📖 The lesson
Chemical fundamentals
Corrosion is the deterioration of a metal by chemical or electrochemical reaction with its environment — the metal returning to a more stable compound (oxide, hydroxide, salt).
- Direct chemical attack — a corrosive agent attacks the metal directly with no current flow: spilled battery electrolyte, caustic cleaners, acids, exhaust deposits, residual flux.
- Electrochemical attack — the common case. It works like a battery and needs four things: an anode (the area that corrodes, losing electrons — oxidation), a cathode (where electrons are consumed — protected), an electrolyte (moisture with salts or acids) and an electrical path between anode and cathode.
The anode and cathode may be two different metals, or simply different areas of the same metal (different grain structure, stress, oxygen concentration or impurity). Factors that speed corrosion: salt, humidity, heat, industrial pollution, dissimilar metals, trapped moisture and dirt, and damaged protective finishes.
The galvanic series and dissimilar metals
- Metals are ranked from anodic (active) to cathodic (noble). In contact, the more anodic metal corrodes and the more cathodic one is protected.
- A small anode with a large cathode is the worst combination — for example an aluminium rivet in a stainless panel.
- Prevent galvanic corrosion with insulation between the metals: primer, wet-installed sealant, barrier tape, sacrificial coatings (cadmium plating on steel bolts in aluminium), and suitable fastener materials (titanium in carbon fibre).
Types of corrosion
- Surface (uniform) corrosion — general roughening and dulling, sometimes with powdery deposits.
- Pitting — localised deep pits, often in aluminium and magnesium; small surface spots can hide large cavities.
- Filiform — thin worm-like filaments spreading under paint films, usually where surface preparation before painting was poor and humidity is high.
- Intergranular — attack along the grain boundaries, typically from incorrect heat treatment (slow quench). Hard to see; found with ultrasonic or eddy current.
- Exfoliation — an extreme form of intergranular corrosion in rolled or extruded material (especially 7xxx alloys), where the grains are flattened: the metal swells and lifts in layers like the pages of a book, often near fastener holes.
- Stress-corrosion cracking (SCC) — cracks caused by sustained tensile stress (from assembly, interference fits, residual stress) together with a corrosive environment. Can cause sudden failure.
- Corrosion fatigue — cyclic loading in a corrosive environment; cracks start earlier than in clean air.
- Fretting — slight relative movement between parts in close contact (rivets, fittings, bearings) wears away protective films. On aluminium, dark streaks of oxide flowing back from rivet heads — "smoking rivets" — indicate fretting and loose rivets.
- Concentration cell (crevice) corrosion — different concentrations of metal ions or of oxygen in the electrolyte. Areas starved of oxygen — under seals, washers, debris, in lap joints — become anodic and corrode.
- Galvanic — dissimilar metal contact (above). Microbial — bacteria and fungi growing in water at the bottom of fuel tanks produce acids and sludge that attack tank structure.
- Hydrogen embrittlement — hydrogen absorbed by high-strength steel during plating or acid cleaning makes it brittle.
Recognising corrosion products
| Metal | Corrosion product |
|---|---|
| Aluminium | White or grey powder; blistering of paint |
| Magnesium | White, powdery, snow-like deposits; spots and raised paint |
| Steel | Red-brown rust (iron oxide) |
| Copper and alloys | Blue-green deposits |
| Cadmium plating | White deposits (it corrodes sacrificially to protect steel) |
Corrosion-prone areas
Engine exhaust trails, battery compartments and vents, lavatories and galleys, bilges and lower fuselage, wheel wells and landing gear, engine inlets and cowlings, flap and spoiler recesses, lap joints and faying surfaces, piano hinges, control cables, spot-welded assemblies and anywhere water or fluid can collect. Drain holes must be kept clear.
Prevention and treatment
- Prevention: keep aircraft clean and dry; anodising and chemical conversion coatings (chromate/Alodine) on aluminium; primers and paint; wet-installed fasteners and sealants; corrosion-inhibiting compounds (water-displacing) in hidden areas; Alclad; plating on steel.
- Removal: clean the area, then remove all corrosion products mechanically — abrasive paper or nylon pads on aluminium (aluminium oxide, never steel wool or steel brushes, which embed particles and cause new corrosion), glass-bead blasting where approved — or with approved chemical removers. Blend smoothly and measure the material removed against SRM limits.
- Treatment: neutralise, apply conversion coating, prime and refinish; apply corrosion inhibitor.
- Corrosion levels in a corrosion prevention and control programme: Level 1 is corrosion within allowable limits between inspections; Level 2 exceeds limits and needs repair or replacement; Level 3 is an urgent airworthiness concern, reported to the manufacturer and authority.