Corrosion: types, detection, prevention and treatment

A persistent threat to airworthiness

Corrosion is one of the most persistent and costly threats to aircraft airworthiness. Left unchecked, it can weaken structures, damage systems, reduce fatigue life and lead to expensive repairs. Because corrosion often begins out of sight and progresses slowly, technicians must understand how it forms, where it is likely to occur, how to identify it early, and how to treat it correctly before significant damage develops.

Corrosion control is not simply a maintenance task. It is a continuous process of inspection, prevention, detection, treatment and protection.

Why aircraft corrode

Corrosion is a natural electrochemical process that causes metal to return to a more chemically stable state, typically an oxide, hydroxide or salt. If any one of the four elements below is removed, the corrosion process is interrupted. For corrosion to occur, four elements are generally required:

• An anode: the area where metal is lost.
• A cathode: the area receiving electrons.
• An electrical path between them.
• An electrolyte such as water, salt, cleaning chemicals or industrial pollutants.

Why aircraft are vulnerable

Water naturally collects in low points, joints, cavities and hidden structural areas, making these locations particularly vulnerable. Aircraft operate in environments that are especially favorable to corrosion:

• Rain and humidity
• Coastal salt air
• De-icing chemicals
• Temperature changes causing condensation
• Industrial pollution
• Battery electrolyte contamination
• Galley and lavatory fluids

Common corrosion hotspots

Technicians pay special attention to areas where moisture or contaminants can accumulate. These areas often require enhanced inspection during scheduled maintenance. Common corrosion-prone locations include:

• Bilges and fuselage low points
• Under floor panels
• Galley and lavatory areas
• Wheel wells
• Landing gear bays
• Battery compartments
• Engine nacelles
• Exhaust paths
• Control cable areas
• Lap joints and riveted structures
• Areas beneath damaged paint
• Locations where dissimilar metals contact one another

Common forms of corrosion

Different environments produce different forms of corrosion. Some are obvious and easy to detect, while others can hide significant structural damage beneath a relatively small surface defect.

Surface (uniform) corrosion

Uniform corrosion is the most common form. The corrosion attack is distributed relatively evenly across the exposed surface. It is usually easier to detect and treat than localized forms of corrosion, but extensive surface corrosion can still reduce structural thickness if left untreated. It appears as:

• General roughening
• Surface etching
• Dull or discolored metal
• Paint blistering or peeling

Pitting corrosion

Pitting corrosion creates small, localized cavities in the metal surface. A pit can penetrate deep into the metal while appearing minor from the surface, and pits can act as stress concentrations that contribute to fatigue cracking. Because the visible damage often understates the true damage, pitting requires careful evaluation. Characteristics include:

• Tiny holes or pits
• Significant depth beneath a small surface opening
• Localized material loss

Galvanic (dissimilar metal) corrosion

Galvanic corrosion occurs when two dissimilar metals are in electrical contact and an electrolyte is present. The more active metal becomes the anode and corrodes preferentially. Examples include steel fasteners in aluminum structures, and aluminum components attached to magnesium parts. Common preventive measures include:

• Protective coatings
• Sealants
• Isolation washers
• Proper material selection

Intergranular corrosion

Intergranular corrosion attacks along the grain boundaries within an alloy. Rather than attacking the surface uniformly, corrosion progresses internally between microscopic grains. Because significant damage may exist below the surface, it requires careful assessment. Warning signs include:

• Limited visible surface damage
• Reduced structural strength
• Unusual cracking
• Surface swelling

Exfoliation corrosion

Exfoliation is an advanced form of intergranular corrosion. As corrosion products expand, the metal layers separate and lift apart. Exfoliation can severely reduce the strength of structural members and often requires substantial repair or replacement. The surface may appear:

• Swollen
• Bulged
• Flaked
• Laminated

Concentration cell corrosion

This form develops when differences in oxygen concentration or chemical composition exist across a metal surface. Because these areas are hidden from view, concentration cell corrosion is often discovered only during inspections. Common locations include:

• Under gaskets
• Beneath sealants
• Lap joints
• Moisture traps

Filiform corrosion

Filiform corrosion appears as thread-like tracks beneath painted surfaces. It often develops when paint coatings are damaged and moisture penetrates underneath. The corrosion spreads beneath the paint film while remaining hidden until the coating begins to lift. Common signs include:

• Fine worm-like lines
• Paint bubbling
• Thin branching corrosion trails

Stress corrosion cracking (SCC)

Stress corrosion cracking occurs when three factors combine: sustained tensile stress, a susceptible material and a corrosive environment. The result is cracking that may develop with little visible warning.

Stress corrosion cracks can propagate rapidly and significantly reduce structural integrity. Affected components often require careful inspection and evaluation.

Fretting corrosion

Fretting occurs when two parts experience small repeated motions against one another, such as in bolted joints, control system interfaces and structural attachments. The movement removes protective coatings, allowing corrosion and wear to develop together. Typical signs include:

• Dark debris
• Surface wear
• Oxidation products

Detecting corrosion: visual inspection

Early detection is one of the technician's most important responsibilities. Visual inspection is the most common method. Technicians look for:

• Discoloration
• Paint blistering
• Surface roughness
• White powdery deposits on aluminum
• Rust on steel
• Surface cracking

Magnified inspection

Magnification may reveal:

• Small pits
• Microcracks
• Filiform corrosion
• Fastener deterioration

Nondestructive inspection (NDI)

Where necessary, corrosion damage may be evaluated using the techniques below. They help assess damage that is not visible on the surface.

• Eddy current inspection
• Ultrasonic testing
• Radiography
• Dye penetrant inspection

Corrosion prevention

The most effective corrosion control strategy is prevention.

• Moisture control: since moisture often acts as the electrolyte, controlling water is essential. Methods include drain maintenance, ventilation, sealant integrity checks and prompt leak repair.
• Protective coatings: protective finishes provide a barrier between metal and the environment, such as primers, paint systems, chemical conversion coatings and corrosion-inhibiting compounds.
• Regular cleaning: removing contaminants such as salt, dirt, exhaust deposits, hydraulic fluid and de-icing chemicals reduces the likelihood of corrosion developing.
• Scheduled inspections: many corrosion issues are identified long before they become structural problems through routine inspection programs.

Corrosion treatment sequence

Always follow the applicable maintenance manual, structural repair manual (SRM), corrosion control manual and approved procedures. However, the general treatment process is usually similar across aircraft types.

1. Inspect and expose the damage

The objective is to reveal the full extent of the corrosion. Hidden damage cannot be evaluated accurately. Begin by:

• Cleaning the area
• Removing contaminants
• Removing paint or coatings where required

2. Remove all corrosion products

Corrosion products must be completely removed using approved methods. Incomplete removal allows corrosion to continue beneath the surface. Possible techniques include:

• Abrasion
• Mechanical cleaning
• Chemical treatment
• Blending and material removal procedures

3. Assess remaining damage

These limits are typically found in the structural repair manual (SRM). If damage exceeds allowable limits, repair or replacement is required. After corrosion removal:

• Measure material loss
• Determine pit depth
• Evaluate structural integrity
• Compare findings with allowable limits

4. Treat and protect the bare metal

Once the corrosion is removed, the exposed metal must be protected. For aluminum structures, chemical conversion coatings are commonly used to improve corrosion resistance and paint adhesion. Protection may include:

• Chemical conversion coatings
• Corrosion inhibitors
• Surface treatments
• Approved primers

5. Restore protective finishes

The final step is restoring all protective barriers. Without proper refinishing, corrosion may quickly return to the repaired area. This often includes:

• Primer application
• Topcoat paint
• Sealants
• Protective compounds

Key takeaways

• Corrosion is an electrochemical process that requires an anode, cathode, electrical path and electrolyte.
• Moisture and contaminants are major drivers of corrosion growth.
• Aircraft are particularly vulnerable in hidden areas where water and contaminants accumulate.
• Pitting, intergranular corrosion and exfoliation can be far more severe than they initially appear.
• Early detection is often the difference between minor treatment and major structural repair.
• The standard approach is: inspect, remove, evaluate, treat and protect.
• Always compare corrosion damage against approved structural limits before returning the aircraft to service.

The bottom line

Corrosion is inevitable, but serious corrosion damage is largely preventable. Effective technicians understand where corrosion forms, recognize its various forms, inspect vulnerable areas carefully, and follow approved treatment procedures without shortcuts. The goal is not simply to remove visible corrosion, but to stop the underlying process, restore protection, and preserve the aircraft's structural integrity for years to come.