Cleaning and Corrosion Control
Corrosion is an electrochemical process that needs a metal, oxygen, and moisture. Questions ask you to name the type, choose the cleaner, and know how much metal removal is too much.
Types. Uniform (surface) corrosion attacks evenly. Galvanic corrosion occurs where dissimilar metals meet in the presence of an electrolyte. Concentration-cell corrosion happens in stagnant pockets, under dirt, or at lap joints. Pitting is localized and deep. Intergranular corrosion eats along grain boundaries, and exfoliation is its visible flaking form. Fretting is wear between mated surfaces. Stress corrosion cracking requires tensile stress plus a corrosive environment and is a leading cause of sudden failure in high-strength aluminum and steel.
Cleaning. Use the cleaner specified for the surface: mild soap and water for painted surfaces, approved alkaline or solvent cleaners for engines and wheel wells, and only approved products for acrylic windows, never solvents or abrasive pads. Rinse thoroughly, because dried cleaner attracts moisture.
Removal and protection. Remove corrosion with the mildest method that works, staying inside structural repair manual limits, then apply a conversion coating such as alodine, prime, and paint. Alclad surfaces have a finite cladding thickness; polishing through it exposes the core alloy.
Traps. Never use steel wool or a wire brush on aluminum, since embedded steel particles start galvanic corrosion. A small pit may still exceed limits; the repair manual, not the eye, sets the allowable.
Rule of thumb. Wash more often in coastal and winter-salt environments, and inspect lap joints, battery compartments, and wheel wells first.