Identifying & Preventing Airframe Corrosion

Metal corrosion is one of the most persistent and costly threats to light aircraft. Unlike obvious structural damage, corrosion often develops slowly and out of sight—beneath paint, inside lap joints, or in hidden cavities—compromising structural integrity long before it becomes visible. For owners, operators, and maintenance personnel, early identification and proactive prevention are essential to preserving airworthiness, safety, and aircraft value.

What Is Corrosion?

Corrosion is the deterioration of metal caused by various reactions to its environment. In aircraft, it most commonly affects aluminum alloys, but steel, magnesium, and other metals are also vulnerable.

Light aircraft are particularly susceptible due to:

• Thin aluminum skins

• Riveted lap joints

• Exposure to moisture, salt air, and temperature changes

• Long periods of inactivity

Even aircraft stored in hangars are not immune. Humidity alone can initiate and perpetuate corrosion.

Common Types of Corrosion in Light Aircraft

Understanding the different forms of corrosion helps in identifying and addressing problems early.

1. Surface (Uniform) Corrosion

• Appears as dulling, discoloration, or light pitting on metal surfaces

• Common on aluminum skins and control surfaces

• Often found where paint has been chipped or scratched

This is usually the earliest and most treatable stage.

2. Pitting Corrosion

• Small, deep cavities in the metal surface

• Can penetrate beyond the surface layer

• Often found on aluminum in humid or coastal environments

Pitting can quickly become structural if left untreated.

3.Filiform Corrosion

• Thread-like corrosion beneath paint

• Appears as worm-like lines under painted surfaces

• Common in areas with poor surface preparation before painting

Often cosmetic at first—but indicates coating failure.

4. Galvanic Corrosion

• Occurs when dissimilar metals are in contact in the presence of an electrolyte (water)

• Common between aluminum and steel fasteners

• Frequently found in landing gear components and attachment points

Proper material isolation is key to prevention.

5. Intergranular Corrosion

• Occurs along grain boundaries within metal

• Often invisible externally until advanced

• Can severely weaken structural components

This type requires careful inspection and sometimes non-destructive testing (NDT).

6. Exfoliation Corrosion

• Advanced form of intergranular corrosion

• Causes metal layers to lift or “flake”

• Common in thicker aluminum extrusions

This is structurally serious and often requires part replacement.

Where to Look: High-Risk Areas

During inspections, special attention should be paid to:

• Wing roots and spar carry-through structures

• Lap joints and rivet lines

• Battery compartments (acid fumes accelerate corrosion)

• Landing gear and wheel wells

• Bilge areas and belly skins

• Control cable pulleys and turnbuckles

• Under insulation and floorboards

• Around antennas and external fittings

Aircraft operating in coastal or humid regions require more frequent and detailed inspections.

How to Identify Corrosion

Visual Inspection

The first line of defense. Look for:

• Paint bubbling or blistering

• Discoloration or white powder (aluminum oxide)

• Rust on steel components

• Flaking or lifting metal

Use bright lighting and mirrors for confined areas.

Physical Inspection

• Gently probe suspected areas with a non-metallic tool

• Check for soft spots or metal thinning

• Tap lightly to detect changes in sound (experienced inspectors only)

Non-Destructive Testing (NDT)

When corrosion is suspected internally:

• Eddy current inspection

• Ultrasonic thickness measurement

• Borescope inspections

These are particularly valuable for spar and structural components.

Preventing Corrosion

Prevention is far more cost-effective than repair. A structured corrosion control program should include:

1. Environmental Control

• Store aircraft in a dry hangar when possible

• Use dehumidifiers in humid climates

• Avoid long-term outdoor parking

• Rinse aircraft exposed to salt air with fresh water

Humidity is the primary catalyst for corrosion.

2. Regular Cleaning

• Wash aircraft regularly to remove dirt and contaminants

• Clean belly skins frequently

• Neutralize battery acid spills immediately

• Avoid harsh chemicals that strip protective coatings

Dirt traps moisture—clean metal corrodes less.

3. Protective Coatings

• Maintain paint integrity

• Touch up chips promptly

• Apply corrosion-inhibiting compounds (CICs) in internal cavities

• Use proper primers when repainting

Modern corrosion inhibitors can significantly extend structural life.

4. Proper Drainage and Ventilation

• Ensure drain holes are clear

• Verify water cannot pool inside control surfaces or fuselage cavities

• Inspect seals and gaskets

• Fly the aircraft regularly to promote airflow and moisture evaporation

Trapped moisture is a common root cause of hidden corrosion.

5. Material Isolation

• Use proper washers and sealants between dissimilar metals

• Apply anti-corrosion compounds to fasteners

• Replace incorrect hardware with approved materials

Galvanic corrosion is preventable with correct installation practices.

6. Routine Inspections

• Follow manufacturer corrosion inspection intervals

• Increase frequency in high-humidity or coastal environments

• Document and trend findings

Early detection prevents structural escalation.

The Cost of Neglect

Unchecked corrosion can result in:

• Reduced structural strength

• Increased inspection findings

• Expensive spar or skin replacement

• Lower resale value

• In extreme cases, structural failure

For aging light aircraft fleets, corrosion management is often the deciding factor in long-term airworthiness.

Building a Corrosion Control Mindset

Effective corrosion prevention is not a one-time action—it is an ongoing process built into regular maintenance culture. Owners and operators who:

• Keep aircraft clean

• Inspect carefully

• Act quickly on early signs

• Protect exposed metal

This approach will significantly extend the life, value, and safety of their aircraft.

Final Thoughts

Corrosion may be inevitable, but structural damage is not. With disciplined inspection practices, environmental control, and preventative maintenance, light aircraft can remain structurally sound for decades.

In aviation maintenance, vigilance is safety—and nowhere is that more true than in the fight against corrosion.