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By Tony Bingelis (originally published in EAA Sport Aviation, October 1995)
Steel rusts. Aluminum corrodes. And yet, you’d be correct in referring to either conditions as "corrosion." Well, so much for the educational stuff… now, let’s look at the problem of protecting steel part from rust.
Understand The Problem
Those steel parts you received for your project, if left unprotected, will soon begin to rust unless, of course, you do something to prevent that from happening. Steel oxidizes when exposed to the chemical-like influence of moisture in the atmosphere. The more humid the climate in your territory, the sooner unprotected steel surfaces begin to rust.
Rusting first becomes noticeable when small freckle-like red surface spots appear on the bare metal. Sometimes the entire steel surface will take on a light reddish tint that, if ignored, will intensify. In time, the rusting process may become so severe that some concentrations often deepen and develop as pits in the metal.
If you are building from one of the more complete kits, you will undoubtedly receive the steel parts already formed, welded, heat treated, and just about ready for installation.
I hate to tell you this but it is most likely your steel components, such as the engine mount and other weldments, as received, will be without any rust protection other than perhaps a somewhat oily surface. Most certainly they will not have been primed or painted.
Before you even think of starting work on your project, I recommend that you first isolate all of your steel parts and prepare them for storage. That’s right, storage.
Face it, you won’t be needing those steel parts for a year or more. In that length of time steel parts, if neglected, could acquire a good bit of rust.
Naturally, builders who live in the dry southwest regions will not be as concerned over the tendency for steel parts to rust as will the builders who live in the Houston, Washington, DC, or in similar regions of high humidity. No matter where you live though, you will nevertheless be faced with three options:
Option 1: Don’t do anything with the steel parts and assume they won’t rust before you need them.
Option 2: Provide temporary protection for the steel parts and put them away until needed.
a. For smaller steel parts, simply grease or oil them and store in sealed plastic bags.
b. For larger components, such as the landing gear and engine mount, wrap them in plastic after oiling and, likewise, seal them. The re-oiled steel surfaces should provide sufficient temporary protection against corrosion for as long as a couple of years. As for the clear plastic wrap, it will permit easy periodic inspections to verify that the parts continue to remain free of rust.
You realize, of course, this quickie treatment merely postpones, until some future date, the real work of cleaning, priming and finish coating your steel components.
Option 3: Provide permanent protection by cleaning, priming and even painting the steel parts, now…before putting them away.
This is a very practical option because you will not have to later interrupt construction to clean, prime and paint each steel part or component as you need it for installation.
Actually, this "do it now, all at one time technique" is a good one because you will be using less primer, less solvents, and less paint. This means you will also be saving time and money. Not only that, you will have gotten a rather unpleasant chore out of the way.
Cleaning Steel and Removing Rust
No matter how or when you intend to finish your steel parts for installation, the very first step is always the same. Clean the parts! They must be free of:
• Grime and oily residue;
• Welding scale;
• Old surface coating or film;
• Surface rust.
Your welded components, heat treated parts, and normalized 4130 steel sheet pieces, as received, will most likely be coated with an oily film or have a dark stained surface which you must clean away before you can prime, plate or paint the parts.
Note: A number of cleaning methods such as electrolytic cleaning, vapor-degreasing and pickling involve complex procedures and are available only commercially. These cleaning methods are neither cost effective nor suitable for duplication in a home workshop.
The cleaning method you use will depend on the condition of the metal and the nature of the foreign matter you have to remove from the steel parts. Here are a few suggestions.
1. For cleaning steel parts in good condition:
a. Use an organic solvent like naphtha, carbon tetrachloride, or benzol to clean away grease, oil and other foreign matter from the steel surfaces. Apply and work the solvent in with a coarse paint brush, or a Scotch-Brite™ pad.
b. Next, rinse the parts in more of the clean (unused) solvent and wipe dry.
2. For cleaning gas welded tubular steel clusters and lightly rusted steel surfaces:
a. Mechanically remove loose scale and light surface rust using whatever works best in the areas being cleaned…abrasive discs, power driven wire brushes, stainless steel wool, Scotch Brite™ pads, emery cloth, and/or wet/dry sandpaper. Follow this by wiping away the residue with MEK or one of the organic solvents, or . . .
b. Sandblast the steel parts. This is a very effective way to clean the parts and ready them for priming and painting. Or…
c. Chemically treat lightly rusted steel surfaces with one of the commercial cleaners for steel…such as Osphos, Rust Remover, Metalprep, etc. (Follow manufacturer’s printed instructions.)
Most of these cleaner/conditioners remove corrosion, clean and etch the metal surface, and neutralize the rusting elements that otherwise would cause new rust to form. Most types also claim to provide a good bond between metal and paint.
Primers for Steel Parts
Protect your freshly cleaned parts with a suitable primer to prevent further oxidation.
1. Epoxy Primers provide a durable surface that resists most chemicals and is also highly abrasion resistant.
2. Red Oxide Primer, as used in automotive circles, is excellent for use on steel components because of its fast film build, quick dry time, easy sanding, and its ready availability.
3. Zinc Chromate (specification Mil-P-8585A) is preferred by most homebuilders. My favorite corrosion preventative for both aluminum and steel is DuPont’s self-etching Variprime™ because it is a good economical base for most types of paint finishes.
Zinc chromate primer is no longer generally available but can still be obtained from major manufacturers through automotive supply and paint outlets as a professional-use paint product (DuPont, Ditzler, Sherwin Williams, etc.).
Before priming a structure or part with zinc chromate, its surface should be cleaned as described earlier. Spray the zinc chromate on in a thin but wet coat to ensure good adhesion between the base and finish coat.
As always, for reliable results it is important that you do not mix brands and that you closely follow the manufacturer’s instructions. Most paint brands furnish informative pamphlets.
Where appearance is not a consideration, many internal structural components, parts and assemblies are left with the zinc chromate as the final finish.
Protection For Welded Steel Fuselages
Traditionally, the interiors of tubular steel fuselages are shielded from rusting by a protective coating of linseed oil. After all welding has been completed, the tubing interiors are then flushed with hot linseed oil. The oil, heated to approximately 160°F. is forced into the tubular structure by squirting it in through the small holes previously drilled into the tubing. A small capacity trigger type oil pressure pump can works fine (see Figure 1).
The amount of hot linseed oil you introduce into the fuselage’s tubular structure should be measured before hand so you will be able to determine, later, that most of the excess hot linseed oil has been drained out after treatment.
Feeling for the warmth and coolness in various sections of tubing, as the hot oil is introduced, will give you some indication of the presence, or absence, of the heated oil.
Rotate the tubular fuselage as the hot linseed oil is being introduced to ensure its insides will be completely oil coated.
After two or three minutes, you can drain the excess oil. When you have determined that most of the oil has been drained out, plug each hole. Use short cadmium plated self-tapping screws to prevent moisture from re-entering the tubing and nullifying your rust-inhibiting oiling effort.
After your tubular structures and assemblies receive their hot linseed oil treatment and have been sealed, the entire tubular welded structure should be cleaned-preferably, sandblasted.
Note: Sandblasting is the most effective and reliable way to clean and degrease welded steel components…especially fuselages and engine mounts. In the process, the parts are blast cleaned by blowing an abrasive against them. The abrasive most commonly used is plain ol’ sand. Although sand is the best abrasive for general use on steel parts and components, other substances such as glass beads are also effective. The sandblasted parts and components should be primed as soon as practicable after they have been cleaned. Otherwise, rust will begin to appear on the freshly abraded surfaces much sooner than you would expect. If you have a 3 to 4 hp compressor you might consider acquiring a small sandblasting rig and do your own sandblast cleaning of small parts. However, it can be a demanding and very dusty messy operation. If you need to clean something as large as a steel tube fuselage, having the sandblasting job done commercially would, in the long run, be a more feasible alternative.
As For The Engine Mount …
Unfortunately, by the time you complete your engine installation, your freshly painted engine mount will have endured quite a bit of abuse. Consequently, as more and more time passes, the paint will often become chipped and in some places may even begin to show signs of rust.
Therefore, I would highly recommend that your engine mount, and landing gear, for example, be painted with an epoxy primer and epoxy paint. An epoxy finish is very hard and durable so ordinary solvents won’t dissolve and ruin that kind of finish.
A suitable epoxy primer, when used instead of the usual zinc chromate primer, will still provide the corrosion inhibiting quality needed and will also provide a good base for a final epoxy or polyurethane finish.Most builders prefer to prime and then finish their engine mount with a light color top coat to make inspection for cracks easier…a good idea.
Protecting Control Cables
Do not paint your control cables to protect them from corrosion. Instead, for stainless steel or carbon steel control cables apply some grease or oil as a protective lubricant and work in with a cloth. If any broken cable strands are present they will catch on the cloth. This also serves as a good method for periodically inspecting the integrity of your control cables.
Preventing Dissimilar Metal Corrosion
When two dissimilar metals, such as aluminum and steel, are in contact with each other, a weak electric current will flow from one metal to the other any time significant moisture is present. This electrolytic action eats away the softer metal as the corrosion develops.
To prevent this type of corrosion from taking place, avoid placing two dissimilar metals in contact with each other unless both metals are insulated from each other by a protective coating of some sort.
The simplest and most practical treatment for our purposes is the application of a coat of zinc chromate primer to each surface.
A typical example of dissimilar metal corrosion is one where you make an aluminum aileron bellcrank installation with steel aircraft bolts, washers, and nuts in direct contract with the aluminum surfaces.
Although the cadmium plating on the bolts normally affords a degree of rust protection, often some of the plating is scraped off during installation and bare metal-to-metal contact takes place. Add moisture, and soon you will see the effects of corrosion develop as a snow-like powder forming on the aluminum parts.
Although a squirt of WD40, or the application of grease or oil, is a fairly good temporary corrosion control measure for such assemblies, you should, at the next inspection, take the assembly apart, clean the areas of direct contact and coat them with a zinc chromate primer.
Where you have a metal-to-wood contact be sure your metal parts are adequately protected by a coat of zinc chromate primer and possibly a finish coat of paint. In addition, before installation, be sure the wood has at least a couple of coats of polyurethane varnish. Then on final assembly, apply a coat of varnish to each contact surface and dip the assembly bolts in varnish before installing them. This will protect them from the rusting influence of the moisture in the wood.
The Exhaust Pipes
In a homebuilt you may find the exhaust system to be made up of either stainless steel pipes, or with the less expensive automotive type mild steel pipes.
A stainless steel exhaust system resists rust naturally and needs no protection. A mild steel exhaust pipe installation, on the other hand, though durable and long lasting, will nevertheless quickly acquire ugly splotches of rust.
A degree of cosmetic enhancement can be obtained by sandblasting the stacks and painting them with a Hi-Heat Resistant paint. The spray can instructions must be followed carefully to enhance the effectiveness and service life of this type of finish.