14.2 Corrosion Classifications, Mechanical Removal & Chemical Surface Treatments

14.2 Corrosion Classifications, Mechanical Removal & Chemical Surface Treatments

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Corrosion is the progressive chemical or electrochemical degradation of a metal resulting from its natural thermodynamic tendency to return to its lowest energy state (such as oxides, hydroxides, or carbonates). On aircraft structures, corrosion represents an insidious, persistent threat to structural integrity, fatigue endurance, and continuing airworthiness. Aircraft operate in hostile environments characterized by saline marine air, high humidity, industrial sulfur dioxide, de-icing chemicals, and extreme thermal cycling. Certifying maintenance engineers under EASA Part-66 Module 7 must possess an exacting understanding of electrochemical corrosion mechanisms, visual and NDT identification, mechanical blend-out standards, chemical conversion treatments, and protective coating systems.

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The Fundamental Electrochemistry of Aircraft Corrosion

All metallic corrosion on aircraft is fundamentally electrochemical. For an electrochemical corrosion cell to exist and sustain corrosion metal loss, four essential elements must be present simultaneously:

                    THE ELECTROCHEMICAL CORROSION CELL

                             Electrolyte (Liquid film)
                 +---------------------------------------------+
                 |         H2O + O2 + Dissolved Salts          |
                 |                                             |
                 |    Anode               Cathode              |
                 |   (Oxidation)        (Reduction)            |
                 |   Al -> Al3+ + 3e-   O2 + 2H2O + 4e- -> 4OH-|
                 +-------+--------------------+----------------+
                         |                    |
                         +==[ Metallic Path ]=+
                            (Electron Flow)

1. The Anode: The metal or localized site with the lower electrode potential (more electrochemically active / less noble). At the anode, metal atoms lose electrons and oxidize into positive metal ions that dissolve into the electrolyte:

$$M \rightarrow M^{n+} + n e^-\quad (\text{e.g., } Al \rightarrow Al^{3+} + 3e^-)$$

1. The Cathode: The metal or localized site with the higher electrode potential (more noble / less active). The cathode does not corrode; instead, it receives electrons from the anode. In aerated neutral electrolytes, reduction of dissolved oxygen occurs at the cathode:

$$O_2 + 2H_2O + 4e^- \rightarrow 4OH^-\quad (\text{Hydroxide Ions})$$

1. The Metallic Electrical Path: A continuous, low-resistance metallic connection between the anode and cathode that permits electrons to flow freely from anode to cathode.
2. The Electrolyte: A conductive liquid solution (such as moisture, rain, condensation, de-icing fluid, or toilet fluid containing dissolved salts, chlorides, or industrial acids) in contact with both the anode and cathode that supports the conduction of ionic current.

The Fundamental Rule of Corrosion Prevention

Eliminating any ONE of these four elements completely halts the corrosion process. Applying paint, primers, and sealants establishes a physical barrier that prevents the electrolyte from reaching the metal, thereby breaking the corrosion circuit.

The Galvanic Series in Seawater

The driving force of galvanic electrochemical reactions is the potential difference between dissimilar metals. In a galvanic couple, the metal located higher (more active/anodic) in the galvanic series corrodes sacrificially, protecting the metal located lower (more noble/cathodic):

$$\text{Magnesium (Most Active)} \rightarrow \text{Zinc} \rightarrow \text{Aluminium Alloys} \rightarrow \text{Cadmium} \rightarrow \text{Carbon Steel} \rightarrow \text{Stainless Steel (Active)} \rightarrow \text{Titanium} \rightarrow \text{Silver} \rightarrow \text{Gold} \rightarrow \text{Carbon / CFRP (Most Noble)}$$

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Classifications and Mechanisms of Aircraft Corrosion

Aircraft corrosion manifests in various distinct physical forms, each with unique environmental triggers, morphological appearances, and structural consequences.

1. Surface / Uniform Corrosion

Uniform corrosion is a general, even attack across exposed metallic surfaces where microscopic anodic and cathodic sites continuously shift across the surface. It results from atmospheric exposure without an effective protective barrier.

• Appearance: Dulling, surface roughening, and uniform powdery deposits. On aluminium alloys, it forms a grey-white powdery dust (aluminium oxide / hydroxide, $Al_2O_3 \cdot 3H_2O$). On low-alloy steels, it forms reddish-brown rust ($Fe_2O_3$).
• Impact: Easiest to detect visually and treat; uniform reduction in skin thickness without localized stress-concentration notches.

2. Galvanic (Dissimilar Metal) Corrosion

Galvanic corrosion occurs when two dissimilar metals possessing different electrochemical potentials are placed in direct electrical contact in the presence of an electrolyte. The potential difference drives rapid galvanic current, causing aggressive accelerated corrosion of the more anodic metal, while the cathodic metal remains protected.

• Critical Aerospace Hazard: Carbon-Fiber Composites (CFRP) Coupled to Aluminium. Carbon fibers in CFRP structures are electrically conductive and highly noble / cathodic (more noble than stainless steel and close to platinum). When an aluminium airframe skin, bracket, or fastener directly contacts CFRP in the presence of moisture, the aluminium acts as a sacrificial anode, undergoing catastrophic galvanic disintegration. To prevent this, manufacturers mandate an electrical barrier ply (e.g., an isolation ply of fiberglass/epoxy or Tedlar film), wet-sealant installation, and the exclusive use of titanium or A-286 stainless steel fasteners in composite-to-metal joints.

3. Pitting Corrosion

Pitting is an extremely localized, aggressive form of corrosion that produces deep, narrow cavities penetrating vertically into the metal.

• Mechanism: Pitting initiates at microscopic defects in the passive oxide film, under surface dirt deposits, or at chloride ion concentrations. Once a pit forms, the micro-environment inside the pit becomes depleted of oxygen and enriched with positive metal ions ($Al^{3+}$). Negatively charged chloride ions ($Cl^-$) migrate into the pit to maintain electrical neutrality, hydrolyzing into hydrochloric acid ($HCl$). This establishes an autocatalytic acidic concentration cell with a tiny internal anode and a vast external cathode, driving rapid vertical penetration.
• Appearance: Tiny white powdery nodules or blisters on the surface masking deep, sharp-bottomed pinholes.
• Hazard: Pits act as severe geometric stress risers, initiating fatal structural fatigue cracks under cyclic flight loads.

4. Intergranular Corrosion (IGC)

Intergranular corrosion is a selective chemical attack along the microscopic grain boundaries of an alloy, while the interior grain cores remain virtually unattacked.

• Root Cause: Improper heat treatment, inadequate solution heat treatment, or slow, delayed quenching of high-strength precipitation-hardened aluminium alloys (e.g., 2024-T3, 7075-T6). Slow cooling allows intermetallic precipitates (such as $CuAl_2$ in 2000-series alloys or $MgZn_2$ in 7000-series alloys) to segregate and precipitate along grain boundaries. This depletes the adjacent grain boundary margin of copper or zinc, creating a narrow, highly anodic strip alongside the noble precipitate. In the presence of an electrolyte, galvanic micro-cells rapidly destroy the grain boundaries.
• Appearance & Hazard: Extremely difficult to detect visually because it progresses internally through the metallic microstructure with minimal surface discoloration. It causes sudden loss of structural strength and ductile toughness.

5. Exfoliation Corrosion

Exfoliation is an advanced, severe manifestation of intergranular corrosion occurring in wrought aluminium alloys with highly directional, flattened, elongated grain structures produced by rolling, forging, or extrusion processes (e.g., wing spar caps, fuselage stringers, seat tracks).

• Mechanism: Corrosion attacks grain boundaries parallel to the rolled surface. The resulting corrosion products (aluminium oxides/hydroxides) occupy 3 to 5 times the volume of the original parent metal. This immense volumetric expansion exerts massive internal wedging forces, lifting the surface grains and delaminating the metal in distinct, fragile layers.
• Appearance: Severe surface blistering, swelling, flaking, and lifting of leaf-like sheets of metal, culminating in laminar peeling.

                        EXFOLIATION CORROSION

            Expanding Corrosion Products Force Surface Grains Upward
            ===================================
            \\\ Surface Leaf Flakes Lifting ///   <-- Blistering & Delamination
            ===================================
             Directional Elongated Grain Boundaries (Rolled/Extruded)
            ===================================
            Base Parent Metal (Sound Structure)

6. Stress Corrosion Cracking (SCC)

Stress Corrosion Cracking (SCC) is an insidious, catastrophic failure mechanism resulting from the simultaneous interaction of three mandatory factors:

1. A susceptible alloy microstructure (e.g., high-strength 7075-T6 aluminium, 2024-T3, 300M landing gear steel, or titanium alloys).
2. A sustained, static tensile stress acting upon the component (residual manufacturing stresses from interference-fit bushings, over-torqued taper pins, press-fits, cold forming, or sustained flight/pressurization loads; compressive stresses do NOT cause SCC).
3. A specific corrosive environment (moisture containing chloride ions or industrial pollutants).

• Characteristics: Cracks propagate intergranularly or transgranularly with zero macroscopic plastic deformation or necking. The component suffers sudden, brittle structural fracture at loads well below its certified design yield strength.

7. Fretting Corrosion

Fretting corrosion occurs at the interface between two tightly clamped, contacting metallic surfaces subjected to slight relative micro-motion or continuous cyclic vibration (slip amplitude typically 5 to 100 micrometers).

• Mechanism: Microscopic relative motion continuously abrades and strips away the natural protective oxide film on contacting asperities, exposing fresh, bare metal. The exposed metal oxidizes immediately, and subsequent vibration cycles crush and grind the brittle oxide debris into an extremely fine abrasive powder. The abrasive powder accelerates mechanical wear and gouging.
• Appearance on Aircraft: Characteristic black, greasy liquid or dry powder oozing out from under rivet heads and lap joints on aluminium skins. In line maintenance, this condition is universally termed "smoking rivets".

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Aircraft Corrosion Identification Matrix

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Mechanical Removal Standards & Procedures

When corrosion is identified, it must be completely removed mechanically down to sound, uncorroded base metal before protective coatings can be restored.

1. Damage Assessment & Structural Repair Manual (SRM) Limits

Prior to grinding or blending, the technician must measure the maximum depth of corrosion penetration using an optical depth micrometer, needle-point dial indicator, or ultrasonic gauge. The measured depth must be compared against the allowable damage limits published in the Structural Repair Manual (SRM) Chapter 51:

• Allowable Damage: Corrosion depth within the specified percentage of original material thickness (typically 5% to 10% maximum depending on structural classification) that can be blended out without structural reinforcement.
• Non-Repairable Damage: Corrosion exceeding allowable limits requires installation of an engineered structural repair doubler, spliced insert, or complete component replacement.

2. Permissible Mechanical Removal Media

• Approved Tools: Aluminium oxide abrasive paper (graded from 240-grit down to 400-grit for final polishing), non-woven synthetic nylon abrasive pads (Scotch-Brite), rotary flap wheels, rubberized abrasive wheels, and pneumatic glass-bead blasting under strictly regulated low nozzle pressures.
• Blend-Out Geometry Standard: Corrosion must be blended out in a smooth, concave, dish-shaped saucer geometry. The SRM mandates a minimum blend-out taper ratio—typically 20:1 up to 50:1 (length to depth). For example, if a blend removes 0.5 mm of material, the blend depression must extend at least 10 to 25 mm laterally in all directions. Sharp edges, notches, or gouges are strictly prohibited, as they introduce severe stress concentrations that invite fatigue cracking.

                         SRM BLEND-OUT GEOMETRY

      <------------------------ Blend Width (20:1 to 50:1) ------------------------>
      +-------------------------\                       /--------------------------+
      | Original Surface          \                   /           Original Surface |
      |                             \_______________/                              |
      |                               ^ Depth (Max allowable)                      |
      +----------------------------------------------------------------------------+
                               Sound Base Metal (No sharp notches)

3. Strict Tooling Prohibition: Carbon Steel Wire Brushes & Steel Wool

The use of carbon steel wire brushes, steel scrapers, steel wool, or copper tools on aluminium, magnesium, or titanium alloys is STRICTLY PROHIBITED.

• Engineering Rationale: Steel wire brushes break off microscopic carbon steel bristles and embed tiny iron particles into the softer aluminium surface matrix. In the presence of atmospheric humidity, each embedded iron particle creates an intense galvanic micro-cell with the surrounding aluminium. Rapid, severe galvanic pitting occurs around every embedded steel particle, destroying the structural component far more rapidly than the original corrosion.

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Chemical Surface Treatments & Re-protection

Once corrosion has been mechanically removed down to sound metal, the bare metallic surface is chemically active and vulnerable to rapid oxidation. It must be immediately restored through an approved five-step chemical sequence:

                  FIVE-STEP CHEMICAL RESTORATION SEQUENCE

   [1. Solvent Degrease] ---> Removes grease, hydraulic fluid & contaminants
             |
             v
   [2. Acid Deoxidize]  ---> Strips natural oxide; chemically activates surface
             |
             v
   [3. Chemical Conversion] -> Alodine / Chromate film (MIL-DTL-5541); passivates
             |
             v
   [4. Epoxy Primer]    ---> Strontium chromate primer; applied in recoat window
             |
             v
   [5. Topcoat / Seal]  ---> Polyurethane topcoat + polysulfide joint sealant

1. Solvent Degreasing & Cleaning: The bare area is thoroughly cleaned with approved solvent (e.g., methyl propyl ketone, approved aqueous alkaline cleaner) to remove all grease, oils, finger oils, and abrasive grinding dust.
2. Acid Etching & Deoxidizing: A mild phosphoric/chromic acid deoxidizing solution is applied to remove residual natural oxide films, dissolve embedded microscopic contaminants, and etch the metal surface to an active, chemically clean state.
3. Chemical Conversion Coating (Alodine / Chromate Conversion):
   • Applied per MIL-DTL-5541 (or proprietary specifications such as Alodine 1200 / Bonderite). The chemical solution contains chromic acid, dichromates, and fluoride activators.
   • Applied by brushing, swabbing, or immersion. The solution must remain continuously wet on the surface for 1 to 5 minutes until a uniform, iridescent golden, amber, or light green chemical film forms.
   • Engineering Function: The chemical conversion coating chemically converts the metal surface into a thin, complex hydrated chromate gel layer. This layer passivates the metal, provides active corrosion inhibition, and establishes an ideal microscopic key for paint adhesion.
   • Mandatory Water Rinsing: As soon as the proper color forms, the surface must be immediately and thoroughly rinsed with clean demineralized or tap water to stop the chemical reaction. If Alodine is allowed to dry unrinsed, it forms a powdery, non-adherent crust that prevents primer adhesion, causing subsequent paint to peel completely.
4. Epoxy Primer Application: Within the specified overcoat window (typically 4 to 24 hours after Alodine drying), a two-component corrosion-inhibiting strontium chromate ($SrCrO_4$) epoxy polyamide primer is applied. Strontium chromate is slightly water-soluble; if moisture penetrates the topcoat, chromate ions leach into the moisture film and chemically passivate any microscopic bare metal sites.
5. Polyurethane Topcoat & Sealant Application: A durable, flexible polyurethane topcoat is applied to provide resistance against UV radiation, hydraulic fluids (Skydrol), and abrasion. Finally, all joints, fasteners, and skin seams are sealed with polysulfide or polythioether sealant (wet installation of fasteners and fillet sealing along lap joints) to exclude moisture permanently.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

During an annual C-check of a business jet, an engineer inspects the lower fuselage skin beneath the rear passenger lavatory service panel and discovers an area of severe blistering and flaking on a 7075-T6 aluminium alloy stringer. The metal exhibits layered peeling and cracking along the grain direction, with thick white powdery deposits.

The engineer correctly diagnoses exfoliation corrosion, exacerbated by chronic leakage of lavatory sanitary fluids (a potent chloride and ammonium-rich electrolyte). The engineer consults SRM Chapter 51, records a maximum depth of 0.8 mm on a 4.0 mm stringer flange (20% thickness loss), which exceeds the allowable non-structural blend-out limit of 10%. The engineer halts grinding, raises an unscheduled non-routine task card, fabricates an approved structural splice repair per the SRM, applies Alodine 1200 with an immediate fresh-water rinse, applies strontium chromate epoxy primer, wet-installs titanium fasteners with polysulfide sealant, and replaces the defective lavatory service drain valve seals.

Common Exam Traps

• Trap 1: Believing compressive stresses cause Stress Corrosion Cracking. SCC requires sustained tensile stress. Compressive stresses actually prevent SCC; processes like shot peening are deliberately used to introduce beneficial compressive surface stresses that arrest SCC.
• Trap 2: Using steel wool or carbon steel wire brushes on aluminium. Strictly prohibited because embedded steel particles create thousands of aggressive galvanic pitting cells.
• Trap 3: Allowing Alodine / chemical conversion coating to dry without rinsing. Alodine must be rinsed thoroughly with water while still wet; letting it dry unrinsed forms a loose, powdery film that completely destroys primer adhesion.
• Trap 4: Confusing intergranular corrosion with exfoliation. Intergranular corrosion is the microstructural grain-boundary attack; exfoliation is the severe consequence seen in wrought, directional grain structures where expanding corrosion product volumes lift surface layers in flakes.