AET Corrosion Control and Prevention — Questions and Answers
Question 1: Which type of corrosion is characterized by a white or gray powdery deposit on aluminum aircraft structures?
- Galvanic corrosion
- Stress corrosion cracking
- Uniform surface oxidation (aluminum oxide formation) (Correct answer)
- Pitting corrosion
Correct answer: Uniform surface oxidation (aluminum oxide formation)
When aluminum oxidizes, it forms aluminum oxide (Al₂O₃) — a white or gray powdery film. Unlike iron rust, aluminum oxide forms a tight adherent layer that can slow further corrosion, but the powdery appearance is a clear visual indicator of surface oxidation.
Question 2: What condition must be present for galvanic corrosion to occur between two dissimilar metals?
- Both metals must be in direct sunlight for an extended period
- The metals must be in electrical contact with each other in the presence of an electrolyte (Correct answer)
- Both metals must be under mechanical stress simultaneously
- The ambient temperature must exceed 100°F
Correct answer: The metals must be in electrical contact with each other in the presence of an electrolyte
Galvanic corrosion requires three elements: two dissimilar metals (separated in the galvanic series), a metallic electrical path connecting them, and an electrolyte (such as moisture, fuel, or salt water) completing the circuit. Remove any one element and galvanic corrosion stops.
Question 3: Which fastener material used in aluminum aircraft skin would create the greatest risk of galvanic corrosion?
- 2024 aluminum alloy rivets
- Titanium fasteners
- Uncoated carbon steel fasteners (Correct answer)
- Stainless steel (300-series) fasteners
Correct answer: Uncoated carbon steel fasteners
Uncoated carbon steel and aluminum are far apart in the galvanic series, creating a large electrochemical potential difference that drives aggressive galvanic corrosion. Aluminum and titanium are much closer in the galvanic series, making titanium an acceptable fastener in aluminum structure.
Question 4: What is the correct first step when active corrosion is discovered on an aircraft structure during inspection?
- Apply a zinc chromate primer directly over the corroded area to seal it
- Remove all corrosion mechanically or chemically before applying any corrosion-inhibiting treatment or primer (Correct answer)
- Seal the corroded area with epoxy topcoat to exclude oxygen
- Apply a penetrating oil to neutralize the corrosion and prevent further spread
Correct answer: Remove all corrosion mechanically or chemically before applying any corrosion-inhibiting treatment or primer
Primer and sealant applied over active corrosion trap moisture and corrosion products against the metal, allowing the corrosion to continue undetected underneath. The corroded material must be fully removed first, the base metal restored, and then primer and protective coatings applied.
Question 5: What non-destructive testing (NDT) method is most effective for detecting corrosion beneath the outer skin of an aluminum aircraft panel that shows no exterior symptoms?
- Visual inspection with a bright flashlight and mirror
- Eddy current or ultrasonic testing (Correct answer)
- Dye penetrant inspection
- Magnetic particle inspection
Correct answer: Eddy current or ultrasonic testing
Eddy current and ultrasonic testing can detect material loss or density changes beneath the surface of aluminum panels without removing the skin. Dye penetrant requires a surface-breaking defect to be visible; magnetic particle inspection only works on ferromagnetic metals.
Question 6: Intergranular corrosion is considered especially serious in aircraft structures because it:
- Spreads very visibly across the metal surface, making it easy to map and monitor
- Is limited to the outermost layer of the metal and can be polished away
- Attacks along grain boundaries within the metal, causing severe strength loss that may not be apparent from the surface (Correct answer)
- Only affects ferrous metals and can be controlled with standard zinc-chromate primer
Correct answer: Attacks along grain boundaries within the metal, causing severe strength loss that may not be apparent from the surface
Intergranular corrosion propagates along the grain boundaries inside the metal microstructure, physically separating the grains and reducing structural strength dramatically. Because the surface may look intact, the full extent of damage is hidden, making this one of the most dangerous corrosion forms for aircraft structures.
AET Corrosion Control and Prevention
Which type of corrosion is characterized by a white or gray powdery deposit on aluminum aircraft structures?