4.3 Corrosion Inspection, Removal & Protective Treatments

4.3 Corrosion Inspection, Removal & Protective Treatments

Maintaining the structural airworthiness of an aircraft throughout its multi-decade operating lifecycle requires a rigorous defense-in-depth strategy. Aircraft structures must be systematically inspected for early corrosion onset, accurately measured against allowable engineering limits, mechanically cleaned without introducing foreign-object contamination, chemically passivated, and sealed beneath resilient multi-layer protective coating schemes.

Under EASA Part-66 Module 06, maintenance personnel must master both the physical workshop procedures for structural rework and the chemical principles governing conversion coatings, anodising, primers, and corrosion inhibiting compounds.

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1. Inspection Techniques & Non-Destructive Inspection (NDI)

Early detection of corrosion prevents localized surface defects from propagating into major structural repairs or catastrophic fatigue fractures.

┌────────────────────────────────────────────────────────────────────────┐
│                     AIRCRAFT CORROSION INSPECTION METHODS              │
├────────────────────────────┬───────────────────────────────────────────┤
│ METHOD                     │ PRIMARY AIRFRAME APPLICATIONS             │
├────────────────────────────┼───────────────────────────────────────────┤
│ Visual & Optical (5x-10x)  │ External skins, blistered paint, filiform │
│ Tactile & Glancing Light   │ Skin "pillowing" on lap joints, waviness  │
│ Optical Depth Micrometer   │ Accurate pit and excavation depth mapping │
│ Low-Frequency Eddy Current │ Subsurface corrosion in 2nd/3rd lap layers│
│ High-Frequency Eddy Current│ Pitting & radial cracks in fastener holes │
│ Ultrasonic Pulse-Echo      │ Remaining skin/web thickness verification │
│ Radiography (X-Ray)        │ Internal honeycomb core, hollow controls  │
└────────────────────────────┴───────────────────────────────────────────┘

1. Visual and Optical Inspection

Visual examination remains the primary and most cost-effective method for detecting corrosion. Over $80%$ of all structural corrosion is discovered during routine visual checks.

• Preparation: Metal surfaces must be clean and free of heavy dirt, grease, and hydraulic residue. Cleaning must be performed using approved non-corrosive solvents (e.g., MIL-PRF-680) and lint-free cloths.
• Lighting Techniques: Low-angle glancing (raking) light is essential. Pointing a high-intensity inspection light parallel to the airframe skin casts pronounced shadows behind raised areas, revealing subtle surface blisters, bulges, and skin pillowing (the outward swelling of exterior lap joint skins caused by the volumetric expansion of hidden corrosion products between layers).
• Optical Aids: Technicians use $5\times$ to $10\times$ hand magnifiers and optical comparators to distinguish between mechanical surface scratches and active chemical pitting.
• Borescopes: Rigid optical borescopes and flexible articulating video borescopes (with integrated LED lighting and high-definition digital zoom) permit visual inspection of inaccessible internal cavities, wing boxes, flap tracks, engine pylons, and lower fuselage bilges through tiny drain holes or fastener access holes without major structural disassembly.

2. Quantitative Depth Measurement

When corrosion is detected, the technician must measure the exact depth of penetration to determine if the damage falls within the allowable damage limits defined in the manufacturer's Structural Repair Manual (SRM Chapter 51).

• Dial Depth Gauge: A precision dial indicator mounted on a flat reference base with a pointed needle probe that seats at the bottom of the corrosion pit.
• Optical Depth-Measuring Microscope: A specialist microscope featuring a high-magnification objective and a calibrated focus knob with a micrometer scale. The technician focuses first on the uncorroded upper skin surface, zeroes the scale, and then re-focuses on the lowest bottom point of the pit. The vertical travel distance displayed on the micrometer indicates pit depth to within $\pm 0.0001\text{ in}$ ($0.0025\text{ mm}$) without touching or mechanically disturbing the defect.

3. Non-Destructive Inspection (NDI) Technologies

When corrosion is concealed beneath fastener heads, inside closed box sections, or between multiple layers of structural lap joints, NDI techniques are mandatory:

• Low-Frequency Eddy Current (LFEC, $100\text{ Hz to } 10\text{ kHz}$):
  • Eddy currents penetrate deeply into non-ferrous metals at low excitation frequencies. LFEC is the industry standard for inspecting multi-layer fuselage lap splices (e.g., on Boeing and Airbus pressurized hulls).
  • LFEC phase-lag and amplitude signals measure the reduction in electrical conductivity and metal volume, allowing technicians to detect and quantify corrosion thinning in the second and third layers of a joint without removing fasteners.
• High-Frequency Eddy Current (HFEC, $100\text{ kHz to } 2\text{ MHz}$):
  • High-frequency eddy currents are concentrated near the surface due to the skin effect. HFEC is used to inspect fastener hole bores and countersinks after fastener removal, detecting microscopic intergranular fissures and corrosion pitting prior to bolt installation.
• Ultrasonic Thickness Gauging (UT):
  • Pulse-echo ultrasonic compression wave probes emit high-frequency acoustic waves ($2.25\text{ to } 10\text{ MHz}$) that propagate through the skin and reflect off the back wall.
  • By measuring round-trip transit time, digital ultrasonic gauges determine remaining sound metal thickness to within $\pm 0.001\text{ in}$ ($0.025\text{ mm}$) from a single accessible external side.
• Radiographic Testing (X-Ray):
  • Differential absorption of X-rays produces image density variations on digital detector arrays or radiographic film.
  • Used to inspect closed hollow structures—such as rudder and elevator trailing edges, aileron ribs, and composite honeycomb sandwich panels. X-rays clearly reveal internal moisture ingress, corroded aluminium honeycomb core, and hidden corrosion along inner spar flanges.

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2. Corrosion Removal Standards & Blending Procedures

Once corrosion is detected and categorized, it must be completely removed. Structural repair manuals (SRM Chapter 51) specify strict rules for mechanical rework.

                     CORROSION BLEND-OUT REWORK GEOMETRY

  Original Surface Level
 ──────────────────────────┐                               ┌──────────────────────────
                           │   Smooth Blend Radius (R)     │
                           │   (No sharp steps or notches) │
                           └───┐                       ┌───┘
                               │   Taper Ratio:        │
                               │   1:20 to 1:50 Taper  │
                               └───┐               ┌───┘
                                   │◄─────────────►│
                                    Excavation Base
                                    (Depth: 'd')
                                    Total Blend Length: 20d to 50d!

1. Structural Damage Classification (SRM Chapter 51):

• Allowable Damage: Minor surface corrosion within defined depth limits (typically $< 5% \text{ to } 10%$ of original skin thickness). Can be blended out and repainted without requiring reinforcing doublers or structural engineering approval.
• Non-Allowable / Repairable Damage: Corrosion depth exceeds allowable limits but remains within repairable structural limits. Requires permanent engineering reinforcement (e.g., riveting a formed structural doubler or splicing a new frame section) in accordance with the SRM.
• Scrap / Non-Repairable: Corrosion exceeds maximum repair limits, or has penetrated across multiple fastener rows, critical primary attachment lugs, or pressure bulkheads, requiring complete component replacement.

2. The Blend-Out Geometry Standard:

When grinding or sanding out a corrosion pit, technicians must never create a sharp depression, groove, or vertical-sided hole. Sharp edges act as severe stress raisers (notches) that initiate rapid structural fatigue cracking.

• Blend Taper Ratio: The excavation must be smoothly blended into the surrounding sound metal using a specified taper ratio—typically $1:20\text{ to }1:50$.
• For example, under a $1:50$ taper requirement, if a corrosion pit has a depth of $0.010\text{ in}$ ($0.25\text{ mm}$), the blend-out must extend outward across a radius/length of at least $0.50\text{ in}$ ($12.5\text{ mm}$) in all directions, creating an expansive, saucer-shaped, elliptical depression with generous corner radii.
• Following blending, the excavated area must be polished using ultra-fine abrasive paper (400 to 600 grit) to remove all machining scratches, and re-inspected using dye penetrant or eddy current testing to guarantee that $100%$ of intergranular fissures have been eradicated.

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3. Approved vs. Prohibited Mechanical Removal Tools

Selecting the correct abrasive medium is vital. Using improper workshop tools can cause irreparable galvanic damage to aircraft structures.

The Cardinal Workshop Prohibition: Maintenance regulations (FAA AC 43.13-1B, EASA Part-M/Part-145, Boeing/Airbus SRM Chapter 51) strictly prohibit the use of carbon steel wire brushes, rotary steel wire wheels, steel wool, or emery cloth on aluminium, magnesium, or titanium alloys. Microscopic iron particles become mechanically embedded into the soft substrate. In service moisture, every single embedded steel speck acts as a noble cathode against the surrounding anodic aluminium ($-0.65\text{ V}$ steel vs $-0.80\text{ V}$ aluminium). Within days, hundreds of active galvanic micro-cells develop, causing violent, widespread pitting corrosion across the entire reworked zone.

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3. Chemical Surface Treatments & Conversion Coatings

Once corrosion has been mechanically removed and the metal polished to a sound baseline, the chemically active, bare metal surface must be immediately passivated. Bare aluminium spontaneously oxidizes in ambient air, but its natural oxide film is uneven, porous, and non-protective.

1. Chromate Conversion Coating (Alodine / Bonderite 1200 / MIL-DTL-5541)

Chromate conversion coating is a non-electrolytic chemical immersion, spray, or brush-on process that transforms the bare aluminium surface into a protective, complex chemical film.

             CHROMATE CONVERSION COATING PROCESS (MIL-DTL-5541)

  ┌────────────────┐   ┌────────────────┐   ┌────────────────┐   ┌────────────────┐
  │ 1. DEGREASING  │──>│ 2. ALKALINE    │──>│ 3. ACID        │──>│ 4. WATER       │
  │ Solvent Wipe   │   │ CLEANING       │   │ DEOXIDIZING    │   │ RINSE          │
  │ (MIL-PRF-680)  │   │ Non-etching    │   │ Removes Oxides │   │ Clean Tap/DI   │
  └────────────────┘   └────────────────┘   └────────────────┘   └────────────────┘
                                                                          │
  ┌────────────────┐   ┌────────────────┐   ┌────────────────┐            │
  │ 7. FINAL PAINT │<──│ 6. WATER RINSE │<──│ 5. ALODINE 1200│<───────────┘
  │ Epoxy Primer + │   │ Temp < 50°C    │   │ APPLICATION    │
  │ Topcoat        │   │ Air Dry        │   │ 1 to 5 Minutes │
  └────────────────┘   └────────────────┘   └────────────────┘

• Chemistry of the Film: The treatment solution contains chromic acid, sodium dichromate, potassium ferricyanide, and hydrofluoric acid. When brushed onto bare aluminium, it chemically reacts with the metal to produce a continuous, microscopic ($0.25\text{ to } 1.0\ \mu\text{m}$), amorphous, gelatinous film of hydrated chromium chromate ($x\text{Cr}_2\text{O}_3 \cdot y\text{CrO}_3 \cdot z\text{H}_2\text{O}$).
• Key Military/Aerospace Classifications (MIL-DTL-5541 / AMS-C-5541):
  • Class 1A: Thick coating providing maximum corrosion protection, designed as a final protective barrier on unpainted surfaces and as a high-adhesion primer base for organic paint systems. Appearance: Iridescent golden-yellow or brown.
  • Class 3: Thin coating formulated specifically for low electrical contact resistance. Used where electrical bonding and grounding continuity are mandatory (such as avionics equipment mounting racks, radar waveguides, and lightning strike ground bonding paths). Appearance: Very light yellow to clear.
• The Unique "Self-Healing" Property: Unlike inorganic paint films, chromate conversion coatings possess self-healing capability. Residual hexavalent chromium ions ($Cr^{VI}$) trapped within the amorphous gel dissolve slightly when contacted by moisture and migrate through the film to re-passivate freshly scratched bare aluminium, arresting corrosion before it starts.

2. Anodising (Electrolytic Oxidation)

Unlike chemical conversion coatings (which are purely chemical dipping/brushing treatments), anodising is an electrolytic electrochemical process performed in specialized immersion processing tanks.

                        THE ANODISING PROCESS TANK

             DIRECT CURRENT (DC) POWER SUPPLY
                  ( + )              ( - )
                    │                  │
                    ▼                  ▼
       ┌─────────────────────┐    ┌─────────────────────┐
       │   ALUMINIUM PART    │    │    CATHODE PLATE    │
       │      ( ANODE )      │    │ (Lead / Stainless)  │
       └──────────┬──────────┘    └──────────┬──────────┘
                  │                          │
                  ▼                          ▼
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       ~~~   ACID ELECTROLYTE (Chromic / Sulfuric)    ~~~
       ~~~                                            ~~~
       ~~~   Nascent Oxygen (O²⁻) reacts with Al:     ~~~
       ~~~        2Al + 3O²⁻ ──> Al₂O₃ + 6e⁻          ~~~
       ~~~   Forms hard, dense, tubular ceramic oxide ~~~
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Fundamental Operating Principle: In an electroplating bath, the workpiece is made the cathode ($-$ ) to deposit metal onto it. In anodising, the workpiece is deliberately made the ANODE ($+$). Water in the acid bath is electrolyzed, releasing nascent oxygen ions at the aluminium anode surface. The oxygen reacts with the substrate to grow a thick, hard, highly structured hexagonal honeycomb-tubular layer of aluminium oxide ($\text{Al}_2\text{O}_3$).
• Major Aerospace Anodising Specifications (MIL-A-8625 / ISO 7599):

• The Critical Sealing Process: Freshly anodised aluminium oxide contains millions of microscopic, open hexagonal pores per square centimeter. In this unsealed state, the coating is porous and highly stain-prone. The component must undergo sealing immediately after anodising:
  • Boiling Deionized Water Sealing ($98^\circ\text{C to }100^\circ\text{C}$): Water molecules react with the anhydrous aluminium oxide, converting it into boehmite ($\text{Al}_2\text{O}_3 \cdot \text{H}_2\text{O}$). This hydration reaction swells the crystal structure, mechanically plugging and closing the pores.
  • Sodium Dichromate Sealing: Carried out in a hot, dilute sodium dichromate bath. Locks active chromate corrosion inhibitors inside the sealed pores and imparts a characteristic golden-yellow tint.

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4. Fastener & Ferrous Surface Protection

High-strength alloy steels used in landing gear cylinders, flap tracks, engine mounts, and structural bolts cannot form protective passivating films and require active metallic and inorganic surface protection.

1. Cadmium Plating (AMS-QQ-P-416)

Electroplated cadmium remains the premier protective coating for high-strength steel aircraft bolts, pins, and structural fittings:

• Sacrificial Galvanic Protection: Cadmium has an electrode potential of approximately $-0.80\text{ V}$ SCE, making it anodic to structural steel ($-0.65\text{ V}$). If the plating is scratched, the cadmium corrodes sacrificially, galvanically protecting the underlying steel core.
• Non-Seizing Lubricity: Cadmium provides natural lubricity, preventing galling and seizing of threaded fasteners during high-torque tightening.
• Chromate Dip Post-Treatment: Cadmium plating is followed by a chromate conversion dip (Type II yellow or Type I clear) to prevent white cadmium oxidation blooming.
• Mandatory De-Embrittlement Baking: Electroplating steel generates atomic hydrogen that induces catastrophic hydrogen embrittlement. Steels with ultimate tensile strength exceeding $1000\text{ MPa}$ ($145\text{ ksi}$) must undergo mandatory de-embrittlement baking in a calibrated oven at $190^\circ\text{C to }205^\circ\text{C}$ ($375^\circ\text{F to }400^\circ\text{F}$) for $8\text{ to }24\text{ hours}$, and baking MUST commence within $1\text{ to }4\text{ hours}$ of removal from the plating tank.

2. Sacrificial Zinc Primers

Zinc-rich primers contain high concentrations ($>85%$ by weight) of metallic zinc dust suspended in an organic epoxy or inorganic silicate vehicle. When applied to steel airframe members, the zinc particles establish electrical contact with the steel substrate, acting as a sacrificial anode.

5. Organic Protective Coatings (The Multi-Layer Paint Scheme)

Modern aircraft paint schemes represent an engineered, multi-layer barrier and inhibition system designed to withstand extreme environmental fluctuations, aerodynamic erosion, ultraviolet (UV) radiation, and chemical attack.

               CROSS-SECTION OF MULTI-LAYER AIRFRAME COATING

 ┌─────────────────────────────────────────────────────────────┐ ▲ Polyurethane Topcoat
 │        POLYURETHANE TOPCOAT (35 to 50 µm)                   │ │ (Chemical, UV & Skydrol Barrier)
 ├─────────────────────────────────────────────────────────────┤ ▼
 │        EPOXY PRIMER (15 to 25 µm)                           │ ▲ Epoxy Primer
 │        (Active Strontium / Zinc Chromate Inhibitors)        │ │ (Chemical Bonding & Passivity)
 ├─────────────────────────────────────────────────────────────┤ ▼
 │        CHEMICAL CONVERSION COATING (0.5 to 1 µm)            │ ▲ Alodine / Anodise
 │        (Hydrated Chromium Chromate / Alumina)               │ │ (Microscopic Passivity)
 ├─────────────────────────────────────────────────────────────┤ ▼
 │                                                             │ ▲
 │                 STRUCTURAL BASE ALLOY                       │ │ Airframe Substrate
 │                 (e.g., 2024-T3 / 7075-T6)                   │ │ (Protected Metal)
 │                                                             │ ▼
 └─────────────────────────────────────────────────────────────┘

1. Epoxy Primers (MIL-PRF-23377 / MIL-PRF-85582)

Two-part epoxy primers (consisting of an epoxy base resin and a polyamide or amine curing agent) provide the foundational bond between the metal substrate and the exterior topcoat:

• Adhesion: Epoxies possess polar chemical groups that cross-link tenaciously to chromate conversion coatings and anodised surfaces.
• Active Corrosion Inhibition: Aerospace epoxy primers are heavily loaded with strontium chromate ($\text{SrCrO}_4$) or zinc chromate pigments (or modern REACH-compliant non-chromate lithium/zirconium inhibitors). If water penetrates the outer paint film, these sparingly soluble chromate pigments dissolve slightly, releasing chromate ions that migrate to the metal interface and chemically re-passivate any active micro-pits.
• Fluid Barrier: Epoxies form an impermeable physical barrier resistant to water, fuel, and de-icing fluids.

2. Polyurethane Topcoats (MIL-PRF-85285)

Two-component aliphatic polyurethane enamels form the outermost visible layer of the aircraft paint scheme:

• Chemical Resistance: Polyurethanes cross-link into a dense, chemically inert polymer matrix capable of withstanding continuous exposure to aggressive synthetic ester lubricants, jet fuels, and phosphate-ester hydraulic fluids (Skydrol).
• Environmental Flexibility: Polyurethane coatings maintain high elasticity and elongation across extreme temperature ranges—retaining flexibility at $-55^\circ\text{C}$ cruising altitude without cracking under aerodynamic airframe flexing, while resisting degradation at $+70^\circ\text{C}$ on hot tarmac ramps.
• UV Resistance & Gloss: Aliphatic isocyanates provide superior resistance to ultraviolet radiation, preventing chalking, discoloration, and weathering.

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6. Corrosion Inhibiting Compounds (CICs)

Corrosion Inhibiting Compounds (CICs) are specialized organic compounds applied to internal structures, unpainted cavities, and moisture-prone zones to provide supplementary barrier and water-displacing protection.

┌────────────────────────────────────────────────────────────────────────┐
│            CORROSION INHIBITING COMPOUNDS (CICs) COMPARISON            │
├──────────────────────────┬─────────────────────────────────────────────┤
│ CHARACTERISTIC           │ SOFT-FILM / WATER-DISPLACING (LPS-3/AV-8)   │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Film Physical State      │ Soft, oily, or flexible waxy film           │
│ Water Displacement       │ High: actively drives moisture out of joints│
│ Capillary Penetration    │ High: creeps into lap joints & around rivets│
│ Self-Healing Ability     │ High: flows back across scratches           │
│ Service Life & Rework    │ 1 to 3 Years (Requires periodic re-spray)   │
│ Typical Application Zones│ Fuselage bilges, door surrounds, lower skins│
├──────────────────────────┼─────────────────────────────────────────────┤
│ CHARACTERISTIC           │ HARD-FILM / BARRIER (Dinitrol AV-100D)      │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Film Physical State      │ Firm, dry, resilient, non-tacky waxy resin  │
│ Water Displacement       │ Moderate: applied to dry, clean surfaces    │
│ Capillary Penetration    │ Low to Moderate                             │
│ Self-Healing Ability     │ Low: rigid barrier                          │
│ Service Life & Rework    │ 5+ Years (Long-term semi-permanent barrier) │
│ Typical Application Zones│ Wheel wells, flap bays, cargo compartment   │
└──────────────────────────┴─────────────────────────────────────────────┘

1. Soft-Film, Water-Displacing Penetrating CICs (MIL-PRF-16173 Grade 3, Dinitrol AV-8, Boeshield T-9, LPS-3):

• Mechanism: Formulated with complex petroleum waxes, sulfonates, and volatile mineral solvents, these compounds possess extremely low surface tension. When sprayed into joints, they physically displace standing water from the metal surface.
• Capillary Wicking: They creep into microscopic lap joint seams, around rivet shanks, and into faying surfaces via capillary action.
• Self-Healing Waxy Layer: They dry to a pliable, non-drying waxy film that remains flexible and self-heals over minor scratches. However, because they remain soft, they attract dirt and can be washed away by fluid flow, requiring scheduled reapplication every $1\text{ to }3\text{ years}$ during intermediate and heavy maintenance checks.

2. Hard-Film Barrier CICs (MIL-PRF-16173 Grade 1 / 4, Dinitrol AV-100D):

• Mechanism: Dry to a firm, resilient, amber, translucent or opaque wax-resin barrier layer.
• Abrasion Resistance: Unlike soft-film compounds, hard-film CICs form a durable, non-tacky surface that resists mechanical scuffing, stone chipping, and high-velocity water wash-off.
• Application: Applied to high-velocity exposure zones—such as landing gear wheel wells, wing flap track recesses, and cargo hold under-floor structures subject to physical abrasion.

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7. Practical Maintenance Scenarios & Exam Traps

Maintenance Scenario 3: Rework of a Corroded Fuselage Stringer

During a scheduled base maintenance C-check on a commercial transport aircraft, an engineer discovers pitting corrosion along the lower flange of an extruded 2024-T3 fuselage stringer in the forward cargo compartment. The stringer nominal web thickness is $0.080\text{ in}$ ($2.03\text{ mm}$).

Maintenance Execution Sequence:

1. Cleaning: The technician cleans the area using lint-free rags soaked in MIL-PRF-680 solvent to remove grease and dirt.
2. Depth Measurement: An optical micrometer measures the deepest pit at $0.005\text{ in}$ ($0.127\text{ mm}$), representing $6.25%$ thickness loss.
3. SRM Consultation: The engineer references SRM Chapter 51. The allowable blend-out limit for this stringer flange is $10%$ of thickness ($0.008\text{ in}$). The damage is classified as Allowable Rework.
4. Mechanical Removal: Using aluminium oxide abrasive paper (240 grit) supported by a contoured sanding block, the technician blends out the pitting using a $1:50\text{ taper ratio}$. For $0.005\text{ in}$ depth, the blend-out extends across a length of $0.250\text{ in}$ ($6.35\text{ mm}$) in all directions, creating an elliptical depression with smooth radii. The area is polished with 400-grit and 600-grit paper.
5. Verification NDT: The technician performs a dye penetrant inspection to confirm zero residual pitting or micro-cracks remain.
6. Chemical Conversion: The bare blend-out is solvent-degreased, treated with a mild acid deoxidizer, thoroughly rinsed with water, and coated with Alodine 1200 (MIL-DTL-5541 Class 1A) using a clean nylon brush for 3 minutes until a uniform golden-yellow iridescent film develops. The area is rinsed with clean water and air dried.
7. Protective Primer & Topcoat: A two-part strontium-chromate inhibited epoxy primer (MIL-PRF-23377) is applied and allowed to cure, followed by a polyurethane topcoat (MIL-PRF-85285).
8. CIC Sealing: After full paint cure, a penetrating, water-displacing CIC (Dinitrol AV-8) is sprayed across the entire stringer pocket to prevent future moisture entrapment.

Common Exam Traps:

1. The Forbidden Tool Trap: Questions frequently test prohibited cleaning media: "Which tool is acceptable for removing corrosion from an aluminium skin?" Carbon steel wire brushes, carbon steel wool, and emery cloth are NEVER acceptable. Only non-metallic Scotch-Brite, aluminium oxide paper, or glass bead blasting are approved.
2. Class 1A vs. Class 3 Alodine Trap: Questions often switch the roles of Class 1A and Class 3: Class 1A is for general corrosion protection and paint adhesion (thick, yellow/brown); Class 3 is strictly for low electrical contact resistance / electrical bonding (thin, clear/light yellow).
3. Anodising Polarity Trap: Remember that in anodising, the aircraft part is made the ANODE ($+$). If you make it the cathode, you are electroplating, not anodising!
4. Type I Anodising Trapped Acid: An exam favorite: "Why is chromic acid anodising (Type I) permitted on complex spot-welded assemblies?" Because residual trapped chromic acid does not corrode aluminium, unlike sulfuric acid (Type II).