Summary of Aircraft Corrosion and Maintenance Practices

## Introduction Corrosion prevention and surface finishing are essential for maintaining the safety, performance, and longevity of aircraft structures and components. This guide breaks down common corrosion forms, their causes, preventive maintenance practices, and recommended surface-finishing and cleaning techniques. Practical examples and actionable steps are provided so you can apply these topics during inspections and maintenance tasks. > **Definition:** Corrosion is the chemical or electrochemical reaction between a metal and its environment that produces a deterioration of the metal and its properties. ## Common Forms of Corrosion ### 1. Surface (uniform) corrosion - Affects large exposed areas uniformly. - Often results from prolonged exposure to moisture and contaminants. - Example: Fading and general thinning of protective coatings on an exterior skin panel. ### 2. Dissimilar metal corrosion (galvanic corrosion) - Occurs where two different metals are in electrical contact in the presence of an electrolyte (moisture). - Most likely where different metals meet and water can collect. - Example: Corrosion at a fastener made of steel passing through an aluminum fitting. ### 3. Intergranular corrosion - Progresses along grain boundaries within an alloy, weakening the structure. - Often caused by improper heat treatment or sensitization. - Example: Corrosion along grain lines of an alloyed component near welded or heat-affected zones. ### 4. Stress corrosion cracking (SCC) - Appears when a metal under tensile stress corrodes selectively and cracks. - Occurs in parts under constant tensile loads; holes around pressed-in bushings and highly stressed fittings are vulnerable. - Example: Cracking around a loaded hole in a casting with a pressed-in bushing. ### 5. Fretting corrosion - Caused by slight oscillatory motion between two mating surfaces normally at rest with respect to one another. - Characterized by pitting and fine powdered debris between contacting surfaces. - Example: Wear and pitting at hinge pin contact surfaces that experience small movements. > **Definition:** Fretting corrosion is localized surface damage and corrosion caused by repeated small-amplitude relative motion between contacting surfaces. ## Factors Affecting Corrosion Formation - **Atmospheric conditions:** humidity, salt spray, temperature cycles - **Geometry:** size and shape can trap moisture (crevices, recesses) - **Metal type:** different alloys have different corrosion resistances - **Corrosion-producing agents:** salts, acids, pollutants, battery electrolytes Did you know that areas where water can collect or be trapped are disproportionately likely to develop corrosion because moisture and contaminants concentrate there? ## Corrosion-Prone Areas (Practical Examples) - Battery compartments (electrolyte exposure) - Exhaust systems and exhaust trails (high temperature + corrosive deposits) - Wheel wells and bilge areas (moisture accumulation) - Piano-type hinges (dissimilar metal contact and contaminant trap) - Metal fittings near lavatories and external openings ## Inspection and Corrosion-Preventive Maintenance ### Key operations for corrosion prevention 1. Adequate cleaning 2. Periodic lubrication of moving parts 3. Detailed, regular inspection 4. Prompt treatment of detected corrosion 5. Touch-up of damaged paint and protective finishes 6. Sealing seams and joints to prevent moisture ingress 7. Use of protective covers where feasible 8. Daily wipe-down of exposed critical areas > **Definition:** Corrosion-preventive maintenance includes cleaning, lubricating, sealing, inspecting, and promptly treating corrosion to prevent structural damage and functional degradation. ## Steps for Corrosion Removal (Sequential) 1. Clean and strip the corroded area of loose paint, grease, and contaminants. 2. Mechanically remove as much corrosion as possible (see approved tools below). 3. Neutralize any residual corrosion products or trapped elec