Corrosion is one of the most relentless threats to aircraft structural integrity. Left untreated, it can silently degrade load-bearing members, control cables, and skin panels to the point of catastrophic failure — all without any obvious external indication. For Aviation Maintenance Technicians (AMTs), understanding how to properly treat active corrosion and apply chemical conversion coatings is not just a test requirement; it is a fundamental airworthiness skill practiced every time an aircraft enters the hangar for scheduled maintenance.
This article covers the full treatment process — from identifying and removing active corrosion to restoring protective surface treatments — with emphasis on the chemical conversion coating process that forms the backbone of corrosion prevention on aluminum aircraft structures.
Understanding Corrosion Before You Treat It
Corrosion is an electrochemical process in which metal is oxidized by reacting with oxygen, moisture, or other contaminants in the environment. Before any treatment can begin, the technician must correctly identify the type of corrosion present, because the treatment approach differs significantly depending on the corrosion form and the metal involved.
The most common forms encountered in airframe maintenance include surface corrosion (a general etching or pitting of the exposed metal surface), intergranular corrosion (which attacks the grain boundaries within the metal and is particularly dangerous in high-strength aluminum alloys), filiform corrosion (a thread-like corrosion that travels beneath paint or coatings, common on aluminum and magnesium), galvanic corrosion (which occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte), and stress corrosion cracking (a combination of sustained tensile stress and a corrosive environment).
The FAA's Aviation Maintenance Handbook — Airframe (FAA-H-8083-31) makes clear that the type and extent of corrosion damage determines whether the part can be treated in place, requires blending, must be replaced, or demands an engineering disposition. AMTs must evaluate whether remaining material thickness after cleanup still meets the manufacturer's minimum structural requirements before returning a part to service.
The Corrosion Treatment Process
Step 1 — Cleaning
All treatment begins with thorough cleaning. Contaminants such as grease, oil, hydraulic fluid, and dirt must be removed before corrosion products can be properly assessed and treated. Approved cleaning agents — typically aliphatic naphtha, isopropyl alcohol, or approved aqueous cleaners — are applied and the area rinsed thoroughly. Residual cleaning agents can interfere with chemical conversion coatings, so complete removal and drying of the area is critical.
Step 2 — Mechanical Removal of Corrosion Products
Active corrosion products (oxides, hydroxides, and salts) must be mechanically removed before chemical treatment. Approved methods include hand sanding with aluminum oxide abrasive paper, non-metallic abrasive pads (such as those approved by the aircraft manufacturer), or rotary wire brushes made of the same metal as the base material being cleaned. Steel wire brushes must never be used on aluminum or magnesium — embedded steel particles will cause immediate galvanic corrosion.
For aluminum, the goal is to remove the white, powdery corrosion product (aluminum oxide and aluminum hydroxide) down to bright, clean metal. For steel components, rust scale is removed down to bare metal using approved abrasive methods. Blending is performed carefully with fine abrasive paper, feathering the edges of any blended area to avoid stress risers. All material removal must be documented, and the remaining thickness checked against the structural repair manual (SRM) limits.
Step 3 — Chemical Treatment
Once bare metal is exposed, it must be treated promptly. Aluminum begins to re-oxidize within minutes of exposure. The two primary chemical treatment approaches are chemical conversion coating and anodizing, with conversion coating being the most common field-applied method.
Chemical Conversion Coatings Explained
A chemical conversion coating — most commonly known by the trade name Alodine (a Henkel product) or generically as a chromate conversion coating — creates a thin, chemically bonded protective layer directly on the aluminum surface. The process causes a controlled chemical reaction between the aluminum substrate and an acidic solution containing hexavalent chromium compounds. The result is a golden-tan to iridescent brown film (the color intensifies with longer dwell times) that is integral to the surface — not a separate coating sitting on top.
This conversion layer serves two critical functions: it provides a barrier against further oxidation and, perhaps more importantly, it provides an excellent bonding surface for primer and topcoat paint systems. A properly applied conversion coating dramatically improves paint adhesion compared to bare or anodized aluminum and also offers some sacrificial corrosion protection in its own right through the slow release of hexavalent chromium ions at any damaged site.
How the Chemical Process Works
The active ingredient in most chromate conversion coatings is chromic acid (or a mixture containing chromates), combined with fluoride compounds that act as activators. The fluoride ions etch the natural aluminum oxide layer just enough to expose fresh aluminum, allowing the chromate compounds to react directly with the metal. The resulting layer is primarily composed of hydrated chromium oxide mixed with aluminum oxide and chromate compounds — it is an extremely thin film, generally described as only a few tenths of a mil or less, making it essentially dimensionless for fit and clearance purposes.
The MIL-DTL-5541 specification governs chromate conversion coatings for aluminum and aluminum alloys. Class 1A coatings provide maximum corrosion protection and paint adhesion, while Class 3 coatings are specified for applications requiring low electrical resistance and electrical conductivity through the surface (such as bonding and grounding paths), rather than for maximum paint adhesion.
Application Procedure
Conversion coating can be applied by brush, swab, or immersion, making it practical for both shop and field repairs. The general sequence is: clean the metal, apply the conversion coating solution, allow the specified dwell time (typically one to five minutes — always follow the specific product technical data sheet), and then rinse thoroughly with clean water. The coating must remain wet throughout the dwell period; dry spots result in uneven or failed coating. After rinsing, the part is air-dried or dried at low heat. The fresh coating is extremely soft and must not be abraded, wiped hard, or touched until it has fully cured — actual cure time is product-specific, so always consult the manufacturer's technical data sheet.
Color is a useful quality indicator: a properly applied Class 1A coating on 2024-T3 aluminum should appear golden-yellow to brown. A completely clear or frosty appearance may indicate insufficient reaction time or contaminated solution, while an excessively dark or powdery surface may indicate over-etching. Any areas that did not coat uniformly must be re-cleaned and re-treated before primer is applied.
Magnesium and Steel Treatments
Magnesium is the most active (and most corrosion-prone) structural metal used in aircraft. Chromate treatment is also used for magnesium, but the process chemistry and specifications differ from aluminum — the DOW 7 or equivalent treatment produces a chrome pickle coat. Because magnesium corrodes so aggressively, even minor surface damage to the protective coating demands immediate attention. Steel corrosion treatment typically involves mechanical removal to bare metal, followed by application of a zinc chromate or epoxy primer immediately — there is no practical field-applied conversion coating equivalent for steel comparable to Alodine on aluminum.
Primer Application After Treatment
Chemical conversion coatings are not a complete corrosion protection system by themselves — they are the foundation for a paint system. After the conversion coat has cured, an approved corrosion-inhibiting primer (typically zinc chromate primer or a modern waterborne/epoxy equivalent per the aircraft manufacturer's SRM) must be applied before the surface is exposed to the environment or reassembled. The primer should be applied as soon as practical, within the working window specified by the SRM or product technical data sheet, because the excellent bonding surface deteriorates over time as additional surface oxidation occurs.
Key Numbers and Rules
- MIL-DTL-5541 is the governing military specification for chromate conversion coatings on aluminum alloys; Class 1A for maximum protection, Class 3 for conductive applications.
- Dwell time for liquid conversion coating is typically one to five minutes — always follow the product data sheet and aircraft SRM.
- Coating thickness is essentially dimensionless — an extremely thin film measured in fractions of a mil — no allowance needed for fit checks; consult the product data sheet for exact figures.
- Do not use steel wire brushes on aluminum or magnesium — embedded steel particles cause immediate galvanic corrosion.
- Primer must be applied within the SRM or product-specified window after conversion coating, as soon as practical — the exact time limit varies by product.
- Fresh conversion coating is extremely soft — do not abrade or hard-wipe until fully cured per the manufacturer's technical data sheet.
- Hexavalent chromium compounds are highly toxic and regulated — proper PPE (gloves, eye protection, respiratory protection) and hazardous waste disposal are mandatory.
Common Test Traps
- Confusing Class 1A and Class 3 coatings: Class 3 is not a lesser version of Class 1A — it is specifically chosen where electrical conductivity must pass through the surface. Class 1A provides maximum corrosion resistance but higher electrical resistance.
- Thinking conversion coating alone is sufficient final protection: Alodine/conversion coating is a surface preparation and base layer — it must be topcoated with primer (and usually finish coat) for complete protection, especially in service.
- Forgetting to check remaining material thickness: Removing corrosion mechanically removes metal. The SRM limits on minimum acceptable thickness must be checked after blending, not assumed to be acceptable.
- Assuming any wire brush is acceptable: Only wire brushes made from the same material as the base metal (e.g., stainless brushes on stainless, non-metallic pads on aluminum) are approved — cross-contamination causes galvanic corrosion.
- Overlooking intergranular corrosion severity: Surface appearance may look minor, but intergranular corrosion can penetrate deeply into high-strength aluminum alloys (2024, 7075). What looks like light surface scaling may indicate a part that requires replacement rather than treatment.