S — Aircraft StructuresChapter 4 · 10 practice questions

Chapter 4: Corrosion Control - Galvanic, Exfoliation, Intergranular, Pitting, Stress Corrosion, Prevention

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Corrosion Control - Galvanic, Exfoliation, Intergranular, Pitting, Stress Corrosion, Prevention

Overview

This chapter addresses the critical topic of corrosion control in aircraft structures, focusing on the identification, assessment, and management of various corrosion types that threaten airframe integrity. Corrosion is an electrochemical process that degrades metal components, and its detection and proper treatment are fundamental to aircraft airworthiness. The chapter covers six primary corrosion mechanisms: galvanic, exfoliation, intergranular, pitting, stress corrosion cracking, and general corrosion, along with their prevention strategies. Understanding these mechanisms enables maintenance personnel to make correct decisions regarding repair, replacement, or continued service of affected components.

Diagram — Corrosion Control - Galvanic, Exfoliation, Intergranular, Pitting, Stress Corrosion, Prevention CORROSION CONTROL — Aircraft Maintenance Engineering LÉGENDE Corrosion active Protection/Prévention Flux de processus Inspection/Contrôle Six mécanismes de corrosion sur aéronefs CORROSION Galvanic Deux métaux dissemblables Exfoliation Délaminage en couches Intergranular Attaque aux joints de grain Pitting Piqûres localisées Stress Corrosion Fissuration sous tension General Uniforme PRÉVENTION & CONTRÔLE Inspection régulière Détection précoce visuelle/NDT Revêtements & Inhibiteurs Peinture, alodine, inhibiteurs de corrosion Contrôle environnemental Drainage, ventilation, scellants TC Approved — Aircraft Maintenance Engineer Training — Corrosion Control Chapter

Key Concepts Explained

The 5 Types of Corrosion — Visual Identification The 5 Types of Corrosion — Visual Identification 1. Galvanic Electrolyte Anode (Al) (Corroded) Cathode (Protected) e- Flow Contact of 2 dissimilar metals + electrolyte. The anodic metal (less noble) corrodes. Ex: Mag/Al, Al/Steel 2. Pitting Depth Small deep cavities. Surface often intact. Stress concentration. Limit: Often 10% of thickness. 3. Intergranular Grain boundaries attacked Attack along grain boundaries. Invisible on surface. Loss of cohesion. Common: 7075-T6, 2024-T3 4. Exfoliation Lifted layers (flaking) Severe form of intergranular corrosion. Metal delamination. Voluminous corrosion products. Not repairable: Replacement required 5. Stress Cracking Tension Perpendicular crack Tensile stress + corrosive environment. Sudden cracking. No plastic deformation. Not repairable: Replacement required

Galvanic Corrosion

Galvanic Cell — Mechanism and Prevention Galvanic Cell — Mechanism and Prevention CORROSION MECHANISM Electrolyte (Moisture / Salt Water) ANODE Aluminum (Corrodes) CATHODE Stainless Steel (Protected) Electron Flow (e-) PREVENTION METHODS Electrical Isolation Non-conductive gaskets, washers, coatings, or paint. Sacrificial Anodes Adding a more anodic metal (e.g., zinc) that corrodes first. Inhibiting Compounds Application of pastes or liquids to exclude moisture (electrolyte). Ref: CARs Standard 571.06 | AC 43.13-1B AME Training - Corrosion Control

Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, creating a galvanic cell. The more anodic metal corrodes preferentially, while the cathodic metal is protected. This is governed by the galvanic series, which ranks metals according to their electrochemical potential in a given environment.

Critical relationships:

  • Magnesium is highly anodic to aluminum and will corrode preferentially when in contact
  • Aluminum is anodic to steel and copper alloys
  • The greater the separation in the galvanic series, the more severe the corrosion

Prevention measures:

  • Use of insulating gaskets or coatings between dissimilar metals
  • Application of sealants to prevent electrolyte ingress
  • Proper drainage to prevent moisture accumulation
  • Selection of metals close together in the galvanic series

Common locations: Battery compartments, landing gear assemblies, wheel assemblies, and any area where dissimilar metals contact without proper isolation.

Intergranular Corrosion

Intergranular vs Exfoliation Corrosion — Progression Intergranular Corrosion vs Exfoliation Wrought alloys 2024-T3 / 7075-T6 — Attack along grain boundaries Intergranular Corrosion Cross-section view — attack along grain boundaries Surface virtually intact Characteristics: Often invisible on surface, penetrates deeply Reduces mechanical strength with no external sign Not stopped by chemical treatment or sealing Detection: NDT (ultrasound, eddy current) Affects 2024-T3, 7075-T6 in corrosive environments Exfoliation (severe form) Delamination into layers parallel to the surface Lifted surface Characteristics: Delamination into layers parallel to the surface Structural strength considerably reduced Surface grinding does not remove the damage Stop holes ineffective to halt progression Component must be REPLACED Maintenance Actions GRINDING (blending) if localized Remove down to sound metal, check thickness VS REPLACEMENT mandatory Structure compromised — not repairable

Intergranular corrosion attacks the grain boundaries of a metal while leaving the grain interiors relatively unaffected. This occurs due to compositional differences at grain boundaries, often resulting from improper heat treatment or sensitization. In aluminum alloys, particularly 7075-T6 and 2024-T3, intergranular corrosion can propagate rapidly along grain boundaries, significantly reducing mechanical properties.

Characteristics:

  • Microscopic attack along grain boundaries
  • Can be difficult to detect visually in early stages
  • Reduces ductility and strength
  • May lead to exfoliation in wrought products

Treatment requirements:

  • Complete mechanical removal of all corroded material
  • Verification of remaining thickness against manufacturer's limits
  • Chemical conversion treatments alone are insufficient
  • Replacement is required if removal exceeds allowable limits

Exfoliation Corrosion

Exfoliation corrosion is a severe form of intergranular attack that occurs in wrought aluminum alloys with elongated grain structures. The corrosion propagates along grain boundaries parallel to the surface, causing the metal to delaminate or "leaf" outward. This is particularly common in extruded and rolled products.

Key characteristics:

  • Visible delamination and lifting of surface layers
  • Significant reduction in structural strength
  • Typically not repairable due to extensive intergranular damage
  • Common in 2024-T3 and 7075-T6 alloys

Required action: Exfoliation corrosion generally requires component replacement. Mechanical removal is ineffective because the intergranular damage extends beyond visible surface indications.

Pitting Corrosion

Pitting corrosion produces localized cavities or pits on metal surfaces. It is often initiated by breakdown of protective oxide films, particularly in chloride-containing environments such as marine atmospheres. Pitting can be difficult to detect because pits may be small and covered by corrosion products.

Assessment criteria:

Pitting Corrosion — Assessment and Limits Decision Pitting Corrosion: Measurement and Limits Decision tree — Pit depth vs. limit (10% of thickness) Pitting corrosion detected Visual inspection / NDI STEP 1 — Measure depth Depth gauge / optical microscope Measured depth vs. limit? Limit = 10% thickness YES ≤ 10% WITHIN LIMITS Document only No repair required ✎ Maintenance report NO > 10% OUT OF LIMITS Grinding / blending Remove all pits STEP 2 — Verification Measure residual thickness Residual thickness ≥ min. allowable? YES REPAIR ACCEPTABLE Protection + documentation NO REPLACEMENT Component not repairable Replace per manual Example: 0.040 in skin → limit = 0.004 in. Pit of 0.008 in → out of limits, grinding required. Surface D Within limits Out of limits Verification step
  • Pit depth measurement using calibrated probes or microscopes
  • Comparison against manufacturer's allowable limits (often expressed as percentage of skin thickness)
  • Consideration of pit location relative to stress concentrations
  • Documentation of all corrosion findings

Acceptance limits: Pitting within manufacturer's allowable limits requires no repair but must be documented. Pitting exceeding limits requires blending to remove pits and verification of remaining thickness against minimum allowable values.

Stress Corrosion Cracking (SCC)

Stress Corrosion Cracking — Mechanism Stress Corrosion Cracking (SCC): Mechanism Combination of tensile stress + corrosive environment → intergranular cracking REQUIRED CONDITIONS ① Tensile stress Applied or residual ② Corrosive environment Chlorides, humidity, sweat ③ Susceptible alloy 7075-T6, high-strength steels, titanium STRESS CONCENTRATORS Fastener holes Fillets, notches, scratches Pitting (possible initiation sites) INTERGRANULAR PROPAGATION Along grain boundaries Without plastic deformation Sudden fracture possible SCHEMATIC REPRESENTATION OF THE MECHANISM Microscopic view — grain boundaries: ← crack intergranular σ σ ← → Corrosive environment: Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ Initiation point (concentrator): hole NON-REPAIRABLE — REPLACEMENT REQUIRED Stop-drilling, welding, cold working: ineffective ① Tensile stress: applied (loads) or residual (machining, forming) ② Environment: chlorides, humidity ③ Susceptible alloys: 7075-T6, high-strength steels

Stress corrosion cracking results from the combined action of tensile stress and a corrosive environment. High-strength aluminum alloys (particularly 7075-T6) and some steel alloys are susceptible. SCC typically initiates at corrosion sites or stress concentrations and propagates as intergranular or transgranular cracks.

Critical factors:

  • Tensile stress (applied or residual)
  • Specific corrosive environment
  • Susceptible material condition
  • Time-dependent propagation

Treatment requirements:

  • SCC is typically not repairable by stop-drilling or other methods
  • Component replacement is usually required
  • Approved repair data is necessary for any alternative action
  • Cracks exceeding manufacturer's allowable limits mandate replacement or approved repair

General Corrosion and Fretting Corrosion

General corrosion produces uniform attack over a surface, while fretting corrosion occurs at faying surfaces under cyclic loading. White powdery deposits on aluminum indicate aluminum oxide/hydroxide formation, often from battery acid fumes or galvanic action.

Important Procedures and Regulations

Corrosion Removal Procedures

Corrosion Removal Procedure — Sequence Corrosion Removal Procedure 1. INSPECTION Inspect the area Identify the type of corrosion • Measure depth/extent • Compare to limits • Document (photos, records) 2. REMOVAL Complete grinding of the corroded area • Remove ALL pits • Feather the edges • Do not mask the attack 3. VERIFICATION mm Residual thickness ≥ minimum thickness allowable? If < limit → replace 4. PROTECTIVE TREATMENT Chemical conversion (alodine) Primer (primer) Corrosion inhibitor ⚠ CRITICAL RULE Apply the inhibitor ONLY after COMPLETE corrosion removal. CARs Standard 571.06 — AC 43.13-1B — Complete removal before protective treatment

Per AC 43.13-1B and FAA Airframe Handbook:

  1. Inspection and documentation:
  • Measure and record all corrosion dimensions
  • Photograph affected areas
  • Reference manufacturer's allowable limits
  1. Mechanical removal:
  • Use abrasive blasting, sanding, or grinding
  • Remove all corrosion products completely
  • Feather edges to prevent stress concentrations
  • Verify complete removal using chemical indicators
  1. Thickness verification:
  • Measure remaining material thickness
  • Compare against minimum allowable values
  • Replace if below limits
  1. Protective treatment:
  • Apply conversion coating (alodine)
  • Prime and paint as required
  • Apply corrosion-inhibiting compounds

Regulatory Requirements (CARs Standard 571.06)

  • Any corrosion on flight control cables requires replacement
  • Corrosion within manufacturer's limits requires documentation
  • Corrosion exceeding limits requires repair or replacement using approved data
  • All repairs must be performed using approved methods and materials

Corrosion Inhibitor Application

Critical rule: Corrosion inhibitors are not a substitute for corrosion removal. All corrosion products must be completely removed before applying any protective treatment. Failure to do so results in recurrence of corrosion under the inhibitor.

Common Relationships Between Concepts

Galvanic series and material selection:

  • Magnesium (most anodic) → Aluminum → Steel → Copper alloys → Stainless steel (most cathodic)
  • The closer metals are in the series, the lower the galvanic corrosion risk

Corrosion types and material susceptibility:

  • 7075-T6 aluminum: Susceptible to intergranular corrosion and SCC
  • 2024-T3 aluminum: Susceptible to exfoliation corrosion
  • Magnesium alloys: Highly susceptible to galvanic corrosion
  • Steel alloys: Susceptible to general corrosion and SCC

Environmental factors:

  • Marine environments accelerate pitting and galvanic corrosion
  • Battery acid fumes cause galvanic corrosion on aluminum
  • Chlorides promote pitting corrosion
  • High humidity provides electrolyte for all corrosion types

Corrosion severity and required action:

Corrosion Severity — Required Action Matrix Corrosion Severity: Action Required Corrosion Type Severity Action Required Pitting within limits Depth ≤ limit (e.g., ≤ 10% thickness) Slight Acceptable 📋 Document corrosion Record location, depth, and extent in the maintenance log. Pitting out of limits Depth > limit (e.g., > 10% thickness) Moderate Repair required 🔧 Grind and verify 1. Grind (blend) to remove pits 2. Measure residual thickness Intergranular Grain boundary attack Often invisible on surface Moderate Removal required ⚙️ Mechanical removal Grind to sound metal, check residual thickness. Inhibitor alone = ineffective. Exfoliation Layer delamination Severe form of intergranular Severe Not repairable ⚠️ Mandatory replacement Surface grinding does not remove underlying intergranular damage. Stress corrosion cracking Cracking + stress + corrosive environment Critical Sudden failure possible ⚠️ Mandatory replacement Stop-drilling, welding, and mechanical treatment ineffective. Replace the component. Flight control cables Corrosion on cables Critical Flight safety ⚠️ Mandatory replacement Regulatory requirement — CARs Std 571.06 SLIGHT MODERATE SEVERE CRITICAL Reference: AC 43.13-1B Ch.6 — CARs Standard 571.06 — Always consult the manufacturer's manual
  • Pitting within limits → Document only
  • Pitting exceeding limits → Blend and verify thickness
  • Intergranular corrosion → Mechanical removal required
  • Exfoliation → Replacement typically required
  • SCC → Replacement typically required
  • Corrosion on flight control cables → Replacement required

Prevention strategies:

  • Proper drainage and ventilation
  • Regular cleaning and inspection
  • Application of protective coatings
  • Use of corrosion-inhibiting compounds
  • Proper material selection and isolation
  • Timely removal of corrosion products

Diagram

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