Aluminum alloys are the backbone of most aircraft primary and secondary structures, prized for their excellent strength-to-weight ratio. However, many high-strength aluminum alloys — particularly those in the 2000 and 7000 series — are inherently susceptible to corrosion when exposed to moisture, salt, and atmospheric contaminants. The aviation industry solved this problem decades ago with a product called Alclad, and it continues to rely on a family of carefully specified surface treatment processes to protect aluminum structures throughout their service lives. For any Aviation Maintenance Technician (AMT) working on airframes, a thorough understanding of Alclad construction and surface treatment chemistry is not optional — it is foundational to producing airworthy, corrosion-resistant repairs.
What Is Alclad?
Alclad is a composite sheet product consisting of a high-strength aluminum alloy core metallurgically bonded on one or both sides to a thin layer of commercially pure aluminum (typically 1100 series or an unalloyed aluminum). The cladding layer is applied during the rolling process, producing an intimate, oxide-free bond between the core and the surface layers. This bond is not a coating that can peel like paint — it is an integral part of the metal itself.
The cladding layer is kept intentionally thin, typically comprising approximately 2.5 to 5 percent of the total sheet thickness on each side. This small fraction is enough to provide meaningful corrosion protection without significantly degrading the mechanical properties of the overall sheet. The high-strength core — commonly 2024-T3 (used widely in tension-loaded skins) or 7075-T6 (used in compression structures) — retains its load-bearing characteristics while the outer pure-aluminum layer handles corrosion defense.
How Alclad Provides Corrosion Protection
Pure aluminum naturally forms a thin, stable aluminum-oxide film on its surface when exposed to air. This oxide layer is tightly adherent, self-healing if scratched, and electrically resistant, making it an excellent barrier against further oxidation. Because the pure-aluminum cladding is anodic relative to the high-strength alloy core, it also provides a degree of galvanic (sacrificial) protection — if the cladding is breached, the cladding material surrounding the breach will preferentially corrode before the core alloy is attacked, similar in principle to how zinc protects steel in galvanized products.
This dual mechanism — barrier protection plus sacrificial galvanic action — makes Alclad far superior in corrosion resistance to bare high-strength aluminum sheet. For this reason, AMTs must be careful never to sand through the cladding layer during polishing or surface preparation. Excessive sanding that removes the cladding exposes the vulnerable core alloy directly to the environment, negating the entire corrosion protection strategy.
Common Aluminum Alloy Designations in Airframe Use
Understanding the alloy designations helps the technician select the correct repair material and understand its properties:
- 2024-T3: The most common structural skin alloy. High strength, used in fuselage skins, wing lower skins (tension-loaded). Alclad version is 2024-T3 Alclad. Susceptible to stress-corrosion cracking if improperly treated.
- 7075-T6: Very high strength, used in upper wing skins, spars, and highly loaded structural members. More susceptible to stress-corrosion cracking than 2024. Alclad version is common.
- 6061-T6: Good corrosion resistance and weldability; used for less critical structural and non-structural parts. Often used in bare (non-Alclad) form because the alloy itself has better corrosion resistance than 2000 or 7000 series.
- 3003: A low-strength aluminum-manganese alloy with excellent corrosion resistance and formability, used for cowlings, oil tanks, fairings, and other non-structural parts.
Surface Treatment Processes
Regardless of whether a part is Alclad or bare aluminum, surface treatment is required before applying primers or protective coatings, and after any repair work that disturbs the existing surface. The FAA Aviation Maintenance Handbook (AMT Airframe) describes several processes, each serving a specific purpose in the corrosion protection system.
Cleaning
Every surface treatment process begins with thorough cleaning. Oils, greases, hydraulic fluids, and other organic contaminants must be completely removed before any chemical treatment can work properly. Approved cleaning methods include solvent wiping with methyl ethyl ketone (MEK) or equivalent, and alkaline cleaning solutions. The technician must ensure no contaminant residue remains, as even fingerprint oils can interfere with chemical conversion coatings and cause adhesion failures in primer.
Anodizing
Anodizing is an electrochemical process that deliberately thickens the natural aluminum-oxide layer on the surface. The part is immersed in an electrolytic bath (commonly sulfuric acid) and connected as the anode in an electrical circuit, which causes controlled oxidation that builds a porous, relatively thick oxide layer into the metal surface — not simply deposited on top of it. The resulting film is hard, electrically non-conductive, and highly resistant to corrosion.
Anodizing is typically a shop or depot-level process because it requires an electrolytic tank setup. The two types most commonly referenced in aviation maintenance are chromic acid anodize (Type I) and sulfuric acid anodize (Type II); a harder, thicker Type III (hard anodize) is also used in some wear-resistant applications. Chromic acid anodize produces a thinner, more flexible film well-suited to fatigue-critical parts and Alclad material. Sulfuric acid anodize produces a thicker film with excellent paint adhesion and abrasion resistance. After anodizing, parts should be sealed (typically with hot water or a dichromate seal) to close the porous oxide and maximize corrosion protection, then primed as soon as practical.
Chemical Conversion Coating (Alodine / Chromate Conversion)
Chemical conversion coating — commercially known by trade names such as Alodine or Iridite — is the most common field-level surface treatment process for aluminum airframe repairs. In this process, a chromate-containing chemical solution is applied to the cleaned aluminum surface, where it reacts with the aluminum to form a thin, adherent layer of complex chromium compounds. The resulting film is typically golden to iridescent brown in color and provides three important benefits:
- Corrosion resistance: The chromate film passivates the surface and slows electrochemical corrosion reactions.
- Paint adhesion: The conversion coating creates a chemically active surface that bonds well with epoxy and zinc-chromate primers.
- Electrical conductivity: Unlike anodize, a properly applied thin conversion coating maintains reasonable electrical conductivity, making it suitable for bonding and grounding applications.
Application can be done by brush, swab, or immersion. The solution is left in contact with the surface for a prescribed dwell time (typically one to five minutes depending on concentration and desired coating weight), then rinsed with clean water and allowed to dry. The coating must not be abraded after application. Technicians must observe safety precautions when working with chromate compounds, as hexavalent chromium is a known health hazard requiring proper PPE and ventilation.
Alodine After Repair: Restoring Protection
When a structural repair involves cutting, drilling, sanding, or any mechanical work on aluminum, the existing surface protection is disrupted. The technician must restore corrosion protection to all bare metal surfaces before applying primer. For field repairs, applying Alodine (or equivalent conversion coating) to the affected area is the standard approach. This is why AMT airframe technicians keep chemical conversion coating materials in their kit — it is routine, not exceptional.
Key Numbers and Rules
- Alclad cladding thickness is approximately 2.5% to 5% per side of total sheet thickness.
- Never sand through the cladding — the core alloy beneath has much lower corrosion resistance.
- Chromic acid anodize (Type I) produces a thinner film; sulfuric acid anodize (Type II) produces a thicker, harder film.
- Chemical conversion coating dwell time is typically 1 to 5 minutes for brush/swab application.
- All bare aluminum exposed during repair must be treated with conversion coating before primer application.
- Hexavalent chromium compounds require PPE and proper ventilation — OSHA and shop safety rules apply.
- Do not abrade or disturb the conversion coating surface after application and before priming.
Common Test Traps
- Alclad cladding is anodic, not cathodic: The cladding sacrifices itself to protect the core — students sometimes get the galvanic relationship backwards and think the cladding is more noble than the core.
- Anodizing vs. conversion coating conductivity: Anodized surfaces are electrically non-conductive (or very high resistance), while a thin chemical conversion coating retains conductivity. Confusing the two leads to errors in bonding/grounding repairs.
- Sanding through Alclad: The test may present a scenario where aggressive polishing is performed — the correct answer is that sanding through the cladding defeats the corrosion protection strategy and the part must be evaluated accordingly.
- Chromic vs. sulfuric anodize: Type I (chromic) produces a thinner film suitable for Alclad and fatigue-sensitive parts; Type II (sulfuric) is thicker. These are easy to mix up on the written test.
- Conversion coating must come before primer: Applying primer to bare, untreated aluminum without a conversion coating first is a common shortcut error that fails to provide adequate adhesion and corrosion protection.