Corrosion Control

Corrosion Control Controlling corrosion damage through effective inspection and treatment By Joe Escobar September 2001 The average age of the aircraft in use today is getting older. Many mechanics are maintaining aged aircraft and...


Dye penetrant
Dye penetrant inspections aid in inspecting for large stress-corrosion or corrosion fatigue cracks. In this process a dye is applied to the clean surface to be inspected. The penetrant is absorbed into flaws by capillary action. Once it has been allowed to dwell the allotted time, the excess dye is removed and a developer is applied. The dye that was absorbed in the flaws is then drawn to the surface by the developer, giving a visual indication of the fault.

Magnetic particle
This type of NDI can be used for detecting cracks or flaws on or near the surface of ferromagnetic metals. A portion of the metal is magnetized and finely divided magnetic particles are applied to the object. Any surface faults create discontinuities in the magnetic field and cause the particles to accumulate on or above the imperfections.

Eddy current
Eddy current testing (primarily low frequency) is useful in detecting thinning of material due to corrosion and cracks in multi-layered structures. It can also be used to some degree for detecting corrosion on the hidden side of aircraft skins when used with a reference standard. High frequency eddy current testing is useful in detecting cracks that penetrate the surface of the structure.

X-ray inspection
X-ray inspection has limited uses for detecting corrosion due to the difficulty in obtaining the sensitivity required to detect minor or moderate corrosion. In an X-ray inspection, X-rays are passed through the material. A film placed on the opposite side of the material is exposed to these X-rays. The film is then developed and inspected. Areas of high density are indicated as underexposed areas, while areas of low density are indicated as overexposed areas. Trained personnel can interpret whether or not defects are present.

Ultrasonic inspection
Ultrasonic inspection can detect corrosion damage on some surfaces. It is commonly used to detect exfoliation and stress-corrosion cracks.

Preventing corrosion
Although corrosion will always be present on aircraft, especially on older ones, there are measures that can be taken to ensure that it is kept to a minimum.
An effective corrosion control program incorporates the following components:
• Inspection for corrosion on a scheduled basis.
• Thorough cleaning, inspection, lubrication, and preservation at prescribed intervals.
• Prompt corrosion treatment after detection.
• Accurate record keeping and reporting of material or design deficiencies to the manufacturer and the FAA.
• Use of appropriate materials, equipment, and technical publications.
• Maintenance of the basic finish systems.
• Keeping drain holes and passages open and functional.
• Replacing deteriorated or damaged gaskets and sealants to avoid water intrusion and entrapment, which leads to corrosion.
• Minimizing the exposure of aircraft to adverse environments, such as hangaring away from salt spray.

Landing gear and adjacent structure are particularly prone to corrosion.
Landing gear and adjacent structure are particularly prone to corrosion.

As mentioned earlier, in order for corrosion to occur, four conditions must exist: presence of an anode, presence of a cathode, presence of an electrolyte, and electrical contact between the anode and cathode.
Since it is not usually an option to remove either the anode or the cathode, the two common ways to prevent corrosion are to remove the electrolyte or to prevent physical contact between the anode and cathode.

Electrolyte
Keeping the electrolyte from contacting the metal is one way to stop corrosion from forming. A good primer and/or paint coating is a good start. Not only is a good initial coating essential, but it should be inspected and maintained to minimize chips and scratches from allowing the corrosion process to begin.
Removing corrosive deposits like exhaust trail residue and salt spray are extremely helpful. Turbine and reciprocating engine exhaust are very corrosive, and regular cleaning aids in preventing corrosion from from setting up in these hot spots. These areas require more attention, especially at lap joints and access panels. Paint chipPaint coatings should be inspected thoroughly, touching-up scratches and chips to prevent corrosion.

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