4.4 Cleaning, Corrosion Control, and Finishing

Corrosion Theory

Cleaning and corrosion control are not cosmetic subjects. Corrosion is the deterioration of a metal by chemical or electrochemical action, and it requires an anode, a cathode, an electrolyte, and a metal path — remove any one and the cell stops. Moisture, oxygen, salt, industrial pollutants, spilled battery electrolyte, exhaust residue, and trapped dirt all accelerate attack. A protective barrier slows corrosion, but a scratch, failed sealant, missing primer, or contaminated lap joint creates a local corrosion cell. The authoritative reference is AC 43-4B (Corrosion Control) and AC 43.13-1B Chapter 6.

The Forms of Corrosion

Galvanic corrosion needs dissimilar metals plus an electrolyte; the more anodic (active) metal is sacrificed — for example, aluminum corrodes when bolted to steel or in contact with carbon-fiber composite. Intergranular attack runs along grain boundaries and is worst in 7000-series (zinc) high-strength alloys such as spars and stringers; when corrosion products lift the surface grains it becomes exfoliation, a serious structural condition.

Filiform corrosion forms wormlike threads beneath an organic coating when relative humidity is roughly 78-90% and the surface is slightly acidic; the prevention is to store the aircraft below 70% RH, use low-permeability coatings, and wash off acidic contaminants. Stress corrosion develops in highly stressed parts under sustained tension, frequently starting from a scratch or surface pit, and pitting is dangerous less for lost cross-section than because each pit becomes a fatigue-crack nucleus.

Corrosion-Prone Areas and Inspection

Corrosion inspection concentrates on prone areas where moisture and contaminants collect: battery compartments (spilled electrolyte), bilges and lower fuselage skins, wheel wells and landing gear, lavatory and galley surroundings, exhaust trail areas, lap joints and around fasteners, control-surface hinges, engine mounts, and any place where dissimilar metals meet or where water is trapped behind insulation or under floorboards.

Detection ranges from visual inspection of telltale powder, blistered paint, or stains, through magnifiers and borescopes, to NDT such as eddy current for cracks growing from corrosion pits. A surface usually must be clean before an accurate corrosion inspection is possible.

Cleaning Compatibility

Cleaning starts with material compatibility. Strong alkaline or acidic cleaners attack aluminum and magnesium and strip protective finishes; solvents can craze transparencies and acrylics or soften composite resin; abrasive blasting or wire brushing with the wrong media can gouge soft metal or embed particles that start new galvanic cells; and high-pressure water can drive contamination into bearings, seams, bushings, and electrical connectors.

Match both the product and the method to the surface — aluminum, magnesium, steel, titanium, composite, transparency, rubber, or painted finish — and protect adjacent sensitive areas, bearings, and openings before cleaning begins.

Removal, Treatment, and Coatings

Removal and treatment must follow approved data — removing too much base metal creates a structural defect, so corrosion damage is classified against the manufacturer's limits as negligible, repairable, or non-repairable, and any rework is held within the allowable depth and area.

Removal methods are matched to the metal and the corrosion severity: light surface corrosion on aluminum is removed by hand with abrasive mats, fine abrasive paper, or nylon pads and a chemical inhibitor; heavier corrosion may need controlled abrasive blasting, rotary files, or grinding within limits; magnesium requires its own gentle methods and a dichromate treatment; and steel corrosion (rust) is removed mechanically and the bare steel re-protected promptly.

After removal, the affected area must be neutralized, cleaned, and dried before any coating is applied. Excess removal that exceeds the negligible-damage limit converts a cleaning task into a repair requiring approved data.

After mechanical or chemical removal, restore the protective barrier in layers: a conversion coating (such as chromate or Alodine on aluminum, or anodize on new parts) provides corrosion resistance and a paint-bonding base; then primer (often epoxy or wash primer); then topcoat (polyurethane or enamel).

Corrosion-preventive compounds (CPC) — soft-film water-displacing types or harder waxy films — are applied into joints, recesses, faying surfaces, and box structure that paint cannot fully protect, and are reapplied on an interval set by exposure and the manufacturer's program. Finishing depends on surface prep and ambient conditions (cleaning, etching, conversion coating, correct mixing/thinning, temperature, humidity, flash time); poor control yields runs, orange peel, lifting, and poor adhesion.

Fire prevention and ventilation are critical because finishing materials are flammable and toxic — the SDS drives PPE.

Finally, refinishing affects more than appearance. Identification plates, registration marks, placards, and required markings must be protected or replaced, and control surfaces must be re-balanced after refinishing because added or uneven paint weight changes the static/dynamic balance and can reduce flutter resistance. A flawless finish is unacceptable if it leaves a control surface out of balance.