# Aircraft Fatigue and Corrosion: Causes, Detection, and Prevention

> Corrosion mechanisms, environmental drivers, stress corrosion, fatigue cycles, and preventive maintenance actions to preserve airworthiness.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-01 — System Design, Loads, Stresses, Maintenance
- **Topic code:** 021-01-03
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/system-design-loads-stresses-maintenance/fatigue-and-corrosion/

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Aircraft structures are continuously exposed to environmental factors and operational stresses that can lead to degradation over time. Two primary forms of degradation are corrosion, a chemical deterioration of materials, and fatigue, a weakening caused by repeated loading. Understanding the causes, signs, and prevention of these phenomena is essential for maintaining airworthiness.

## Corrosion

Corrosion is the deterioration of a metal due to a chemical or electrochemical reaction with its environment. The rate and severity depend on the metal type and environmental conditions.

### Indicators of Corrosion

- **Aluminium and Magnesium:** Corrosion deposits typically appear as a **white or grey powder**. Small white spots may indicate **pitting**, which can be very structurally damaging to an airframe.
- **Ferrous Metals:** Corrosion varies from **red to dark reddish-brown stains** (rust).
- **Paintwork:** Discolouration of paint can also be an indicator.
- **Visual Signs:** Paint wear, bulges, and impact dents are often cosmetic but can hide or lead to localized stress.

### Factors Affecting Corrosion Rate

Five main factors influence the speed of deterioration in atmospherically exposed metals:

1. **Temperature:** Generally, a **10°C increase in temperature doubles the corrosion rate**.
2. **Humidity:** Has a direct effect; higher humidity leads to a higher corrosion rate.
3. **Rainfall:** Increases humidity, accelerating corrosion.
4. **Pollution (Sulphur Dioxide):** Accelerates corrosion damage.
5. **Air Salinity:** High over oceans at low altitude. Salt build-up mixed with water creates electrolytes that aggressively attack **aluminium and magnesium**.

### Corrosive Substances

Certain substances are inherently corrosive to aircraft materials:

- **Mercury:** Highly corrosive to aluminium.
- **Battery Acid & Toilet Waste:** Highly corrosive mixtures.
- **Water:** Causes oxidation and rust.
- **Hydraulic Fluids:** Can be corrosive to paintwork, sealing compounds, and rubber materials (though designed to be non-corrosive to metals). They should not be mixed and are skin/eye irritants.

### Stress Corrosion

**Stress corrosion** is the result of the **combined effect of sustained tensile stresses and a corrosive environment**.

- It is particularly characteristic of **aluminium, copper, and high-strength alloy steels**.
- It can cause premature failure and visible cracking.
- Cracking and buckling (indicating compression) are signs of continuous stress exceeding yield strength, often resulting from events like hard landings.

## Prevention

Operators must follow a constant cycle of cleaning, inspection, and lubrication to prevent corrosion.

- **Regular Washing:** Essential to remove acidic contaminants and salt deposits, especially when operating in **marine atmospheres** or warm, humid climates.
- Washing the aircraft (or at least sensitive areas like engines and landing gear) with fresh water helps prevent corrosion significantly.

## Fatigue

**Fatigue** is the weakening of a material caused by **repeated (cyclic) stresses or loadings**, which can lead to failure at a load much lower than the material's static strength.

- **Cyclic Loads:** Examples include the **pressurization cycle** (fuselage expansion/contraction) and landing impacts.
- **Life Limit:** An aircraft that performs **more cycles** (e.g., short flights) will reach its fatigue life limit sooner than one flying fewer, longer sectors. For example, an aircraft doing six 30-minute flights per day accumulates fatigue faster than one doing fewer long flights.
- **Detección:** Fatigue cracks are typically detected **visually during scheduled maintenance**. Systems like HUMS (Health and Usage Monitoring Systems) monitor mechanical health but are not the primary method for detecting airframe fatigue cracks.
- **Effect of Ageing:** Ageing leads to an increase in scratches, dents, and dirt on the airframe surface, which increases **skin friction drag** by disturbing laminar airflow.

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