4.2 Forms & Mechanisms of Aircraft Corrosion

4.2 Forms & Mechanisms of Aircraft Corrosion

Corrosion does not attack aircraft structures in a single, uniform manner. Depending on alloy composition, internal crystal microstructure, fabrication methods, residual stresses, coating integrity, and environmental exposure, corrosion manifests in multiple distinct physical forms.

For aircraft maintenance engineers, correctly identifying the specific form of corrosion during scheduled inspections is paramount. While some forms represent broad, predictable surface wear, others are insidious, localized, and subsurface—capable of causing sudden, catastrophic structural separation with little or no advance visual warning.

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1. Uniform (General Surface) Corrosion

Uniform or surface corrosion is the broad, continuous, even chemical or electrochemical attack across the entire exposed surface of a metal component.

Characteristics & Morphology:

• Manifests visually as a general dulling, etching, or roughening of an originally polished or painted surface.
• On aluminium alloys, it forms a powdery white or gray deposit of aluminium oxide and hydroxide.
• On carbon and low-alloy steels, it forms reddish-brown iron oxide (rust, $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$).
• On copper-base alloys, it produces a characteristic green patina of basic copper carbonate.

Engineering Significance:

• Uniform corrosion is caused by direct exposure to acidic atmospheric moisture, industrial pollutants ($\text{SO}_2$), chemical wash cleaners, or unmitigated weathering.
• From an airworthiness standpoint, uniform corrosion is the least dangerous form of corrosion. Because metal loss is evenly distributed, the remaining structural strength and skin thickness can be readily measured using standard ultrasonic thickness gauges or optical micrometers.
• It progresses at a relatively constant, predictable rate without creating sharp stress-concentration notches. Treatment involves standard mechanical blend-out, chemical conversion, and repainting.

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2. Galvanic Corrosion (Dissimilar Metal Attack)

Galvanic corrosion occurs when two metals possessing different electrochemical potentials are placed in direct electrical contact while immersed in a common conductive electrolyte.

Mechanism & Prevention:

• The more active metal dissolves at an accelerated rate as an anode, while the more noble metal is electrochemically protected as a cathode.
• The severity depends on the potential difference ($\Delta V$), the electrolyte conductivity, and the cathode-to-anode surface area ratio.
• Aviation Prevention Strategies:
  1. Dielectric Isolation: Inserting non-conductive shims (such as fiberglass cloth impregnated with epoxy, or PTFE washers) between mating surfaces.
  2. Interfacial Sealants: Applying flexible polysulfide or polythioether sealant (e.g., MIL-PRF-8116 or AMS 3277) to all faying (mating) surfaces prior to assembly ("wet assembly").
  3. Protective Metallic Platings: Plating steel hardware with cadmium or zinc to shift its potential closer to that of aluminium.
  4. Paint Encapsulation: When painting a galvanic joint, both metals must be painted. If only the active anode is painted, a scratch in the paint creates a dangerous small anode / large cathode condition. If only one metal can be painted, paint the cathode to restrict the available cathodic reduction area.

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3. Pitting Corrosion (The Autocatalytic Acid Pit)

Pitting corrosion is an extremely localized, aggressive form of attack that produces deep, narrow cavities or pits penetrating vertically into the metal. The depth of a pit is typically much greater than its surface diameter.

                    BULK ELECTROLYTE (Aerated Water + Cl⁻)
 ─────────────────────────────────────────────────────────────────────────────
       SURFACE: CATHODE (Passivated Al₂O₃) ── O₂ + 2H₂O + 4e⁻ ──> 4OH⁻
 ═════════════════════════╤═════════════════════════╤═════════════════════════
                          │  Narrow Pit Mouth       │
                          │ (Plugged by Al(OH)₃)    │
                          │                         │
                          │   PIT INTERIOR (ANODE)  │
                          │   Al ──> Al³⁺ + 3e⁻     │
                          │                         │
                          │   Cl⁻ ions migrate in   │
                          │   Al³⁺ + 3H₂O ──>       │
                          │      Al(OH)₃ + 3H⁺      │
                          │                         │
                          │   pH DROPS TO 1.5 - 2.0 │
                          │   (Hydrochloric Acid!)  │
                          └─────────────────────────┘
                          AUTOCATALYTIC ACCELERATION!

The Autocatalytic Pitting Mechanism:

1. Initiation: Pitting initiates at microscopic defects, scratches, or intermetallic inclusions (such as iron-copper particles in 2024 aluminium) where the protective passive oxide film ($\text{Al}_2\text{O}_3$) is breached by chloride ions ($\text{Cl}^-$).
2. Electrochemical Cell Formation: The tiny exposed metal area inside the microscopic pit is starved of dissolved oxygen and becomes the anode (metal dissolution: $\text{Al} \rightarrow \text{Al}^{3+} + 3e^-$). The expansive, oxygen-rich external metal surface acts as the cathode ($\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$).
3. Ionic Migration: As positive aluminium cations ($\text{Al}^{3+}$) accumulate inside the pit, negative chloride ions ($\text{Cl}^-$) migrate rapidly into the pit from the bulk electrolyte to maintain electrical neutrality.
4. Hydrolysis & Acidification: The concentrated aluminium chloride solution inside the pit undergoes chemical hydrolysis: $\text{Al}^{3+} + 3\text{H}_2\text{O} \longrightarrow \text{Al(OH)}_3 \downarrow + 3\text{H}^+$
5. Autocatalytic Runaway: The generation of free hydrogen ions ($3\text{H}^+$) depresses the internal pH inside the pit to $1.5\text{ to }2.0$—creating a pocket of concentrated hydrochloric acid! This high acidity dissolves the metal at an ever-increasing rate.
6. Mouth Plugging: Insoluble white aluminium hydroxide precipitates at the mouth of the pit, forming a microscopic crust or blister that restricts fresh oxygen entry while sealing the acidic electrolyte inside.

Structural Danger:

Pits represent severe, sharp stress-concentration notches. Under flight cyclic tensile loading, fatigue cracks rapidly nucleate from the base of corrosion pits. Many catastrophic aircraft fatigue failures originate from a single unnoticed corrosion pit.

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4. Intergranular Corrosion (IGC)

Intergranular corrosion is the selective, microscopic attack that propagates along the crystal grain boundaries of an alloy, while the interior of the grains remains largely unaffected.

Metallurgical Mechanism:

IGC is caused by microscopic compositional differences between the grain boundaries and the adjacent grain cores, resulting from improper thermal processing or heat treatment:

• In 2024 Aluminium (Al-Cu): If the alloy is cooled too slowly during solution heat treatment (excessive **quench delay exceeding $7\text{ to }15\text{ seconds}$), copper atoms diffuse to the grain boundaries and precipitate as coarse intermetallic copper aluminide particles ($\text{CuAl}_2$). This depletes copper from the narrow zone immediately adjacent to the grain boundaries. Because the copper-depleted zone has an electrode potential ($-0.83\text{ V}$) significantly more negative than the copper-rich core ($-0.68\text{ V}$), the grain boundary zone becomes an active anode and dissolves rapidly in the presence of moisture.
• In Austenitic Stainless Steels (300 Series): Heating between $425^\circ\text{C}$ and $850^\circ\text{C}$ ($800^\circ\text{F}$ to $1550^\circ\text{F}$) causes sensitization (weld decay). Carbon diffuses to grain boundaries and precipitates as chromium carbides ($\text{Cr}_{23}\text{C}_6$). This depletes chromium below the critical $10.5%$ passivity threshold along grain boundaries, triggering rapid intergranular attack in exhaust manifolds and welded ducting.

Inspection Hazard:

IGC is exceptionally treacherous because it penetrates deeply into the interior of structural forgings, extrusions, and sheets without producing noticeable surface thinning or voluminous deposits. A component suffering advanced IGC can lose over $80%$ of its structural load-bearing capacity while appearing pristine to visual inspection.

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5. Exfoliation Corrosion (Lamellar / Layer Corrosion)

Exfoliation corrosion is an advanced, severe form of intergranular corrosion that occurs specifically in wrought aluminium alloys that have been heavily rolled, extruded, or forged, producing highly elongated, flattened, directional grain structures (e.g., 2024-T351, 7075-T6 extrusions, spar caps, floor beams).

        EXFOLIATION CORROSION IN EXTRUDED / ROLLED ALUMINIUM

  ▲ Surface blisters and flakes outward ("leafing")
  │ ┌─────────────────────────────────────────────────────────────┐
  │ │ //////////////////// LIFTED SURFACE LAYER ///////////////// │
  │ └─────────────────────────────────────────────────────────────┘
  │   ▲▲▲▲▲  WEDGE PRESSURE (> 300 MPa) FROM EXPANDING CORROSION PRODUCTS!
  │ ┌─────────────────────────────────────────────────────────────┐
  │ │ //////////////////// INTERMEDIATE GRAIN /////////////////// │
  │ └─────────────────────────────────────────────────────────────┘
  │   ▲▲▲▲▲  Hydrated Alumina (Al₂O₃·3H₂O) occupies 3x to 7x volume of metal!
  │ ┌─────────────────────────────────────────────────────────────┐
  │ │ //////////////////// BASE BULK ALLOY ////////////////////// │
  │ └─────────────────────────────────────────────────────────────┘
  └─────────────────────────────────────────────────────────────────

The Delamination Mechanism:

1. Corrosion initiates at exposed grain boundaries—most commonly at sheared sheet edges, countersunk fastener holes, or unsealed fastener counterbores.
2. The corrosion attack propagates horizontally along the elongated grain boundaries parallel to the rolling surface.
3. The insoluble corrosion product—hydrated aluminium oxide ($\text{Al}_2\text{O}_3 \cdot 3\text{H}_2\text{O}$)—occupies $3\text{ to }7\text{ times the physical volume}$ of the original parent aluminium metal from which it formed.
4. This immense volumetric expansion generates massive internal mechanical wedge pressures exceeding $300\text{ MPa}$ ($43\text{ ksi}$).
5. The force physically forces the uncorroded surface grains apart, causing the surface of the metal to blister, flake, swell, and delaminate. The surface lifts up in thin, brittle layers, resembling the pages of a wet book ("leafing").

High-Risk Airframe Zones:

• Countersunk rivet holes on upper wing skin panels (especially alloys 7075-T6 and 7178-T6).
• Extruded cargo seat tracks, fuselage stringers, and door sill support beams.
• Step fittings and piano hinges exposed to de-icing salts.

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6. Stress Corrosion Cracking (SCC)

Stress Corrosion Cracking (SCC) is the sudden, catastrophic brittle failure of a ductile alloy that occurs under the simultaneous confluence of three mandatory factors:

                      STRESS CORROSION CRACKING TRIAD

                             SUSCEPTIBLE ALLOY
                            (e.g., 7075-T6, 2024-T3,
                             Steels UTS > 1000 MPa)
                                    ▲
                                   / \
                                  /   \
                                 / SCC \
                                /       \
                               /         \
                              /           \
     SUSTAINED TENSILE STRESS ───────────── SPECIFIC CORROSIVE ENVIRONMENT
   (Residual or Applied; e.g.,              (Moisture, Marine Chlorides,
    Interference Fits, Shimming)             Industrial Atmosphere)

The Three SCC Pillars:

1. A Susceptible Metallic Alloy: High-strength alloys with high yield-to-tensile ratios, such as 7075-T6, 2024-T3, 7178-T6, high-strength martensitic steels ($UTS > 1000\text{ MPa}$ / $145\text{ ksi}$), and titanium alloys exposed to chlorinated solvents or red fuming nitric acid.
2. A Sustained Static Tensile Stress: The stress must be tensile; compressive stresses completely prevent SCC. The stress does not need to be an operational flight load; it is most commonly a residual internal stress resulting from:
   • Interference press-fit bushings or bearings.
   • Overtightened tapered fasteners (Taper-Lok / Hi-Lok bolts).
   • Forced alignment and poor structural shimming during assembly.
   • Residual quenching stresses from solution heat treatment.
   • Cold-straightening or bending of extrusions and forgings.
3. A Specific Corrosive Environment: The environment does not need to be highly aggressive. Ambient atmospheric moisture containing trace chlorides or industrial vapors is sufficient to trigger SCC in highly stressed 7075-T6.

Crack Morphology & Prevention:

• Grain Direction Sensitivity: SCC propagation occurs most readily along the short transverse (ST) grain direction of heavy forgings and thick plate, where grain boundary cohesion is lowest.
• Prevention Methods:
  • Overaging Heat Treatments: Replacing peak-aged -T6 tempers with specially overaged tempers—most notably 7075-T73 or 7050-T7451. Overaging reduces peak yield strength by approximately $10%$, but provides virtual immunity to SCC.
  • Shot Peening: Bombarding the component surface with spherical steel shot or ceramic beads. Shot peening plastically deforms the surface, inducing a residual compressive stress layer ($0.2\text{ to }0.5\text{ mm}$ deep). Because cracks cannot propagate in compression, SCC initiation is prevented.
  • Precise Shimming: Enforcing strict SRM gap tolerances (e.g., max allowable unshimmed gap $< 0.005\text{ in}$ or $0.13\text{ mm}$) to eliminate assembly pre-stress.

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7. Fretting Corrosion (Chafing / False Brinelling)

Fretting corrosion occurs at the interface between two tightly fitted, load-bearing metal surfaces subjected to microscopic cyclic relative slip or vibration under high contact pressure.

Mechanical-Chemical Sequence:

1. The two surfaces are clamped together under high normal force, with contact occurring exclusively at microscopic surface asperities (peaks).
2. Cyclic mechanical vibration or flexing produces microscopic oscillatory slip (amplitude typically $1\text{ to }100\ \mu\text{m}$).
3. This micro-motion mechanically rubs off and strips the protective passive oxide film from the contacting asperities, exposing bare, chemically clean metal.
4. Under contact pressure, bare asperities adhere and micro-weld together, only to be sheared apart by the next vibratory cycle.
5. The dislodged metallic micro-debris immediately oxidizes in the presence of air. Because metal oxides (such as alumina, $\text{Al}_2\text{O}_3$) are exceptionally hard ceramic abrasives, the trapped oxide debris acts as a grinding paste (lapping abrasive), accelerating wear and gouging deep microscopic pits.

Visual Identification Debris:

• On Steel Surfaces: The fretting debris manifests as a fine, reddish-brown or black powder commonly termed "cocoa" or "blood" (finely divided $\alpha\text{-Fe}_2\text{O}_3$).
• On Aluminium Surfaces: Fretting generates a fine, black powdery debris consisting of aluminium oxide and finely divided metallic aluminium.

The "Smoking Rivet" Phenomenon:

On commercial airliner fuselages, cabin pressurization cycles cause the fuselage diameter to expand and contract cyclically. If a countersunk rivet experiences slight shank looseness or hole elongation, the rivet head and skin undergo cyclic micro-slip. The resulting fretting generates fine black aluminium oxide powder. In-flight boundary airflows and condensation wash this black powder out from beneath the rivet head, forming dark, horizontal streaks or "smoke trails" across the skin. In aviation inspection, "smoking rivets" are the classic visual indicator of fastener looseness, joint fretting, and imminent hole fatigue cracking.

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8. Filiform Corrosion (Under-Film Worm-Like Attack)

Filiform corrosion is an unusual, specialized form of differential aeration corrosion that develops beneath thin organic coating systems (polyurethane, epoxy, or lacquer topcoats). It appears as a dense network of fine, meandering, worm-like or thread-like filaments.

                        ORGANIC PAINT FILM (Polyurethane / Epoxy)
 ═══════════════════════╤══════════════════════════════════╤══════════════════
                        │                                  │
  AIR (O₂ Diffuses)     │    FILAMENT TAIL (Cathode)       │ ADVANCING HEAD
  ─────────────────►    │    Oxygen Reduction              │ (Active Anode)
                        │    O₂ + 2H₂O + 4e⁻ ──> 4OH⁻      │ Deaerated, Acidic
                        │                                  │ pH 1.0 - 2.0
                        │    Precipitates Insoluble        │ Rich in Cl⁻ & Al³⁺
                        │    Al(OH)₃ / Al₂O₃               │ (Dissolves Metal!)
                        │    (Cracks paint over tail)      │
 ═══════════════════════╧══════════════════════════════════╧══════════════════
                   ALUMINIUM SUBSTRATE (2024 / 7075)

Critical Environmental Window:

Filiform corrosion is strictly governed by atmospheric relative humidity:

• It propagates exclusively within a relative humidity range of $65% \text{ to } 90% \text{ RH}$.
• Below $65% \text{ RH}$, moisture diffusion through the paint is too low to maintain the electrochemical cell.
• Above $90% \text{ RH}$, moisture absorption is so massive that the filament morphology breaks down, resulting in broad paint blistering rather than directional filaments.

Electrochemical Mechanism:

1. Initiation: Initiates at coating scratches, stone chips, sheared sheet edges, or unsealed fastener countersinks where airborne chloride salts have deposited on bare metal.
2. The Active Head (Anode): The advancing tip of the filament is an active, deaerated electrochemical anode. Trapped chlorides hydrolyze into acidic aluminium salts, maintaining an internal pH of $1.0\text{ to }2.0$. The head aggressively dissolves the metal substrate, boring forward under the coating.
3. The Inactive Tail (Cathode): Behind the head, oxygen diffuses through the thin paint film. In this oxygenated environment, the cathodic reduction reaction generates hydroxyl ions, precipitating insoluble aluminium hydroxide. The expanding solid corrosion product cracks the paint film along the filament spine, allowing continuous oxygen ingress to sustain the cathodic tail while the head drives forward.

Structural Risk:

Filiform filaments are shallow ($0.05\text{ mm}$ or $0.002\text{ in}$) and rarely compromise airframe structural strength directly. However, if ignored, filiform filaments strip paint adhesion across wide areas, establishing crevice conditions that escalate into severe pitting and intergranular exfoliation.

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9. Crevice Corrosion & Differential Aeration Cells

Crevice corrosion occurs within narrow, shielded gaps, joints, and micro-cavities where stagnant liquid becomes trapped. Classic aircraft locations include:

• Lap joints between fuselage skin panels.
• Beneath fastener heads, washers, and clamp cushions.
• Gasket mating faces and unsealed access doors.
• Beneath cracked or disbonded sealant beads.

Oxygen Concentration Cell Mechanics:

Crevice corrosion is primarily driven by oxygen concentration gradients:

1. Initially, uniform oxidation occurs both inside and outside the narrow crevice, consuming dissolved oxygen.
2. Outside the crevice, dissolved oxygen is continuously replenished by atmospheric circulation.
3. Inside the tight crevice (typically gaps less than $0.1\text{ mm}$ wide), the stagnant liquid cannot circulate, and dissolved oxygen is completely depleted.
4. This sets up a differential aeration cell: the oxygen-starved metal deep inside the crevice becomes an intense anode, while the oxygen-rich exterior metal acts as the cathode.
5. Metal inside the crevice dissolves rapidly, drawing in chloride ions that hydrolyze into acidic compounds, accelerating localized metal destruction.

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10. Summary: Aircraft Corrosion Forms at a Glance

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11. Practical Maintenance Scenarios & Exam Traps

Maintenance Scenario 2: Upper Wing Skin Fastener Inspection

During a heavy C-check on a commercial transport aircraft, an engineer inspects the countersunk fastener rows on the upper wing skin (7075-T6 extrusion). Around several titanium fastener heads, the paint is cracked and bulging. Upon scraping away the paint, the technician observes white powdery corrosion product and noticeable flaking of the aluminium skin, with metal layers lifting up like pages of a book.

Engineering Analysis:

• The morphology is classic exfoliation corrosion propagating from the fastener counterbore along the elongated rolling grains of the 7075-T6 alloy.
• The damage was triggered by moisture penetrating a failed fastener seal, combined with the extreme exfoliation susceptibility of peak-aged 7075-T6.
• Mandatory Action: Fasteners in the affected zone must be removed. The depth of exfoliation must be determined using an optical depth micrometer. The technician must consult SRM Chapter 51 to verify if the damage falls within allowable blend-out limits. If allowable, blend out using a 1:50 taper ratio, confirm complete removal of grain boundary fissures via High-Frequency Eddy Current (HFEC) or penetrant testing, apply MIL-DTL-5541 conversion coating, prime, and install new fasteners wet with polysulfide sealant. If beyond blend-out limits, an engineered doubler repair or spar cap replacement is mandatory.

Common Exam Traps:

1. The Compressive Stress Trap: Questions frequently ask: "Which stress state causes Stress Corrosion Cracking?" The answer is strictly sustained tensile stress. Compressive stresses prevent SCC completely.
2. The Filiform Humidity Window: Remember the exact numbers: $65% \text{ to } 90% \text{ RH}$. Questions will tempt you with $30%-50%$ or $>95%$. Neither supports filiform.
3. Fretting Debris Color Trap: Questions test your knowledge of debris color: steel fretting produces reddish-brown "cocoa", while aluminium fretting produces black powder ("smoking rivets").