Bare-metal leading edges on business jets—wing leading edges, nacelle lips, horizontal stabiliser tips—oxidise faster than any other exterior surface, often turning dull and chalky between quarterly washes. Understanding why aluminium corrodes and how mechanical polishing removes that oxide layer helps flight departments plan realistic turnaround times and set expectations for ramp appearance work.
Why do leading edges oxidise faster than painted surfaces?
Aluminium alloys (typically 2024-T3 or 7075-T6 on leading edges) react with atmospheric oxygen the moment they're exposed, forming aluminium oxide (Al₂O₃) within nanoseconds. Unlike iron oxide (rust), which flakes away and exposes fresh metal, aluminium oxide forms a thin, self-limiting passive layer roughly 2–5 nanometres thick under normal conditions. This protects the underlying alloy—but it's also what makes the surface look dull.
Leading edges oxidise faster because:
- Aerodynamic heating: even at cruise speeds below Mach 0.90, ram air compression raises leading-edge skin temperature 10–30 °C above ambient, accelerating oxidation kinetics
- Moisture and salt: flight through coastal or winter-treated airspace deposits hygroscopic salts (NaCl, CaCl₂) that hold moisture against the metal, promoting pitting corrosion
- Abrasion: ice crystals, dust and insects mechanically wear the passive layer, exposing fresh aluminium that oxidises immediately
- Cold-soaking: after descent, cold-soaked metal below the dew point condenses atmospheric moisture, creating an electrolyte film
The visible chalky white or grey film is hydrated aluminium oxide mixed with atmospheric contaminants—essentially tarnish.
What's actually happening at the molecular level?
The oxidation reaction is straightforward:
4Al + 3O₂ → 2Al₂O₃
In the presence of water (humidity, rain, decon rinse left to air-dry), this becomes more complex:
2Al + 6H₂O → 2Al(OH)₃ + 3H₂↑
Aluminium hydroxide is the white, powdery residue you see. It's porous and non-protective, unlike the dense oxide layer. Salt accelerates this by providing chloride ions (Cl⁻) that penetrate the passive film and create localised anodic sites—pitting corrosion. EASA and FAA both classify this as a "significant corrosion risk area" in continuing-airworthiness guidance, requiring regular inspection on transport-category aircraft.
Once the hydroxide layer thickens beyond about 50–100 nanometres, it scatters light diffusely rather than reflecting it specularly—the metal looks dull.
How does polishing remove oxidation?
Mechanical polishing uses abrasive compounds suspended in a carrier (water, oil or solvent) to physically shear away the oxide layer and a microscopic amount of the underlying aluminium, leaving a smooth, reflective surface.
A typical single-stage polish for ramp work uses:
- Abrasive: aluminium oxide (ironic, but harder than the tarnish), cerium oxide or fine silicon carbide, graded by particle size (3–10 microns for cutting, <1 micron for finishing)
- Carrier: light petroleum distillate or water-based emulsion
- Surfactants: to suspend particles and prevent re-deposition
The process:
- Cutting: abrasive particles remove 0.5–2 microns of tarnished metal, creating a uniform scratch pattern
- Levelling: finer abrasives (or the same compound under lighter pressure) smooth the scratch pattern to below the wavelength of visible light (~400 nm)
- Burnishing: residual compound and pad friction work-harden the surface slightly, improving reflectivity
You're not "sealing" the metal—you're creating a surface so smooth that the inevitable new oxide layer (which forms in seconds) is thin, uniform and transparent, allowing specular reflection.
What's the difference between cutting and finishing compounds?
| Compound type | Abrasive size | Material removal | Typical use |
|---|---|---|---|
| Heavy cut | 10–20 microns | 2–5 microns | Severe oxidation, corrosion prep |
| Medium cut | 3–10 microns | 0.5–2 microns | Routine tarnish, single-stage ramp polish |
| Finishing polish | <1 micron | <0.5 microns | Final gloss, show finish |
For line maintenance or FBO ramp work, a medium-cut compound is standard—it removes visible oxidation in one pass without excessive material removal. OEMs like Gulfstream and Bombardier specify maximum allowable material removal over the aircraft's life (typically 0.25–0.50 mm total) to preserve skin gauge and bucking-bar dimples, logged in the tech log.
Heavy cutting is a hangar job, often requiring engineering sign-off if you're approaching those limits.
How long does a polished leading edge stay bright?
Unprotected polished aluminium begins to tarnish visibly within 48–72 hours in humid or coastal environments, faster if the aircraft is flying. The new oxide layer grows logarithmically—rapid at first, then slowing—but never stops.
Factors affecting durability:
- Flight hours: an aircraft flying 50 hours/month will show tarnish in a week; one flying 10 hours/month may stay bright for three weeks
- Basing: a jet on the apron at EGLF (Farnborough, coastal) tarnishes faster than one at LSZS (Samedan, high-altitude dry air)
- Protection: some operators apply a sacrificial wax or polymer sealant (Rejex, CorrosionX) immediately after polishing, which delays oxidation by 2–4 weeks but requires removal before the next polish
No coating is permanent on leading edges—aerodynamic shear and thermal cycling remove anything within a few flights. Polishing is inherently a recurring task.
Can you polish leading edges on the ramp, or does it need a hangar?
You can absolutely polish on the ramp, and most operators prefer it—no hangar slot, no downtime, and you can do it during a routine turnaround or overnight stop. Requirements:
- Lighting: adequate apron lighting or portable LED work lights to see the surface finish
- Power: 230V GPU or airside mains for rotary polishers (cordless tools exist but lack torque for aluminium)
- Weather: no active precipitation (compounds wash away), ideally <80% humidity (flash tarnish), temperature >5 °C (some compounds thicken or separate when cold)
- Access: leading edges are within arm's reach from a stable stepladder; no scaffolding needed for anything up to a G650 or Global 7500
Wind is the main constraint—above 15 knots, polishing compound and airborne dust contaminate the work. Early morning or late evening is often best.
How Terso approaches brightwork on the ramp
We carry medium-cut aluminium polish and finishing compound in our ramp kit, along with wool and foam pads for different surface conditions. Leading-edge work is typically done during a dry wash or decon service, adding 30–60 minutes depending on aircraft size and oxidation severity. We log the work in the TLB if the operator requests it, and we'll advise if tarnish has progressed to pitting that needs engineering review before we touch it.
Sources
- Corrosion Control and Treatment Manual, Chapter 6: Aluminium Alloys
- EASA Part-M Continuing Airworthiness Requirements, AMC M.A.302
- Gulfstream G650 Aircraft Maintenance Manual, Chapter 51-00-00: Structures – General
- Aluminium Oxide Formation and Properties, ASM International Handbook
- Bombardier Global 7500 Maintenance Planning Guide, Section 5: Corrosion Prevention
- ICAO Annex 8: Airworthiness of Aircraft, Chapter 4: Continuing Airworthiness