The depth of the groove shall be of sufficient depth to retain the hardening type sealing compound, which shall be subsequently applied and smoothed flush with the surface of adjacent dissimilar metals.

Recommendations for Home Built Aircraft

The basic procedure of aluminum to aluminum sheet metal structures is given here. The recommendations are a compromise for reasonable corrosion control, time to fabricate, and life cycle cost (you have heard of, "Pay me now, or pay me later").

There are many variations on the basic procedure. A few will be mentioned now with others cropping up in future discussions. From time to time each person will have to make "design compromises" between corrosion control and other requirements.

For illustration purposes, Figure 1 shows a sheet metal clip attached to two other pieces of sheet metal. For clarity of construction no sealant is shown. This represents attaching a wing rib to a spar or attaching a frame to a skin, or installing an intercostal between frames for mounting a component.

Figure 1 - Illustrative Clip (Don't Use for Rivet Spacing)

After the rivet holes have been drilled and cleaned, prepare Class 1A chemical conversion coating on both pieces that are to be joined. Be careful and not over do the chem film dwell time (Overdone it can weaken the edge of a countersunk hole). When the surfaces have hardened (cured) it is a good time to put on a thin primer coat to assist in protecting the chemical film.

(Some people will chem film then drill and clean the holes. They dab primer on the bare places. I question why do just a good job when it is easy to do an excellent job of corrosion control.)

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. It is a skill.

Figure 2 shows why it is good to prepare the rivet holes before chem film and why the rivets should be installed wet. After drilling the holes of both sheets, the holes need to be deburred. We always used a large twist drill. The deburring 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.

Figure 2 - Sealing a Rivet

Figure 3 shows how to seal the edges of clip with a sealant. This should be accomplished whether the joint is inside or will be exposed to the outside.

Figure 3 -- Sealing the Clip

Note in the view of Figure 4 that the corners of the clip are chamfered (they could be rounded). It will be difficult to effectively seal a 90-degree corner. There are other design reasons for chamfering corners. You probably know a few if you have worked with sheet metal.

Figure 4 - Top View of Sealing the Clip

Lap joints - Many designs have lap joints (Figure 5) in the fuselage skins. If the outside is to remain natural aluminum color, then mask the panel so you can put a chemical conversion coat where the panel is overlapped on the outside.

^outside
Figure 5 - Outside Lap Joint

I would chem film the entire inside skin. The coating can be accomplished after all of the holes are drilled and cleaned. Carefully mask the outside of the rivet holes if natural finish on the outside is to be maintained. Apply a thin coat of primer on the cured chem film.

Just before installing the rivets, apply a thin coat of sealant to each fayed surface. Install the rivets wet with sealant. As in Figure 5, seal the inside edge of the lap joint. On the outside clean off any sealant squeeze-out before it cures. For the rivets where moisture might collect on the inside, even though they were installed wet, I would also seal them as shown in Figure 2.

There are many ways to prepare the outside surface and keep the natural finish. There is a clear chemical conversion coating but I am not yet familiar with it.

Faying Surfaces of Dissimilar Materials

Sealing faying surfaces of dissimilar materials has much more to it than the simple statements made in MILF7179 Requirements. It will be best to present the subject in the future with the discussion of the galvanic table and electrical bonding.

Electrical Bonding

By now, what with all the sealing of faying surfaces, you are probably curious about electrical bonding for static electricity, lightning, grounded electro-mechanical components and EMI (Electromagnetic Interference). "Electrical bonding is an art."

Please wait for a future article: "Three Rivets in the Bulkhead." What I will have to say (write) can not be sung to "Three Coins in the Fountain," but remember it because we will talk about it more. For now, think about installing three rivets, next smaller size, "dry" in locations where there is likely to be corrosion. Some of these locations might be on the bottom of the tail boom of a tail dragger and at the low spot in the fuselage for an aircraft with tricycle gear.

Caveat

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. I would insist on chem film in factory aircraft, but I perceive that Homebuilts receive more Tender Loving Care.


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Revised -- 20 December 1997