Corrosion never shows up on a flight schedule. It works quietly in the places we rarely see, such as under fairings, inside bilges, behind galleys and between skin layers, and it keeps working whether the aircraft is flying or parked. For aircraft maintenance engineers, technicians, inspectors, and even pilots who do a good walk-around, understanding corrosion is a core airworthiness skill.
What Is Corrosion?
Corrosion is the electrochemical deterioration of a metal caused by its reaction with the environment. For it to happen, four things must be present at the same time (an electrochemical cell):
- Anode, the metal that corrodes (gives up electrons)
- Cathode, the metal that is protected (receives electrons)
- Electrolyte, usually moisture, salt, condensate, or spilled fluids
- Electrical path, the metal contact that links anode and cathode
Remove any one of the four and corrosion stops. Almost all corrosion control comes down to this idea: keep water away, separate dissimilar metals, and keep protective finishes intact.
Why It Matters
Corrosion reduces the cross-section of a structural member and creates stress concentrations that become starting points for fatigue cracks. A part can look acceptable on the surface while losing strength underneath.
The 1988 Aloha Airlines Boeing 737 fuselage failure is a well-known reminder. Investigators found that the maintenance program had failed to detect disbonding and fatigue damage in the lap joints of an aging aircraft. Ageing structure, moisture ingress, and missed findings are a dangerous combination.
Types of Corrosion in Depth
Corrosion is not one problem. It is a family of related processes, and each type has its own cause, appearance, and danger. Different alloys also behave differently. Aluminium typically leaves white or gray powder. Steel produces red-brown rust. Magnesium, the most vulnerable of the common aircraft metals, forms white, fluffy, bulky deposits and can deteriorate quickly once its finish is breached. Knowing what you are looking at tells you how far it may have gone.
1Uniform (General Surface) Corrosion
This is the most common type and the easiest to recognise. The whole exposed surface reacts with the environment at a roughly even rate, leaving a dull, etched, or roughened appearance. On aluminium you see a gray or white powdery film. On steel you see red rust.
It forms where the protective finish has worn away or never reached, for example on unpainted areas, worn paint on leading edges, or scratched surfaces. Because the loss is spread out, it is the least dangerous type when found early. The risk is that it is often the first sign of a failing finish, and it creates rough spots that become starting points for pitting and crevice attack.
2Galvanic (Dissimilar-Metal) Corrosion
When two different metals touch in the presence of an electrolyte, the more "active" (anodic) metal corrodes faster and the more "noble" (cathodic) metal is protected. Aluminium alloys are anodic to steel, titanium, copper, and brass, so aluminium is usually the victim.
The damage concentrates at the joint, usually around fasteners, fittings, and wherever a different material is attached to skin or structure. It appears as a ring or patch of heavy corrosion products right at the contact point, often with paint lifting around it.
Key factors:
- Distance on the galvanic series. The further apart the metals, the stronger the attack.
- Area ratio. A small anode connected to a large cathode corrodes very fast. A small aluminium part held by a large steel or titanium fitting is in trouble. A small steel fastener in a large aluminium panel is much less severe.
- Electrolyte presence. No moisture, no galvanic cell.
3Pitting Corrosion
Pitting is localised attack that makes small, deep holes while the rest of the surface looks almost normal. On aluminium and magnesium you typically see white or gray powder in and around tiny pits. On stainless steel it appears as small dark spots and rust stains.
It starts where the natural protective oxide film breaks down, often at a scratch, an inclusion, or contamination, and chlorides (salt) make it much worse. Once a pit starts, its inside becomes more acidic and the attack accelerates downward. A pit then acts as a stress concentrator, which is why pitting is a common starting point for fatigue cracks.
4Crevice Corrosion
This occurs inside narrow gaps where liquid gets in and cannot escape or dry out, such as under lap joints, washers, gaskets, seals, clamps, and bonded parts. The gap has less oxygen than the open surface, which creates an oxygen concentration cell. The area inside the crevice becomes the anode and is attacked, while the exposed surface is protected.
It is dangerous because the damage is hidden. The outside can look fine while metal is lost inside the joint. Signs include corrosion products seeping out of the seam, bulging or "pillowing" of the skin at a lap joint, and paint cracking along the joint.
5Filiform Corrosion
Filiform is a special type of crevice attack that happens under a paint or coating. It looks like fine, worm-like threads or tracks spreading across the surface, with a "head" and "tail". It is most often seen on painted aluminium, but it can also affect steel and magnesium.
It starts at damaged or thin points in the coating, such as around fasteners, along skin edges, at scratches, and at poorly sealed seams. High humidity and contaminants such as salts encourage it. Aircraft that operate in warm, humid, and coastal areas see more of it.
Usually, filiform is superficial and does not weaken the structure by itself. But it lifts the paint and exposes the metal, and it can lead to deeper pitting and intergranular attack, especially near fasteners and joints. Do not ignore it just because it looks cosmetic.
6Intergranular Corrosion
Metals are made of tiny crystals (grains). In some alloys, especially certain heat-treated aluminium alloys and some stainless steels, the grain boundaries differ chemically from the grain interiors because of how the alloy was heat treated or because of welding. The boundaries then become anodic and corrode, while the grains themselves are largely untouched.
The attack runs along the grain boundaries below the surface. From the outside you may see little more than a few surface pits or a rough area, while inside the metal the grains are being separated. Strength and ductility drop significantly with little visible warning.
7Exfoliation Corrosion
Exfoliation is an advanced form of intergranular corrosion. It occurs in wrought products with elongated, flattened grains, such as extrusions, plate, and forgings. The corrosion products take up more volume than the metal they replace, so they push the layers of metal apart. The surface lifts, flakes, and swells, like the pages of a book.
You see raised, layered blisters or flaking, often at the edges of extruded sections, around fastener holes, and along the grain direction. By the time it is visible, a lot of metal has been affected underneath.
8Stress Corrosion Cracking (SCC)
SCC is cracking that occurs when three things are present together:
- A susceptible material (for example, some high-strength aluminium alloys, high-strength steels, and some stainless steels).
- A sustained tensile stress, whether from applied loads or locked-in residual stress (from press fits, forming, misaligned assembly, interference fits, or over-torqued parts).
- A corrosive environment.
The cracks are fine, often branched, and can grow with no visible deformation. The part may look fine until it fails suddenly. In aluminium forgings and extrusions, SCC typically runs through the thickness direction of the grain (the "short transverse" direction), which is why part design and grain direction matter.
Typical locations include press-fit bushings, bolts and fittings under sustained load, landing gear components, and assembled parts with interference fits or forced alignment.
9Corrosion Fatigue
Fatigue is damage from repeated cycles of stress. In clean air, many metals have a fatigue limit below which they can run indefinitely. In a corrosive environment that limit effectively disappears, and the metal fails at a much lower stress and fewer cycles. Corrosion pits form on the surface and become crack starters, and the cracks are then driven by the cyclic loads.
It affects parts under repeated loading in humid or salty conditions, such as landing gear, propeller parts, engine mounts, wing and control attachments, and fuselage joints. Because the crack starts at a corrosion pit, finding and treating pitting early is one way to prevent fatigue failures.
10Fretting Corrosion
Fretting happens where two tightly fitted surfaces rub against each other with tiny, repeated movement, such as in riveted or bolted lap joints, splines, bearings, hinges, and clamped fittings. The movement wears away the protective oxide film, and the exposed metal reacts, creating abrasive oxide particles that make the wear worse.
On aluminium the debris is often black powder. On steel it is reddish-brown. A telltale sign is staining or "smoking rivets", where black or dark streaks trail from fasteners or seams.
11Microbiological Corrosion (Fuel Tanks)
Water settles at the bottom of fuel tanks. Where it meets the fuel, microorganisms such as bacteria and fungi can grow, forming a slimy, dark, or brownish sludge. The organisms produce acidic byproducts and create deposits on tank surfaces, where oxygen differences create further corrosion cells under the sludge.
The result is pitting and corrosion of tank structure and damage to tank sealants and coatings. The sludge can also block filters and fuel system components and contaminate fuel quantity readings.
Types Often Occur Together
In practice, you rarely find one type alone. Filiform may sit above pitting. Crevice attack may lead to exfoliation. Fretting may seed a corrosion fatigue crack. A good inspector does not stop at the first thing found. After cleaning, look for the next layer of the problem.
High-Risk Areas
Experience shows that the following areas deserve extra attention:
- Lavatories, galleys, and floor areas below them. Spilled liquids and cleaning chemicals get trapped here.
- Bilge and lower fuselage. Water, hydraulic fluid, and debris collect and sit.
- Battery compartments and vents. Electrolyte fumes and spillage attack nearby structure.
- Wheel wells and landing gear. Exposed to runway contaminants, stone damage, and moisture.
- Exhaust trail areas and engine exhaust-adjacent skin. Hot, aggressive residue.
- Flap and spoiler wells, control surface hinges, and trailing edges. Moisture traps and poor drainage.
- Skin lap joints and areas under fasteners. Sealant breakdown allows moisture in.
- Integral fuel tank bottoms. Water settles at the lowest points.
- Cargo floors and areas near animal or wet cargo carriage.
Operating Environment
Aircraft working in coastal and tropical conditions (high humidity, salt-laden air, frequent rain) corrode faster than those in dry inland climates. For operators on coastlines, low-level utility flying, and regional operations, increased washing, inspection, and preservation intervals are not optional extras. They are part of the cost of keeping the aircraft airworthy.
Detection
Corrosion is found through a combination of methods:
- Visual inspection. Look for paint bubbling, blistering, discoloration, powder deposits, skin bulging, and loose or distorted fasteners. Good lighting, a mirror, and a clean surface make a big difference.
- Borescope. For enclosed structure and tubes.
- NDT methods. Eddy current, ultrasonic, and radiography find hidden material loss and cracks. Methods and procedures must follow the approved data.
- Tap testing and feel. Useful for detecting disbonds and skin lifting.
Take every inspection opportunity seriously. When a panel is opened for another task, look around it.
Classification and Reporting
Large-aircraft maintenance programs normally include a Corrosion Prevention and Control Program (CPCP). Findings are typically classed by severity:
- Level 1. Damage within allowable limits that can be cleaned up without exceeding limits.
- Level 2. Damage that exceeds allowable limits and requires repair, reinforcement, or replacement.
- Level 3. A finding the manufacturer or authority considers an urgent airworthiness concern, needing prompt action and reporting.
Exact definitions and reporting requirements depend on the aircraft type, the program, and the regulator (ICAO Annex 6 and Annex 8 principles, FAA, EASA, or the local authority). Always follow the approved Structural Repair Manual (SRM), Aircraft Maintenance Manual (AMM), and the operator's maintenance program.
Corrosion Removal and Treatment: The General Process
The specific procedure depends on the material, the part, and the approved data. In general:
- Clean and strip the area of paint, dirt, and oil so the damage can be assessed.
- Remove corrosion products with approved methods only (approved abrasives, mechanical or chemical means). Never use tools or materials that damage the base metal or contaminate it, such as steel wool on aluminium.
- Measure the material loss and compare to the SRM limits. Take care to blend out smoothly and not leave sharp edges or notches.
- Decide on repair, rework, or replacement if limits are exceeded.
- Apply a conversion coating to restore the surface treatment of the metal.
- Prime and topcoat per the approved scheme.
- Apply corrosion-inhibiting compound (CIC) where specified.
- Document everything. The task card, the measurements, the materials used, and the release to service.
Prevention: What Actually Works
- Wash regularly with approved products. Salt and dirt hold moisture against the skin.
- Keep drain holes open. A blocked drain is a swimming pool in a place that was designed to be dry.
- Protect finishes. Touch up scratches and paint damage early, before moisture gets under it.
- Control dissimilar metals with proper isolation, sealant, and approved fasteners.
- Maintain seals and sealants. Damaged sealant at lap joints and fairings invites water in.
- Clean up spills fast, especially lavatory, galley, battery, and hydraulic fluids.
- Use hangar and parking discipline. Covers, plugs, and blanks help, but stop moisture being trapped under them.
- Train people to report. The best corrosion program is one where everyone who opens a panel tells someone what they saw.
Safety Notes for the Hangar
Corrosion treatment involves hazardous chemicals, dust, and sometimes confined spaces. Use correct PPE (eye protection, gloves, respirators as required), follow the product safety data sheets, ventilate the area, and handle the waste properly.
Key Takeaways
- Corrosion needs an anode, cathode, electrolyte, and metallic path. Break one and you stop it.
- It is usually found where moisture collects and where nobody looks.
- Surface appearance can understate the real damage, so measure and assess against approved limits.
- Prevention (washing, drains, finishes, sealant) is far cheaper than repair.
- Always work to the approved manuals and document every step.
