5.2 Corrosion Removal, Chemical Conversion Coatings & Surface Treatments

5.2 Corrosion Removal, Chemical Conversion Coatings & Surface Treatments

Once corrosion initiates on an aircraft structure, maintenance personnel must take prompt, approved corrective action to mechanically eliminate all active corrosion products, treat the bare substrate with chemical conversion coatings, and restore protective barrier finishes. Performing improper corrosion removal—such as using incompatible abrasive tools or unapproved chemical reagents—can cause catastrophic structural weakening or accelerate electrochemical destruction far beyond the original defect.

All maintenance procedures must conform strictly to FAA-H-8083-30B, FAA AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair), AC 43-4A, and the applicable aircraft manufacturer's Structural Repair Manual (SRM).

1. Mechanical Corrosion Removal by Alloy Substrate

Mechanical corrosion removal involves physically abrading away oxidation products down to clean, sound, uncorroded parent metal while maintaining structural cross-sectional limits and smooth aerodynamic blend contours.

                  MECHANICAL CORROSION REMOVAL MEDIA
  ┌─────────────────────────────────┼─────────────────────────────────┐
  │                                 │                                 │
  ▼                                 ▼                                 ▼
ALUMINUM & ALCLAD               MAGNESIUM ALLOYS                HIGH-STRENGTH STEELS
• Aluminum Oxide Paper (240-400) • Non-metallic scrapers         • Glass Bead Blasting
• Scotch-Brite (Al₂O₃ web)       • Stiff non-metallic brushes    • Stainless wire brushes
• Nylon Abrasive Flap Wheels     • Glass bead blasting           • Flap wheels / sanding
• Controlled Glass Bead Blast    • Follow with Chromate (Dow 19) • Post-bake stress relief
  [NO STEEL WOOL / BRUSHES]        [NO FERROUS / METALLIC TOOLS]   [NO STRONG ACID STRIPPERS]

1. Aluminum and Aluminum Alloys

• Approved Mechanical Media:
  • Aluminum oxide abrasive paper (grit sizes 240, 320, 400, and 600 for final blending).
  • Non-woven nylon abrasive pads impregnated with aluminum oxide or silicon carbide (Scotch-Brite pads: maroon general-purpose, grey ultra-fine).
  • Nylon abrasive filament rotary wheels and flap brushes.
  • Direct dry abrasive blasting using fine glass beads (MIL-G-9954) or walnut shell media under strictly controlled low air pressure ($30–45\text{ psi}$) to prevent surface peening and dimensional distortion.
• STRICT PROHIBITION — The Steel Wool Rule:
  • Carbon steel wire brushes, carbon steel wool, rotary steel wire wheels, and emery cloth containing iron oxide are strictly forbidden on aluminum, Alclad, and magnesium alloys.
  • Mechanism of Failure: Steel wool and wire brushes shed microscopic particles of elemental carbon steel that embed permanently into the soft aluminum surface. Each embedded iron particle establishes an active galvanic micro-cell with the surrounding aluminum. In the presence of ambient humidity, the aluminum corrodes sacrificially around every embedded particle, resulting in millions of rapid, destructive galvanic pits that destroy the component.
• Alclad Aluminum Protection Rules:
  • Alclad consists of a high-strength aluminum alloy core (such as 2024 or 7075) laminated on both outer surfaces with a thin layer of pure (99.3%) aluminum (representing approximately $5%$ of total sheet thickness per side).
  • The pure aluminum cladding provides galvanic sacrificial protection to the core. Mechanics must exercise extreme care during mechanical polishing and corrosion blending to avoid cutting or buffing through the cladding layer. If cladding is breached, the exposed 2024/7075 core must be chemically conversion-coated with Alodine and primed immediately.

2. Magnesium Alloys

• Magnesium is the most chemically active structural metal used in aviation (found in gearbox casings, helicopter transmissions, wheel rims, and flight control hinges).
• Removal Procedures: Mechanics must use non-metallic scrapers (phenolic scrapers, hardwood wedges), stiff natural-bristle brushes, aluminum oxide paper, or low-pressure glass bead blasting.
• Chemical Treatment (Chrome-Pickle / Dow 19): Cleaned bare magnesium oxidizes almost instantaneously. Bare areas must be immediately passivated with an approved chromate chemical solution such as Dow 19 (MIL-M-3171 Type VI / Potassium Dichromate - Nitric Acid solution) to establish a protective golden-brown conversion film.

3. Ferrous Metals (Carbon and High-Strength Low-Alloy Steels)

• Removal Procedures: Glass bead blasting, stainless steel wire brushes, aluminum oxide grinding discs, and flap wheels.
• Hydrogen Embrittlement in High-Strength Steels (180 ksi or greater):
  • High-strength aircraft landing gear components, wing attach pins, and turbine engine bolts (such as 4340 and 300M steel heat-treated to 180 ksi or greater tensile strength) must NEVER be cleaned using unapproved strong mineral acids (hydrochloric or uninhibited sulfuric acid) or harsh chemical paint strippers.
  • Mechanism: Strong acids react with iron to liberate atomic hydrogen ($\text{H}^+$). Atomic hydrogen rapidly diffuses into the interstitial crystalline lattice of the stressed steel. Under static structural load, the trapped hydrogen migrates to microscopic stress concentrations, causing sudden, catastrophic brittle fracture (hydrogen embrittlement cracking).
  • Mitigation: Heavy high-strength steel corrosion must be removed mechanically by glass bead blasting. If chemical pickling or electroplating is required, the steel parts must undergo a mandatory hydrogen embrittlement relief bake at $375^\circ\text{F} \pm 25^\circ\text{F}$ for $4\text{ to }24\text{ hours}$ in an oven within 4 hours of chemical exposure.

4. Structural Blending Limits

When blending out corrosion damage using rotary flap wheels or sanding blocks, the technician must create a smooth, saucer-shaped depression with a minimum blend radius of $4:1$ to $10:1$ (length to depth ratio) per AC 43.13-1B, ensuring no sharp step-down notches or gouges remain that could concentrate fatigue stresses.

2. Chemical Conversion Coatings: Alodine (MIL-DTL-5541)

A chemical conversion coating is a non-electrolytic, aqueous chemical process that transforms an active metallic surface into a dense, insoluble, complex chromate-containing protective film.

                      4-STEP ALODINE APPLICATION SEQUENCE
  ┌────────────────────────────────────────────────────────────────────────┐
  │ STEP 1: PREPARATION & ACID CLEANING (Alumiprep 33)                     │
  │ • Phosphoric acid cleaner removes soils and oxides                     │
  │ • Verify with WATER-BREAK-FREE TEST (continuous film for >=30 sec)     │
  └───────────────────────────────────┬────────────────────────────────────┘
                                      ▼
  ┌────────────────────────────────────────────────────────────────────────┐
  │ STEP 2: CHEMICAL CONVERSION APPLICATION (Alodine 1201)                 │
  │ • Apply by brush, sponge, spray, or immersion                          │
  │ • Maintain wet chemical contact for 2 TO 5 MINUTES                     │
  │ • CRITICAL: DO NOT ALLOW SOLUTION TO DRY ON SURFACE                    │
  └───────────────────────────────────┬────────────────────────────────────┘
                                      ▼
  ┌────────────────────────────────────────────────────────────────────────┐
  │ STEP 3: COLD WATER FLUSH & RINSE                                       │
  │ • Flush thoroughly with clean cold water to remove unreacted acids     │
  └───────────────────────────────────┬────────────────────────────────────┘
                                      ▼
  ┌────────────────────────────────────────────────────────────────────────┐
  │ STEP 4: INSPECTION & DRYING                                            │
  │ • Proper Finish: Iridescent Golden-Yellow to Light Amber               │
  │ • Defect (Dark brown / powdery): Over-exposure / dried chemical (strip)│
  │ • Air dry completely (1-2 hours) prior to applying epoxy primer        │
  └────────────────────────────────────────────────────────────────────────┘

Specifications & Functions of Alodine

Chemical conversion coatings on aircraft aluminum alloys are governed by military specification MIL-DTL-5541 (formerly MIL-C-5541) and Qualified Products List QPL-81706. In civil aviation, the process is widely known by the Henkel/Bonderite trade name Alodine 1201.

• Four Primary Functions of Alodine:
  1. Imparts substantial intrinsic atmospheric corrosion resistance to bare aluminum.
  2. Provides an extraordinary chemical adhesion anchor for subsequent primer and paint coats.
  3. Preserves electrical conductivity for radio frequency (RF) shielding and static grounding (Class 3 coatings).
  4. Does not alter critical dimensional tolerances of precision-machined parts (adding negligible thickness: $<0.00001\text{ inch}$).

Standard 4-Step Field Application Protocol

1. Step 1: Degreasing and Acid Cleaning (Alumiprep 33):
   The abraded bare aluminum surface is thoroughly degreased with solvent and washed with an acidic conversion cleaner (such as Alumiprep 33, containing dilute phosphoric acid, glycol ethers, and wetting agents) to dissolve residual surface oxides and chemically activate the metal.
   • The Water-Break-Free Test: The surface is rinsed with clean water and observed. If the water forms a continuous, unbroken liquid sheet across the entire metal surface for at least 30 seconds, the metal is chemically clean. If the water beads up, separates, or forms dry islands, residual oil or silicone grease remains, and acid cleaning must be repeated.
2. Step 2: Alodine Application (Alodine 1201):
   Apply the acidic chromate conversion solution liberally to the clean, wet aluminum using a clean synthetic bristle brush, non-linting sponge, or low-pressure spray.
   • Contact Time: The solution must remain continuously wet on the surface for 2 to 5 minutes to permit the chemical conversion reaction to complete.
   • CRITICAL OPERATING RULE: Never allow the Alodine solution to dry on the metal surface prior to rinsing. Allowing the solution to dry produces a loose, powdery, non-adherent chromate layer that prevents primer bonding and must be completely stripped back to bare metal.
3. Step 3: Thorough Cold Water Rinse:
   Immediately flush the treated area with generous volumes of clean, cold fresh water to wash away all unreacted chromates and free acid. Do not use hot water, as excessive heat can leach hexavalent chromates from the newly formed film.
4. Step 4: Visual Quality Inspection and Drying:
   • Acceptable Quality: A properly processed conversion coating displays an iridescent golden-yellow, light amber, or bronze color that is firmly bonded and does not rub off on a clean cloth.
   • Unacceptable Defects: A pale, clear, or silvery finish indicates under-exposure (insufficient contact time) or depleted solution. A dark brown, muddy, or dusty/powdery crust indicates over-exposure or allowing the solution to dry, which ruins paint adhesion.

3. Anodizing (Electrolytic Oxidation)

Anodizing is an electrolytic electrochemical process that artificially grows a dense, highly controlled layer of aluminum oxide ($\text{Al}_2\text{O}_3$) on the surface of aluminum parts in a factory tank immersion setup.

                      ELECTROLYTIC ANODIZING PROCESS
  ┌────────────────────────────────────────────────────────────────────────┐
  │                       ACID ELECTROLYTE BATH                            │
  │         (Chromic Acid - Type I  or  Sulfuric Acid - Type II)           │
  │                                                                        │
  │         Cathode (- Terminal)            Anode Part (+ Terminal)        │
  │         [ Lead / Steel Plate ]          [ ALUMINUM COMPONENT ]         │
  │                   │                                │                   │
  │                   │  2H⁺ + 2e⁻ ──► H₂↑             │  2Al + 3O²⁻ ──►   │
  │                   │  (Hydrogen Gas at Cathode)     │  Al₂O₃ + 6e⁻      │
  │                   │                                │  (Oxide Ceramic)  │
  │                   └───────────┐        ┌───────────┘                   │
  └───────────────────────────────┼────────┼───────────────────────────────┘
                                  │        │
                           (-) ───┴────────┴─── (+)  DC POWER RECTIFIER

Specifications & Types of Anodizing (MIL-A-8625)

• Type I — Chromic Acid Anodizing: Produces a thin ($0.00005–0.0002\text{ inch}$), ductile, highly corrosion-resistant oxide film with a dark grey or olive cast. Preferred for structural parts subjected to fatigue or parts with complex rolled joints where trapped acid would not cause rapid corrosion.
• Type II — Sulfuric Acid Anodizing: Produces a thicker ($0.0002–0.001\text{ inch}$), highly porous oxide film that readily absorbs organic dyes (providing vibrant colors for cockpit controls, fittings, and handles).
• Type III — Hard Anodize (Hardcoat): Thick ($0.001–0.003\text{ inch}$), dense ceramic layer exhibiting extreme wear resistance, hardness (approaching case-hardened steel), and erosion protection for landing gear pistons, hydraulic cylinders, and actuator slides.

Sealing Process

Freshly anodized aluminum oxide contains billions of microscopic hexagonal pores. To lock in corrosion resistance, the parts are sealed by immersion in boiling deionized water or a hot sodium dichromate solution ($200^\circ\text{F}–212^\circ\text{F}$). The boiling water hydrates the aluminum oxide into aluminum monohydrate ($\text{Al}_2\text{O}_3 \cdot \text{H}_2\text{O}$), which swells and permanently seals the pores.

Electrical Properties & Aircraft Maintenance Grounding Rules

• Dielectric Insulation: The aluminum oxide layer produced by anodizing is a pure ceramic dielectric that does not conduct electricity.
• MANDATORY FAA RULE FOR ELECTRICAL BONDING:
  Anodized coatings provide complete electrical insulation. At locations where electrical ground studs, lightning bonding jumpers, static discharge wicks, and avionics antennas attach to the airframe, the anodized coating MUST be spot-faced or sanded away down to bare metal. The bare metal is then treated with an electrically conductive chemical conversion coating (Alodine Class 3 per MIL-DTL-5541) to ensure direct metallic conductivity and achieve a bonding resistance of less than $0.003\text{ Ohms } (3\text{ m}\Omega)$ per FAA AC 43.13-1B.

4. Primers, Topcoats & Dissimilar Metal Isolation

Protective paint finishes provide the primary physical and chemical barrier isolating structural aircraft alloys from atmospheric moisture, salt spray, and operational fluids.

1. Aviation Primer Systems

• Zinc Chromate Primer (TT-P-1757):
  • Traditional single-component alkyd-resin primer containing active zinc chromate pigment particles (yellow/green color).
  • Corrosion Inhibiting Mechanism: In the presence of moisture penetrating the paint film, the zinc chromate pigment slightly dissolves, releasing free hexavalent chromate ions ($\text{CrO}_4^{2-}$). These chromate ions migrate to bare metallic scratches and react to passivate the aluminum, actively healing minor scratches.
  • Limitation: Poor resistance to synthetic turbine lubricants and ester-based hydraulic fluids (Skydrol).
• Epoxy Polyamide Primer (MIL-PRF-23377 / MIL-PRF-85582):
  • Standard two-component high-performance primer system (epoxy resin component + polyamide curing agent component).
  • Contains strontium chromate corrosion inhibitors; forms a dense, cross-linked, chemical-resistant barrier.
  • Unsurpassed resistance to hydraulic fluids (phosphate esters / Skydrol), aviation fuels, solvents, and salt spray.
  • Must be topcoated within the manufacturer's specified recoat window (typically $2\text{ to }24\text{ hours}$) to achieve chemical cross-linking with the topcoat.

2. Polyurethane Topcoats (MIL-PRF-85285)

• Two-component aliphatic polyurethane coating system providing a high-gloss, ultra-durable finish.
• Extraordinary flexibility, weather resistance, UV radiation blocking, and chemical barrier properties.
• Extremely dense structure: resists hydraulic fluids, de-icing solutions, and weathering.

3. Dissimilar Metal Isolation per FAA AC 43.13-1B

Whenever structural engineering dictates the assembly of dissimilar metals possessing significant galvanic potential differences (such as attaching stainless steel fittings, titanium hinge brackets, or steel bolts to an aluminum airframe), strict barrier isolation is legally mandatory per AC 43.13-1B, Chapter 6.

• Mandatory Isolation Protocols:
  1. Faying Surface Sealing: Both mating surfaces must be primed with at least two coats of epoxy polyamide primer (MIL-PRF-23377) or zinc chromate primer.
  2. Interposition of Non-Absorbing Barrier: A layer of non-hygroscopic pressure-sensitive vinyl tape, zinc chromate paste, or chromate-inhibited polysulfide sealant (MIL-PRF-81733 / AMS-S-8802) must be interposed between the mating parts before clamping.
  3. Wet Fastener Installation: All structural steel and titanium fasteners passing through aluminum structure must be dipped in wet epoxy primer or wet polysulfide sealant immediately prior to insertion into the fastener hole. Wet installation seals out moisture and eliminates crevice electrolytes around the fastener shank.
  4. Cadmium Plating (QQ-P-416): Aircraft high-strength steel bolts (AN, MS, NAS series) are coated with a sacrificial electroplated layer of elemental cadmium. Cadmium is galvanically very close to aluminum in the galvanic series, eliminating the severe galvanic potential difference that would otherwise exist between steel and aluminum.

5. Comparative Surface Treatments Summary Matrix