One of the most deceptive maintenance challenges facing an aviation maintenance technician (AMT) is corrosion that has already taken hold beneath an apparently intact paint film. Because aircraft coatings are designed to be tough and adherent, they can hold their shape even as the metal beneath them is actively deteriorating. By the time corrosion becomes visible through an exterior finish, the underlying damage may be significantly advanced. Developing a sharp eye for subtle paint irregularities — and understanding the chemistry that drives them — is therefore a foundational skill for anyone maintaining aircraft structures.
This article covers the types of sub-surface corrosion most commonly found under paint, the visual and tactile clues that indicate hidden attack, the inspection methods used to confirm and characterize damage, and the practical and regulatory context that governs how AMTs document and address what they find.
Why Paint Films Hide Corrosion
Modern aircraft finishes consist of multiple layers: a chemical conversion coating applied directly to bare metal, a corrosion-inhibiting primer, and one or more topcoats. Each layer is bonded to the one below it, creating a system that can physically bridge over a corroding surface for a surprisingly long time. The conversion coating (most commonly a chromate or non-chromate treatment) provides a first line of chemical defense by passivating the metal surface, but once that layer is compromised by a scratch, impact, or moisture infiltration through a seam, an electrochemical cell can form entirely out of sight.
The electrochemical nature of corrosion is key to understanding why it hides so well. The anodic reaction — where metal atoms give up electrons and dissolve — and the cathodic reaction — where oxygen and moisture complete the circuit — take place at the metal-to-primer interface. The topcoat above may remain visually intact for weeks or months. It is only when corrosion products build up with enough volume to physically push the paint away from the surface that external clues appear.
Types of Corrosion Commonly Found Under Paint
Filiform Corrosion
Filiform corrosion is arguably the most distinctive sub-surface attack pattern because of its characteristic thread-like or worm-track appearance. It progresses as narrow, randomly wandering filaments beneath the paint, typically originating from a scratch, fastener hole edge, or sheared paint edge. Each filament consists of an active anodic head followed by a trail of dried corrosion products. On aluminum alloys — by far the most common structural metal in light aircraft — these tracks appear reddish-brown or grayish-white when viewed through translucent topcoats, or as raised ridges when the topcoat is opaque. Filiform corrosion is most aggressive in environments with relative humidity between roughly 70 and 95 percent and is a strong indicator that the conversion coating has been locally breached.
Blistering and Paint Lifting
Blistering results when corrosion products accumulate beneath the paint and physically delaminate it from the primer or the primer from the metal. The corrosion products — primarily aluminum oxide and hydroxide compounds on aluminum structures — occupy significantly more volume than the parent metal they replace. This volumetric expansion exerts upward pressure on the paint film, creating domed blisters that range from pinhead size to several centimeters across. Pressing a blister gently will often reveal whether it is filled with a dry, chalky powder (well-advanced corrosion with dried byproducts) or whether it collapses and re-inflates (a moisture-filled void where active corrosion is still occurring). Blistering is particularly common along lap joints, around fastener rows, and at the edges of inspection panels where moisture can wick under the coating.
Galvanic Corrosion Under Paint
When dissimilar metals are in contact — steel fasteners in aluminum structure, for example — galvanic corrosion can proceed aggressively once a conductive electrolyte is present. Under paint, this often manifests as a ring or halo of blistering centered on a fastener, with corrosion spreading radially outward as the anodic metal (usually the aluminum) is consumed. Because fasteners are numerous and often inaccessible without disassembly, galvanic attack under paint can be widespread before it is detected during a standard walkaround.
Intergranular Corrosion
Intergranular corrosion attacks along the grain boundaries of an alloy, often leaving the surface looking intact or showing only slight surface roughness. In high-strength aluminum alloys such as 7075, this type of attack can penetrate deeply into the material while the paint above appears nearly normal. A slightly grainy or matte texture in an area that should be smooth, combined with a barely detectable swelling of the paint, may be the only external evidence. Intergranular corrosion is particularly serious because it preferentially occurs in heat-affected or improperly heat-treated alloys and can dramatically reduce structural strength well before catastrophic appearance changes occur.
Visual and Tactile Inspection Techniques
Effective sub-surface corrosion detection begins with a systematic, close visual inspection performed in good light — ideally both direct and raking (oblique) illumination. Raking light is especially valuable because it dramatically highlights small surface irregularities such as slight paint lifting, hairline cracks in topcoat, or the tiny raised ridges of filiform tracks that would be invisible under direct overhead lighting. A flashlight held nearly parallel to the skin surface is a simple and highly effective technique during hangar inspections.
Tactile inspection complements visual examination. Running a clean fingertip or a fingernail across a suspected area can detect blisters too low-profile to see clearly, as well as the slight roughness of a surface where the paint has separated from its primer but has not yet lifted. Areas around fasteners, along chord-wise seams, and at the lower surfaces of horizontal stabilizers and wing roots — all zones where moisture tends to collect — deserve particular attention during tactile checks.
A wooden or plastic tapping tool (a blunt dowel or the handle of a screwdriver) used gently to tap the surface can reveal delamination by a change in sound. A solid, well-bonded panel produces a clear, sharp tap; a delaminated area produces a dull, hollow sound. This technique, sometimes called coin tapping or tap testing, is widely used and FAA-recognized for detecting delamination in composite and bonded structures, though it is not specifically emphasized in FAA guidance as a method for detecting corrosion under paint on metallic structure.
Instruments and Advanced Aids
When visual and tactile inspection raises suspicion but cannot confirm the extent of damage, AMTs may use additional tools. Ultrasonic thickness gauges can measure remaining metal thickness through intact paint without the need for coating removal, helping quantify material loss before disassembly. Eddy-current inspection can detect sub-surface cracks and corrosion in conductive materials and is often used on high-stress structural areas. Whenever nondestructive inspection (NDI) methods are employed, they must be conducted in accordance with the applicable manufacturer's maintenance manual or a recognized NDI standard, and findings must be documented in the aircraft maintenance records.
Key Numbers and Rules
- Filiform humidity range: Most active between approximately 70% and 95% relative humidity — below this range the reaction stalls; above it, corrosion may transition to other forms.
- Corrosion product volume: Aluminum oxide corrosion products occupy approximately 2.5 times the volume of the original aluminum metal consumed, explaining why even minor sub-surface attack produces visible paint lifting.
- Inspection lighting angle: A raking or oblique light source is the standard technique for revealing paint irregularities; direct perpendicular lighting routinely misses blistering and filiform tracks.
- Airworthiness documentation: Any corrosion found must be evaluated against manufacturer-specified allowable limits (typically stated in the Structural Repair Manual or Corrosion Prevention and Control Program document); damage exceeding limits requires repair before return to service per 14 CFR Part 43.
- Conversion coating: MIL-DTL-5541 (chromate) or approved non-chromate equivalents are the standard chemical treatments on aluminum aircraft structure; their integrity at the metal-primer interface is the primary barrier preventing sub-surface corrosion initiation.
Why It Matters: Safety and Airworthiness
Aircraft structural components are designed to carry specific loads with defined safety margins. Corrosion reduces the effective cross-sectional area of load-bearing members, introduces stress concentrations at pits and grain-boundary voids, and in the case of intergranular attack, degrades the ductility of the alloy. A painted surface that looks acceptable from a few feet away can conceal a fuselage skin, spar cap, or stringer that has lost a significant percentage of its designed thickness. The FAA's Aviation Maintenance Technician Handbook — Airframe emphasizes that corrosion control is inseparable from structural airworthiness, and that identifying corrosion in its earliest possible stage is always preferable to addressing widespread structural damage later.
From a regulatory standpoint, AMTs are responsible under 14 CFR Part 43 for returning aircraft to an airworthy condition following maintenance. Failing to identify or properly document corrosion found during inspection creates both safety risk and legal liability. Conversely, a well-trained technician who catches filiform tracks or a single suspicious blister during a routine walkaround can prevent a minor cosmetic issue from becoming a costly structural repair or, far worse, an in-flight failure.
Common Test Traps
- Assuming intact paint means no corrosion: FAA test questions frequently emphasize that corrosion can be well-advanced beneath paint that shows no obvious external damage — the paint film's physical integrity does not indicate the metal's condition.
- Misidentifying filiform as a cosmetic defect: Filiform corrosion is sometimes dismissed as a paint adhesion failure, but it represents active metal attack; test questions may ask you to distinguish it from simple delamination or paint crazing.
- Overlooking galvanic halo patterns: Blistering centered on a fastener should immediately raise suspicion of galvanic corrosion, not just a localized paint failure; missing this pattern is a common error flagged in AMT exam scenarios.
- Confusing tap-test sounds: A hollow or dull sound indicates delamination or sub-surface void — not necessarily that no corrosion is present at all. The tap test is a screening tool; confirmation requires closer examination or NDI, not dismissal of the finding.
- Applying allowable-damage limits from the wrong manual: Each aircraft model has specific corrosion limits in its manufacturer documentation. Using generic rules of thumb rather than the correct Structural Repair Manual chapter is incorrect procedure and a tested concept on the AMT Airframe exam.