Field-oriented checklist of the practical ways A&P mechanics and MRO shops remove pitted corrosion from aircraft-grade aluminium (2xxx/7xxx Alclad and bare sheet). Each method comes straight from FAA guidance or recent MRO/Lab experience; pick the one that fits the damage depth, location and equipment you have.
1. Verify the damage limits first
• Consult the airframe Structural Repair Manual (SRM). If any pit or blended dish-out would leave the skin or extrusion thinner than the SRM minimum (typically ≈ 10 % of original thickness, or 0.020 in on 0.040 in Alclad), the part is repaired or replaced rather than cleaned. Federal Aviation Administration
2. Mechanical cleaning (hand or low-speed power tools) – the default for light/medium pitting Step Key points References Mask & strip paint Use MIL-spec alkaline stripper; no acid strippers on aluminium. Federal Aviation Administration Remove corrosion products Plastic or Tampico fibre brushes, aluminium wool, or 150- 240 grit AlOx cloth. Never steel/copper brushes or siliconcarbide on clad skins. Federal Aviation Administration Blend (dish-out) Fair the cleaned area to a 20 : 1 length-to-depth ratio; remove material between closely-spaced pits so no isolated gouges remain. Federal Aviation Administration Gauge depth Use a dial-depth gauge or pit gauge to confirm remaining thickness. Federal Aviation Administration Finish Progressively sand to 400 grit, solvent-wipe, then treat (chromate or trivalent conversion), prime (epoxy), and topcoat. Federal Aviation Administration Pros: Low cost, minimal heat input, already approved in every SRM. Cons: Labor-intensive; removes parent metal; risk of uneven surfaces on large areas.
3. Vacuum or plastic-media blasting (for broad, shallow pitting)
• Low-pressure (30–40 psi on clad, 40–45 psi on bare) AlOx or plastic media removes oxide without gouging.
• Engineering approval is required below 0.0625 in (1.6 mm) skin thickness. Federal Aviation Administration
4. Controlled chemical deoxidising (bath or local swab)
• Phosphoric-acid cleaners such as Henkel Alumiprep 33 followed by de-smut and chromate/TCP conversion (MIL-C-81706) are effective when you can immerse or dam the area. Aviation Laser Services © 2025
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• Strictly control dwell time and rinse until the surface is “water-break-free.” Federal Aviation Administration Pros: Uniform oxide removal; no metal loss. Cons: Requires containment, PPE, and waste handling permits.
5. Pulsed-fiber-laser cleaning (FeatherPulse 300 W unit) – very effective for tight pits, rivet lines, or where you want zero media Parameter Typical starting range for 2024-T3 skins* Spot size 0.5–1.0 mm Pulse width 80–120 ns Repetition rate 20–40 kHz Fluence 2–6 J cm−2 Scan speed 100–300 mm s−1 Passes 2–4 (finish with a low-power “polish” pass) Max substrate temp < 120 °C (use an IR thermometer) *Derived from open-access work on AA2024 laser cleaning and recent MRO case studies. ResearchGateHome Why it works
• Nanosecond pulses ablate oxide and corrosion by micro-spallation; the aluminum matrix reflects most energy, so parent metal loss is negligible.
• No grit, no chemicals, and the laser head can track around rivets or inside wheel wells.
• Surface roughness (~Ra 0.8–1.6 μm) is ideal for primer adhesion. Watch-outs
• Stay below the melt threshold; the satin reflection disappears when you reach base metal – stop there.
• Use fume extraction with HEPA/activated-carbon filters (aluminum oxide fumes).
• Run a coupon first and perform an eddy-current thickness check to document zero base-metal loss before you touch the aircraft.
6. Local hole-enlargement or spot-facing When pits are confined to fastener holes, the OEM often allows ream-oversize + Hi-Lok or sleeve repair rather than skin replacement. Federal Aviation Administration
7. Follow-up protection
1. NDI – Eddy-current or borescope to confirm no hidden exfoliation or intergranular attack.
2. Conversion coat – Chromate or TCP within 8 h of cleaning.
3. Primer/top-coat – Per the SRM paint spec. Aviation Laser Services © 2025
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4. Sealants/CPCs – Edge-seal seams and apply CPC (e.g., LPS-3) in bilges or wheel wells. Federal Aviation Administration
8. When to scrap or engineer a repair
• Pit depth after blending exceeds SRM limits.
• Extensive pitting is coupled with subsurface intergranular corrosion.
• High-strength forgings or spars—always consult the type-certificate holder. Quick decision tree
1. Light powdery pits <0.003 in deep? – Hand brush + conversion + paint.
2. Medium (≤10 % thickness) on open skin? – Mechanical blend or plastic-media blast.
3. Complex geometry, rivet lines, environmental concerns? – Pulsed-fiber-laser cleaning.
4. Beyond limits or in critical member? – Engineer repair or replace. Aviation Laser Services © 2025
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