Corrosion
Core concepts
Corrosion needs an anode, a cathode, an electrolyte and a metallic path. Remove any one and the cell stops. Know the forms: general or surface attack; galvanic between dissimilar metals; pitting in passive metals such as stainless and aluminium; crevice attack in trapped moisture; intergranular and exfoliation in high-strength aluminium extrusions; filiform under paint; fretting at joint interfaces; stress corrosion cracking under sustained tensile stress plus a specific environment; and corrosion fatigue.
Prevention: protective finishes, primers and paints, sealants and wet assembly, drainage paths, plated or isolated fasteners, wash-downs and scheduled inspections. The anodic index ranks metals; the further apart two metals are, the faster the anodic one is consumed. Area matters enormously: a small anode joined to a large cathode corrodes very quickly.
Traps in questions
Aluminium is anodic to steel and sacrifices itself; magnesium is anodic to everything common. Mercury, chlorinated solvents and some fire extinguishants attack aluminium. Stainless is not immune - in chlorides it pits and suffers stress corrosion cracking. Stress corrosion needs a sustained tensile stress, so a part that corrodes only when loaded points to SCC. Exfoliation is typical of extruded 2024 and 7075. Fretting appears as dark powder at joints.
Rule of thumb
Keep the anodic index difference below about 0.25 V in a salt or humid environment and below about 0.50 V elsewhere, and never allow a small anode against a large cathode.
Worked example
A steel bolt in an aluminium plate in a wet bay corrodes the plate, not the bolt: aluminium is anodic and becomes the sacrificial anode, and because the anodic area is small the attack is rapid and localised.