Aircraft corrosion and its effective management are critical aspects of aviation maintenance, ensuring the safety and longevity of aircraft. Understanding the various forms of corrosion, their causes, and appropriate maintenance practices is essential for anyone involved in aerospace.
Understanding Aircraft Corrosion: Types and Causes
Aircraft metals are susceptible to several common forms of corrosion, each with unique characteristics and causes. Factors such as atmospheric conditions, the size and shape of the metal, the type of metal, and the presence of corrosion-producing agents all influence how corrosion forms.
Five Common Forms of Aircraft Corrosion
Identifying the type of corrosion is the first step towards effective treatment. The five most common forms include:
- Surface Corrosion: Often appears as general roughening, etching, or pitting on the metal surface.
- Dissimilar Metal Corrosion: Occurs when two different types of metal are in contact, especially in the presence of moisture. This is most likely where different metals meet.
- Intergranular Corrosion: A severe form that attacks the grain boundaries of an alloy. It is most likely to occur in aluminum alloys that have been improperly heat-treated.
- Stress Corrosion: Happens in metal components continuously under tensile stress. Areas around holes in castings with pressed-in bushings are particularly susceptible.
- Fretting Corrosion: Characterized by pitting and the generation of fine debris. It occurs when two mating surfaces, normally at rest, experience slight relative motion.
Specific Causes of Corrosion in Aircraft Components
Understanding the origins of corrosion helps in its prevention and treatment:
- Fretting Corrosion: Caused by slight relative motion between two mating surfaces normally at rest.
- Stress Corrosion: Occurs in components under constant tensile stress.
- Filiform Corrosion: Primarily caused by improper surface preparation for painting, appearing as blistered paint with fern-like patterns underneath.
- Piano-Type Hinges: These are prime spots for corrosion due to dissimilar metal contact (steel pin and aluminum hinge) and their design, which traps contaminants.
Areas Most Prone to Corrosion on Aircraft
Certain areas of an aircraft are inherently more susceptible to corrosion due to design, environmental exposure, or material combinations. These include:
- Areas where water may get trapped or areas under high stress.
- Battery compartments (due to electrolyte spills).
- Exhaust systems and exhaust trails.
- Wheel wells and bilge areas.
- Piano hinges and other metal fittings.
- Lavatories.
- Areas of dissimilar metal contact.
Effective Aircraft Corrosion Maintenance Practices
Preventive maintenance is key to mitigating corrosion. A proactive approach involves a combination of routine tasks and specific treatments.
Essential Corrosion-Preventive Maintenance Operations
Regular and thorough maintenance routines significantly reduce the risk and impact of corrosion:
- Adequate Cleaning: Removing contaminants before they can cause damage.
- Periodic Lubrication: Protecting moving parts and creating barriers against moisture.
- Detailed Inspection: Identifying early signs of corrosion for prompt treatment.
- Prompt Corrosion Treatment: Addressing corrosion as soon as it is discovered.
- Touch-Up of Damaged Paint: Restoring protective coatings.
- Sealing and Use of Protective Covers: Shielding vulnerable areas from environmental exposure.
- Daily Wipe-Down: Cleaning exposed critical areas to remove moisture and contaminants.
Methods for Corrosion Removal from Aircraft Materials
Corrosion removal requires specific tools and techniques tailored to the material and type of corrosion:
General Rust Removal
Except for highly stressed steel surfaces, acceptable methods include:
- Abrasive papers and compounds.
- Small power buffers and buffing compounds.
- Hand wire brushing.
- Steel wool.
Cleaning Corroded Anodized Surfaces
For anodized surfaces, approved tools include:
- Fiber bristle brushes.
- Aluminum wool.
- Aluminum wire brushes.
Removing Surface Corrosion from Titanium and Highly Stressed Metal Parts
Special care is needed for these critical components:
- Stainless steel wool or brush.
- Aluminum oxide paper.
- Scotch-Brite.
- Abrasive blast with glass beads.
- Hand polish with aluminum polish and a soft cotton cloth.
Steps for Comprehensive Corrosion Removal
A systematic approach ensures thorough treatment:
- Clean and strip the corroded area.
- Remove as much corrosion as possible.
- Neutralize any residual material in pits and crevices.
- Restore protective surface films and paint.
Specialized Cleaning and Finishing Techniques
Various materials and finishes on an aircraft require specific cleaning agents and application techniques.
Cleaning Agents for Aircraft Materials
- Plastic Surfaces: Preferred cleaning agent is soap and water.
- Grease from Aircraft Fabric (prior to doping): Methyl Ethyl Ketone (MEK) or lacquer thinner.
- Lead-Acid Battery Electrolyte Neutralization: A solution of sodium bicarbonate of soda and water.
- Nickel-Cadmium Battery Electrolyte Neutralization: A solution of boric acid and water, or vinegar.
Aircraft Finishes and Their Application
- Structural Aluminum Alloy: Kept from forming corrosion with an oxide coating or aluminum cladding.
- Composite Material Finishes: Primary purpose is to protect the composite structure from ultraviolet degradation. Finishes are removed by light sanding or abrasive blast with plastic media.
- Zinc Chromate Primer: Toluol or toluene is used as a thinner.
Key Application Techniques for a Quality Paint Job
- Correct spray pressures and spray gun settings.
- Proper spray gun handling (even, overlapping spray patterns, correct spray gun distance).
- Good lighting for visibility.
Balancing Control Surfaces After Painting
It is crucial to balance control surfaces after painting because the weight or distribution of the paint may have changed, potentially taking the control surface out of tolerance.
Precautions for Pressure Washing Aircraft
When pressure washing, it is essential to avoid or protect areas that may be damaged or contaminated, such as bearings, hinges, sealed areas, and painted surfaces.
FAQ: Common Questions About Aircraft Corrosion and Maintenance
What are the five common forms of corrosion on aircraft?
The five common forms are surface corrosion, dissimilar metal corrosion, intergranular corrosion, stress corrosion, and fretting corrosion.
How can you identify fretting corrosion?
Fretting corrosion is identified by pitting of mating surfaces and the presence of powdered residue between them, resulting from slight relative motion.
Why are piano-type hinges particularly susceptible to corrosion?
Piano hinges are corrosion-prone due to dissimilar metal contact between their steel pin and aluminum hinge, and their design which tends to trap contaminants like moisture and dirt.
What is an acceptable method for removing surface corrosion from highly stressed metal parts like titanium?
For titanium and other highly stressed metal parts, acceptable methods include using stainless steel wool or a brush, aluminum oxide paper, Scotch-Brite, abrasive blasting with glass beads, or hand polishing with aluminum polish and a soft cotton cloth.
What is the principal cause of filiform corrosion?
Filiform corrosion is principally caused by improper surface preparation prior to painting, leading to its characteristic fern-like patterns under blistered paint.