5.1 Corrosion Types, Chemistry & Identification

5.1 Corrosion Types, Chemistry & Identification

Corrosion represents one of the primary threats to the structural integrity, operational safety, and economic lifespan of civil and military aircraft. According to FAA-H-8083-30B (Aviation Maintenance Technician Handbook — General) and FAA Advisory Circular (AC) 43-4A (Corrosion Control for Aircraft), corrosion is defined as the chemical or electrochemical deterioration of a metallic material resulting from its thermodynamic reaction with the surrounding operating environment.

Because all structural metals (with the exception of unalloyed noble metals such as gold and platinum) occur naturally in a low-energy oxidized state (metallic ores such as bauxite or hematite), significant energy is expended during smelting and refining to extract pure metallic elements. When exposed to ambient atmospheric moisture, industrial pollutants, marine salts, and operational chemicals, refined aircraft alloys naturally seek to return to their lowest thermodynamic energy state—transforming back into metal oxides, hydroxides, carbonates, and sulfates. An Aviation Maintenance Technician (AMT) must master the fundamental electrochemistry of corrosion, accurately recognize its diverse visual manifestations, and identify root causes before structural failure occurs.

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1. The Electrochemical Corrosion Cell

Direct chemical attack (such as battery acid spilling directly onto an aluminum battery tray) occurs occasionally, but the vast majority of aircraft corrosion is electrochemical in nature. An electrochemical corrosion reaction operates identically to a chemical primary battery (galvanic cell), converting chemical energy into electrical current flow.

                    THE GALVANIC CORROSION CELL
  ┌─────────────────────────────────────────────────────────────┐
  │                       ELECTROLYTE                           │
  │        (Moisture, Dissolved Oxygen, Chlorides, Ions)        │
  │                                                             │
  │         Anions (OH⁻, Cl⁻) ────────►   ◄─────── Cations (M⁺) │
  │        ┌────────────────────────┐   ┌─────────────────────┐ │
  │        │         ANODE          │   │       CATHODE       │ │
  │        │   (Active Substrate)   │   │  (Noble Substrate)  │ │
  │        │ • High Negative Pot.   │   │ • High Pos. Pot.    │ │
  │        │ • Oxidation Occurs     │   │ • Reduction Occurs  │ │
  │        │ • Loses Electrons (e⁻) │   │ • Protected Site    │ │
  │        │ • METAL DISSOLVES      │   │ • Unattacked        │ │
  │        └───────────┬────────────┘   └───────────┬─────────┘ │
  │                    │                            │           │
  └────────────────────┼────────────────────────────┼───────────┘
                       │   ELECTRICAL RETURN PATH   │
                       │ (Direct Metallic Contact)  │
                       └──────────► e⁻ ─────────────┘

The Four Mandatory Requirements of a Corrosion Cell

For electrochemical corrosion to initiate and sustain itself, all four of the following physical components must be present simultaneously. Eliminating any single component breaks the circuit and halts the corrosion process immediately:

1. The Anode (Oxidation Site): The metal or localized micro-region possessing the higher chemical activity (more negative electrode potential). At the anode, metal atoms give up valence electrons and dissolve into the electrolyte as positively charged metal cations:
   $\text{M} \longrightarrow \text{M}^{n+} + n\text{e}^- \quad (\text{Oxidation})$
   The anode is the component that physically corrodes and loses mass.
2. The Cathode (Reduction Site): The metal or micro-region possessing lower chemical activity (more positive / noble electrode potential). The cathode receives electrons conducted through the metallic return path and transfers them to electron acceptors in the electrolyte (typically dissolved oxygen or hydrogen ions):
   $\text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^- \longrightarrow 4\text{OH}^- \quad (\text{Reduction in alkaline/neutral electrolyte})$
   $2\text{H}^+ + 2\text{e}^- \longrightarrow \text{H}_2 \uparrow \quad (\text{Reduction in acidic electrolyte})$
   The cathode does not corrode; it is electrochemically protected.
3. The Electrolyte (Ionic Conductor): A conductive liquid medium in contact with both the anode and cathode that facilitates the migration of ionic charges. In aviation, the electrolyte is typically condensed ambient moisture, rain, melted snow, or wash water carrying dissolved atmospheric pollutants (sulfur dioxide, carbon dioxide) or coastal salt spray (sodium chloride, $\text{NaCl}$), which ionizes into sodium ($\text{Na}^+$) and chloride ($\text{Cl}^-$) ions.
4. The Electrical Return Path (Electronic Conductor): Direct physical metallic contact (or an external conductive bond) between the anode and cathode that allows the liberated electrons to migrate from the anode to the cathode, completing the closed electrical loop.

Chemistry of Aluminum Corrosion

When pure aluminum or an aluminum alloy corrodes electrochemically in the presence of aerated moisture containing trace chloride ions:

• Anodic Dissolution: $\text{Al} \longrightarrow \text{Al}^{3+} + 3\text{e}^-$
• Cathodic Reduction: $\text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^- \longrightarrow 4\text{OH}^-$
• Corrosion Product Precipitation: Positively charged aluminum ions ($\text{Al}^{3+}$) and negatively charged hydroxide ions ($\text{OH}^-$) migrate through the moisture electrolyte, collide, and precipitate as solid aluminum hydroxide:
  $\text{Al}^{3+} + 3\text{OH}^- \longrightarrow \text{Al(OH)}_3 \downarrow$
• Dehydration / Oxidation: As the gelatinous aluminum hydroxide dries, it converts into hydrated aluminum oxide:
  $2\text{Al(OH)}_3 \longrightarrow \text{Al}_2\text{O}_3 \cdot 3\text{H}_2\text{O}$ This manifests physically as the familiar chalky, white, powdery deposit observed on corroding aircraft aluminum structures.

2. The Galvanic Series of Metals & Alloys

When two dissimilar metals are placed in electrical contact in the presence of an electrolyte, the rate and severity of galvanic attack depend directly upon their relative positions in the Galvanic Series of Metals and Alloys.

Galvanic Series Hierarchy (In Aircraft Operational Environments)

Core Rule — Galvanic Potential Difference: The further apart two metals reside in the Galvanic Series, the greater the electromotive potential difference between them, and the more rapid and severe the corrosion attack will be on the more anodic (active) metal.

The Area Rule (Anode-to-Cathode Surface Area Ratio)

The rate of galvanic corrosion is profoundly influenced by the relative surface area ratio of the anode to the cathode:

$\text{Corrosion Current Density at Anode } (J_a) = \frac{I_{\text{total}}}{\text{Area}_{\text{Anode}}}$

• Small Anode + Large Cathode (CATASTROPHIC DANGER): When a tiny anodic area must supply electrons to satisfy a vast cathodic area, the corrosion current density concentrates intensely on the small anode. The anode dissolves with extreme velocity. Example: An aluminum rivet driven into a large stainless steel or carbon-fiber composite skin panel will suffer rapid structural decapitation within weeks in a marine atmosphere.
• Large Anode + Small Cathode (MANAGEABLE ATTACK): When a large anodic area is coupled to a small cathodic area, the galvanic current is distributed over a broad surface. The corrosion depth per unit time is minimal. Example: A cadmium-plated stainless steel fastener installed in a massive aluminum wing skin.

3. Comprehensive Taxonomy of Aircraft Corrosion Types

FAA knowledge examinations and practical inspections require technicians to distinguish ten distinct forms of aircraft corrosion by visual signature, metallurgical mechanism, and structural susceptibility.

                      AIRCRAFT CORROSION TAXONOMY
  ┌─────────────────────────────────┼─────────────────────────────────┐
  │                                 │                                 │
  ▼                                 ▼                                 ▼
SURFACE & LOCALIZED             METALLURGICAL & STRESS          ENVIRONMENTAL & MECHANICAL
• Surface Oxidation / Attack    • Intergranular Attack          • Fretting Corrosion
• Pitting Corrosion             • Exfoliation Corrosion         • Filiform Corrosion
• Galvanic Corrosion            • Stress Corrosion (SCC)        • Microbiological Attack
• Concentration Cell Crevices   • Hydrogen Embrittlement        • Erosion-Corrosion

1. Surface Oxidation & Uniform Etch Attack

• Mechanism: Direct chemical reaction between oxygen or atmospheric pollutants and the bare metal surface across an entire exposed area.
• Visual Signature: A uniform dulling, etching, frosting, or discoloration of polished metal surfaces (e.g., unpainted aluminum turning dull grey; magnesium developing a dark grey/brown oxide film; carbon steel forming red rust, $\text{Fe}_2\text{O}_3$).
• Severity: Least dangerous form because it occurs uniformly at a predictable, measurable rate without localized structural notch concentrations.

2. Pitting Corrosion

• Mechanism: An extremely localized, aggressive form of attack occurring when the natural protective oxide film (passive barrier) breaks down at microscopic defect sites. Once initiated, the bottom of the pit becomes starved of oxygen, transforming it into an intensely active anode, while the surrounding aerated metallic surface acts as a vast cathode (oxygen concentration cell).
• Visual Signature: Appears as localized clusters of tiny pinholes, craters, or blistered nodules of white, powdery aluminum hydroxide on aluminum alloys, or grey/white mounds on magnesium. When scraped clean, deep, sharp-bottomed microscopic pits are revealed.
• Structural Hazard: Pits act as severe stress risers (notches) that initiate catastrophic structural fatigue cracking under cyclic flight loads.

3. Intergranular Corrosion

• Mechanism: Microscopic attack along the crystalline grain boundaries of an alloy while leaving the interior grain matrix relatively unaffected.
• Metallurgical Cause in High-Strength Aluminum (2024-T3, 7075-T6): Occurs during improper thermal processing. During solution heat treatment, alloying elements (such as copper in 2024 or zinc/magnesium in 7075) dissolve into solid solution. When quenched in cold water, rapid cooling locks these elements uniformly within the crystalline lattice. If an excessive quench delay occurs between the furnace and the quench tank, precipitate compounds (e.g., $\text{CuAl}_2$) nucleate preferentially along grain boundaries. This depletes the immediately adjacent zone of copper, creating an anodic copper-lean micro-zone right next to the noble precipitate and cathode grain core. In the presence of an electrolyte, the grain boundaries corrode violently.
• Visual Signature: Often completely invisible on the surface until the metal reaches advanced stages of structural degradation. Detection requires non-destructive inspection (NDI) methods such as high-frequency eddy current, ultrasonic testing, or dye penetrant inspection on cross-sections.

4. Exfoliation Corrosion

• Mechanism: An advanced, severe subtype of intergranular corrosion occurring primarily in rolled, forged, or extruded aluminum and magnesium shapes (such as spar caps, stringers, wing skin extrusions, and heavy structural bulkheads).
• Metallurgical Cause: Extrusion and rolling processes elongate alloy grains into flattened, plate-like, directional layers. When intergranular corrosion attacks these flattened grain boundaries, the voluminous corrosion products (which occupy up to $300%$ greater physical volume than the original metal) exert massive internal mechanical wedging forces.
• Visual Signature: The surface metal delaminates, lifts, blisters, flakes, and swells upward in distinct leaf-like sheets or layered flakes (resembling the layers of a puff pastry or the pages of an opened book). Fastener rows often exhibit bulging and pop-through.

Exfoliation Corrosion in Directional Extrusions:
  [  Surface Paint Layer  ]   ▲ Swelling / Blistering Upward
  ═════════════════════════  ─┼──────────────────────────
  ◄─── Elongated Grain ───►  /   Delaminating along grain boundaries
  ░░░░░░░░░░░░░░░░░░░░░░░░░ /    Wedging pressure from Al(OH)₃
  ◄─── Elongated Grain ───►      expansion forces layers apart
  ══════════════════════════════════════════════════════════

5. Galvanic (Dissimilar Metal) Corrosion

• Mechanism: Accelerated attack resulting from direct electrical contact between two metals possessing different electrode potentials immersed in a common electrolyte.
• Visual Signature: Severe, rapid dissolution, pitting, and material loss concentrated on the active metal immediately adjacent to the joint line or fastener interface with the noble metal.
• Common Aircraft Hazard Areas: Steel bolts installed dry in aluminum wing skins; stainless steel brackets mounted directly on unprimed aluminum bulkheads; bare carbon-fiber reinforced plastic (CFRP) panels fastened directly to aluminum fuselage framing.

6. Stress Corrosion Cracking (SCC)

• Mechanism: The spontaneous catastrophic cracking of a metal resulting from the simultaneous combined action of sustained static tensile stress and a corrosive environment.
• Three Mandatory Coexisting Factors:
  1. A susceptible high-strength alloy (e.g., 7000-series aluminum, 2024 aluminum, high-strength steels $\ge 180\text{ ksi}$, brass).
  2. A sustained static tensile stress (acting at or near the surface, caused by assembly interference fits, over-torqued fittings, residual stresses from forging, quenching, or welding, or sustained flight/pressurization loads).
  3. A mildly corrosive environment (such as humid air with trace chlorides or industrial sulfur).
• Visual Signature & Progression: Fine microscopic cracks propagating perpendicular to the applied tensile stress vector, predominantly along grain boundaries (intergranular SCC) or across grains (transgranular SCC). Cracks propagate silently until the remaining cross-sectional area can no longer sustain the load, leading to sudden, catastrophic brittle fracture at stresses well below the alloy's normal tensile yield strength.

7. Fretting Corrosion (Chafing / False Brinelling)

• Mechanism: Rapid damage occurring at the interface between two tightly contacting, load-bearing mating surfaces subjected to slight cyclic relative micro-motion (vibrational slip or cyclic elastic deflection).
• Progression: The microscopic sliding motion continuously wears away the natural protective oxide film, exposing fresh, reactive bare metal. The bare metal instantly oxidizes and the oxide debris is abraded into fine grit particles, acting as a grinding compound that accelerates wear.
• Visual Signatures:
  • Aluminum Alloys: Manifests as a characteristic fine black powdery residue or dark smudges radiating from fastener holes (widely known in line maintenance as "smoking rivets").
  • Ferrous / Steel Alloys: Manifests as a fine reddish-brown, purple, or dark red powder commonly termed "cocoa" or "blood."
• Typical Locations: Pressurized aircraft skin lap joints around working rivets, landing gear attachment trunnions, engine mount fittings, flap track guide pins, and bearing races.

8. Filiform Corrosion

• Mechanism: A specialized form of oxygen-concentration cell corrosion occurring beneath dense, low-permeability organic coatings (primarily polyurethane paint systems over chromate primers).
• Root Causes: Occurs when moisture and acidic electrolytes penetrate microscopic pinholes, edge scratches, or coating imperfections. It is heavily aggravated when acidic wash primers (such as DoD-P-15328) are improperly mixed, improperly cured, or exposed to excessive ambient humidity ($65%–90%\text{ RH}$) prior to topcoat application.
• Visual Signature: Appears as fine, worm-like, thread-like meandering filaments or spider-web tracks bulging beneath the paint film. The active head of each filament contains an acidic, oxygen-starved anode dissolving the metal, while the tail contains oxygen-rich inactive corrosion products.
• Characteristics: Does not penetrate deeply into the metallic thickness, but rapidly destroys paint adhesion across vast areas and initiates surface pitting.

9. Microbiological Corrosion in Fuel Tanks

• Mechanism: Biochemical corrosion occurring inside aircraft integral fuel tanks, wing sumps, and fuel system low points where water condensation settles beneath hydrocarbon jet fuel (Jet A, Jet A-1, JP-8).
• Microbiological Agent: Fungal microorganisms, predominantly Cladosporium resinae (reclassified as Hormoconis resinae), alongside sulfate-reducing bacteria.
• Progression: The microbes live in the bottom water layer and feed on the hydrocarbons in the jet fuel at the fuel-water interface. As they metabolize, they produce dense, brown/black gelatinous slime mats (biofilms) and excrete highly corrosive organic acids (such as citric and oxalic acids) and sulfuric acid.
• Visual Signature & Damage: Thick, slimy sludge clinging to fuel tank lower skins, clogged fuel scavenge lines and boost pump inlet screens, and rapid, deep pitting directly penetrating the aluminum tank structure beneath the fungal colonies.

10. Concentration Cell (Crevice) Corrosion

• Mechanism: Corrosion driven by differences in chemical concentration across an electrolyte in contact with a metallic surface.
  • Metal Ion Concentration Cells: Occur in tight crevices where stagnant liquid accumulates. The liquid within the crevice develops a high concentration of dissolved metal ions, while the liquid outside has a low concentration. The metal outside the crevice (in low ion concentration) acts as the anode and corrodes.
  • Oxygen Concentration Cells: The area inside a tight seam or crevice (under lap joints, gaskets, loose washers, or dirt deposits) is starved of atmospheric oxygen, while the exposed surface has high oxygen. The oxygen-starved area within the crevice becomes the anode and dissolves rapidly.

4. Visual Signatures and Diagnostic Matrix by Metal Group

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5. Realistic Exam Scenarios & Case Studies

Scenario 1: The Pressurized Fuselage Lap Joint Inspection

During a scheduled C-check inspection on a transport-category aircraft, an AMT notices dark grey-to-black streaks radiating aft from several flush rivet heads on the lower fuselage longitudinal lap joint. When the rivets are lightly tapped, several feel slightly loose.

• Diagnosis: This is fretting corrosion combined with mechanical joint working (colloquially termed smoking rivets). Cyclic pressurization and aerodynamic buffeting produce microscopic oscillatory slip between the rivet shank, rivet head, and aluminum skin. The microscopic rubbing abrades the aluminum oxide film into a fine black submicron aluminum powder, which is washed out by moisture and air currents.
• Mandatory Corrective Action: The AMT must inspect the skin for hole elongation and crack initiation using high-frequency eddy current (HFEC). Damaged rivets must be drilled out, fastener holes reamed to next-step oversize dimensions per the Structural Repair Manual (SRM), and new oversize rivets installed wet with polysulfide sealant.

Scenario 2: Heavy Wing Spar Cap Delamination

An AMT performing an annual inspection on a high-performance twin-engine aircraft removes the wing root fairing and inspects the 7075-T6 upper spar cap extrusion. Along a row of steel attach bolts, the aluminum extrusion appears swollen, with visible metal layers separating and flaking off in thin, brittle sheets.

• Diagnosis: This is exfoliation corrosion initiated along the directionally rolled grain boundaries of the 7075-T6 extrusion. The high internal volume expansion of hydrated aluminum hydroxide has forced the surface grains apart.
• Metallurgical Root Cause: Susceptibility caused by poor quench delay control during initial solution heat treatment, exacerbated by moisture trapped between the steel bolt heads and the unsealed spar cap.
• Corrective Action: The AMT must consult the manufacturer's Structural Repair Manual (SRM). If the depth of exfoliation exceeds allowable structural blending limits (typically $<10%$ of original flange thickness), the entire spar cap section or wing assembly must be condemned and replaced.