AMT Corrosion Control and Treatment 3 — Questions and Answers
Question 1: Stress corrosion cracking (SCC) in aircraft structures is caused by the combination of:
- High humidity and low temperature
- Tensile stress and a corrosive environment acting simultaneously (Correct answer)
- Cyclic loading and paint failure
- Impact damage and saltwater exposure
Correct answer: Tensile stress and a corrosive environment acting simultaneously
SCC requires the simultaneous presence of tensile stress (residual or applied) and a specific corrosive environment, causing cracks to propagate along grain boundaries.
Question 2: Exfoliation corrosion is characterized by:
- Pitting concentrated at fastener holes
- Laminar separation of metal layers parallel to the surface (Correct answer)
- White filaments running under paint
- Cracking perpendicular to the grain direction
Correct answer: Laminar separation of metal layers parallel to the surface
Exfoliation corrosion causes the metal to delaminate in layers parallel to the rolled surface, producing a flaking or leafing appearance typical in high-strength aluminum alloys.
Question 3: Which chemical is commonly used to neutralize alkaline cleaning residues on aluminum before corrosion treatment?
- Sodium hydroxide solution
- Dilute chromic acid or phosphoric acid wash (Correct answer)
- Trichloroethylene solvent
- Zinc chromate primer
Correct answer: Dilute chromic acid or phosphoric acid wash
A dilute acid rinse (such as chromic acid or phosphoric acid-based wash) neutralizes alkaline cleaner residues and prepares the aluminum surface for subsequent treatment.
Question 4: What is the primary reason magnesium aircraft components require immediate and thorough corrosion treatment?
- Magnesium is the most cathodic metal in the galvanic series
- Magnesium corrodes extremely rapidly and can penetrate deeply in a short time (Correct answer)
- Magnesium corrosion only occurs at temperatures above 200°F
- Magnesium cannot be treated once corrosion begins
Correct answer: Magnesium corrodes extremely rapidly and can penetrate deeply in a short time
Magnesium is one of the most anodic (active) metals used in aircraft construction and corrodes very rapidly, requiring prompt treatment to prevent deep structural damage.
Question 5: Filiform corrosion appears as:
- Broad, flat patches of white powder
- Thin worm-like threads running under intact paint or coatings (Correct answer)
- Deep pits concentrated near rivet heads
- Reddish-brown rust streaks on bare metal
Correct answer: Thin worm-like threads running under intact paint or coatings
Filiform corrosion forms thread-like or worm-like tracks beneath paint or surface coatings, often appearing on aluminum and steel in humid environments.
Question 6: When inspecting a suspected corrosion area on a steel aircraft component, the presence of reddish-brown deposits indicates:
- Galvanic corrosion from contact with aluminum
- Iron oxide (rust), confirming active or past corrosion of steel (Correct answer)
- Chromate treatment that has been properly applied
- Normal surface oxidation that requires no action
Correct answer: Iron oxide (rust), confirming active or past corrosion of steel
Reddish-brown deposits on steel are iron oxide (rust), a product of steel corrosion that confirms corrosive attack has occurred and requires treatment.
Question 7: After mechanical removal of corrosion from a structural aluminum part, the technician should:
- Immediately apply topcoat paint without any primer
- Apply a conversion coating (such as alodine) followed by primer before painting (Correct answer)
- Leave the bare metal exposed for 24 hours to form a natural oxide layer
- Apply engine oil as a temporary corrosion inhibitor permanently
Correct answer: Apply a conversion coating (such as alodine) followed by primer before painting
Following corrosion removal, the bare aluminum must be treated with a conversion coating and primed promptly to restore corrosion protection before topcoating.
AMT Corrosion Control and Treatment 3
Stress corrosion cracking (SCC) in aircraft structures is caused by the combination of: