Aluminum, steel, magnesium, copper and lead are the types of metals most commonly found in general aviation aircraft. These metals in their pure form are not strong enough to meet the loads of an aircraft in flight therefore metals are combined into alloys to achieve the strength and flexibility necessary to build a light (not heavy) and strong machine that will last a long time.

These aircraft metals chemically want to return to their natural state and when combined with oxygen will start doing just that. One of the tasks of the general aviation maintenance technician is to prevent the metals from returning to their natural state which is called corrosion control. I have not kept track of how much of our job is due to corrosion but I feel it is about 1/3 of the total work we have to do.

Corrosion is a complicated subject and cannot be completely described in a short article such as this one. For further information, an excellent source can be found at faa.gov-regulations & guidelines-advisory circulars-AC 43.13-1B Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair-Chapter 6. This Handbook is considered the general aviation maintenance technicians "bible".

If a scratch happens in the protective paint surface on the aircraft, it not only detracts from the appearance of the aircraft, it allows oxygen to gain access to the underlying metal. The oxygen can then begin to combine with the metal in a process called oxidiation and the corrosion begins. Aluminum and magnesium start to show a white powder and steel will have a "rusty" colored powder as the corrosion progresses. "Rust" and "corrosion" sometimes are considered to be all inclusive for all metals but that is not technically correct. Rust is ferrous oxide and comes from steel. I don't intend on getting into the chemical processes of corrosion as I am not a chemist and don't understand all of what is happening. Our training as a general aviation maintenance technician allows us to stop and prevent corrosion.

To repair a scratch through the paint, the affected area would be sanded to remove any visible signs of corrosion, be treated with an etch (we use Alumiprep), be treated with a chemical conversion product (Alodine), primed (zinc phosphate) and then painted to match the original paint scheme. The difficult part is to match the paint. Paint colors fade and fresh paint does not come in a "faded" version. Many times corrosion will occur in places that include decoritive different color stripes and that complicates the touch-up.

Where two different types of metals make contact is another place where corrosion is often found. Many general aviation aircraft have a step that is attached to the side of the fuselage to assist in entering and exiting the aircraft. This step is often made of steel. The steel step is bolted onto the aluminum fuselage with steel bolts (cadmium plated) and on the inside of the fuselage is a steel doubler which is secured to the main structure to support the weight of the person using the step. These parts are all treated when the aircraft is manufactured but after time and useage the treatment will deteriorate. Part of our job is to make sure there is no dis-similar metal corrosion forming. If caught early, the repair is relatively simple. If caught too late, the step could fail and a person may be injured. I have spoken to a person who was on crutches for several weeks due to a broken leg caused by a step failure. Engines are often supported by a similar steel doubler behind a galvanized steel firewall. Corrosion in this area-although very rare-could have much worse consequences than an entrance step failure.

Most general aviation aircraft use lead/acid type of batteries for starting the engine and for emergency power in case the charging system fails. Corrosion around the battery is very common. The battery is contained in a "box" or has its own cover. The box is vented to the outside atmosphere and has a drain for cleaning that protrudes outside of the aircraft. The battery is vented by two tubes that are exposed to air flowing past a flying aircraft where the ends are cut at an angle. One tube forces air through the box and one tube sucks air from the box to remove the hydrogen gas produced by the battery. Most aircraft manufacturers recommend servicing the battery and checking for corrosion every 30 days.

Lead weights are used to provide proper balance for flight control surfaces to prevent aerodynamic flutter. These weights are attached to the aluminum using aluminum rivets or steel bolts and the dis-similar metals may cause corrosion.

Several products are available to coat metals to slow down the corrosion process but nature wants the metal to return to its original form. This will keep the general aviation maintenance technician busy as long as there are metal airplanes flying. Many aircraft are now constructed of "composites" which are man-made materials such as fiberglass, kevlar and carbon fibers. These materials also have their own special problems which will be addressed here at a later date.