Materials, Hardware, Fasteners and Corrosion Control
Materials and heat treatment
Aluminium alloys are common: 2024 has good fatigue strength but lower corrosion resistance; 7075 has high strength but is more notch-sensitive. Heat treatment: solution treatment, quench, then natural or artificial aging. Overaging from excessive temperature reduces strength. Composites need correct lay-up, cure and moisture control. Steel and titanium are used where strength or temperature demands.
Hardware and fasteners
AN and MS hardware are identified by markings and specifications. Grip length must match the total thickness of the parts; too short or too long is a defect. Torque creates clamp-up; use the specified value and a calibrated wrench. Locking methods include locknuts, cotter pins and safety wire. Thread lubricant changes torque-tension relationship.
Corrosion
Corrosion needs an electrolyte, a potential difference and a path. Galvanic corrosion occurs between dissimilar metals; exfoliation lifts grain layers; stress corrosion cracks under sustained tensile stress; fretting occurs at vibrating joints. Control: coatings, sealants, drainage, washing, isolation washers and inspection.
Worked example
A stainless-steel screw is installed in a 2024 aluminium panel in a wet area. Stainless is cathodic to aluminium, so the aluminium becomes the anode and corrodes around the fastener. A cadmium-plated steel fastener is less severe but still needs protection. The fix is to isolate the fastener with a barrier, seal the joint and inspect for blistering.
Traps
Do not judge a bolt by length alone; do not assume 'stainless' means no corrosion in aircraft; do not wire-lock a fastener without correct tension; and do not use heat to dry composites or aluminium without checking limits.