Aircraft Materials, Hardware, and Corrosion Control
Core concepts
Know the common airframe materials: 2024 and 7075 aluminum alloys, 300-series stainless steel, titanium, magnesium, and composites. Understand temper designations (T3, T6, O, H) and how heat treatment affects strength and corrosion resistance. Hardware includes AN and MS bolts, nuts, washers, cotter pins, safety wire, and turnbuckles; know grip length, thread protrusion (typically 1 to 3 threads beyond the nut), and correct torque. Corrosion types include galvanic, pitting, intergranular, exfoliation, stress corrosion, and filiform. Understand the corrosion triangle: susceptible metal, electrolyte, and oxygen. Control measures include protective coatings, sealants, drainage, and avoiding dissimilar metal contact without isolation.
Traps in questions
A frequent trap is confusing galvanic corrosion (dissimilar metals in contact with electrolyte) with stress corrosion (tensile stress plus corrosive environment). Another is assuming all aluminum alloys corrode equally: 7075 is more susceptible to stress corrosion than 2024 in some tempers. Watch for questions on safety wire direction: wire must pull the fastener clockwise (tightening direction). Also remember that magnesium components require special protection and are not interchangeable with aluminum hardware without isolation.
Worked example / rule of thumb
Rule of thumb: remove corrosion by blending to a smooth contour, but never exceed the allowable blend-out depth in the SRM. For a typical skin, blend-out depth is often limited to 10 percent of sheet thickness. If a 1.6 mm skin has 0.16 mm of pitting, blending to 0.16 mm is at the limit; deeper requires engineering disposition. Always re-protect the blended area with alodine and primer.