Corrosion is one of the most persistent and potentially dangerous threats to aircraft structural integrity. Left undetected, even mild surface corrosion can progress to intergranular or exfoliation corrosion that weakens primary structural members far beyond what is visible at the surface. For the aviation maintenance technician (AMT), developing a thorough, methodical approach to corrosion inspection is not merely a regulatory obligation — it is a direct contribution to flight safety. This article covers the techniques, procedures, high-risk areas, documentation requirements, and practical knowledge that every AMT candidate needs to master for the General written knowledge test and for real-world work on the flight line.
Corrosion inspection procedures are grounded in the Aviation Maintenance Technician Handbook — General (FAA-H-8083-30), which describes corrosion types, inspection methods, and treatment philosophies in detail. The technician's job during inspection is to identify active corrosion, assess its severity, determine whether it is within acceptable limits per the manufacturer's structural repair manual (SRM), and document findings accurately so that repair or continued-airworthiness decisions can be made.
Understanding What You Are Looking For
Before beginning any inspection, the technician must understand the forms corrosion can take. Surface corrosion appears as a dull, etched, or pitted surface and is the most common and usually least severe form. Pitting corrosion creates small but deep cavities in the metal that can concentrate stress; it is especially dangerous because the depth of a pit is often greater than its surface area suggests. Intergranular corrosion attacks the grain boundaries within a metal alloy and may produce little visible surface evidence while seriously degrading strength — this makes it particularly treacherous in aluminum alloys. Exfoliation corrosion is an advanced form of intergranular attack that causes the metal to delaminate in thin layers or flakes, producing a characteristic leafing appearance. Galvanic corrosion occurs at the junction of dissimilar metals in the presence of an electrolyte, and is frequently found around steel fasteners in aluminum structures. Filiform corrosion appears as a network of thread-like filaments under paint or primer on aluminum surfaces. Fretting corrosion results from slight relative motion between mating surfaces and produces a fine reddish-brown powder (iron oxide) or dark aluminum oxide debris at joint interfaces.
Preparation and Safety Before Inspection
Effective corrosion inspection begins with proper cleaning. Grease, dirt, paint, and oxidation products can mask active corrosion. The aircraft or component should be cleaned with approved solvents or water-based cleaners per the manufacturer's instructions before the inspection begins. The technician should wear appropriate personal protective equipment, including gloves and eye protection, especially when using solvents or when inspecting areas with chromate-containing primers, which may present health hazards.
Adequate lighting is essential. Natural light is often insufficient, so the technician should use a bright flashlight or drop light oriented at a low angle (raking light) to highlight surface irregularities, pitting, and paint lifting. Raking light causes surface pits and corrosion products to cast shadows that would be invisible under direct overhead illumination.
Inspection Techniques and Tools
The primary inspection method for corrosion is visual inspection, which encompasses both unaided eye examination and the use of magnifying aids. A 10x magnifying loupe is commonly used to examine suspected corrosion more closely and to distinguish surface staining from active pitting. Mirrors on extendable handles allow the technician to inspect areas that are not in direct line of sight, such as the interior of wheel wells, behind brackets, and underneath stringers.
Tactile inspection complements visual methods. Running a gloved finger or a clean cloth across a surface can detect roughness, blistering paint, or raised edges that the eye might miss, especially on complex curved surfaces. Paint blistering or bubbling is a critical indicator of corrosion beneath the coating, because corrosion products occupy more volume than the original metal and push the paint away from the substrate.
Borescopes and videoscopes are used to inspect interior areas such as fuel tanks, closed cavities, tubular structures, and areas accessible only through small inspection ports. These instruments provide illuminated, magnified views of surfaces that are otherwise inaccessible without disassembly. For high-stakes structural areas, nondestructive testing (NDT) methods such as dye penetrant inspection, eddy current testing, and ultrasonic thickness measurement may be required. Eddy current testing is especially valuable for detecting subsurface corrosion and intergranular corrosion in aluminum skins without the need to remove paint or fasteners.
High-Risk Areas and Zones
FAA-H-8083-30 identifies specific areas of the aircraft that are inherently prone to corrosion and that the technician must examine with extra diligence. These include:
- Battery compartments — acid or alkali fumes from batteries attack surrounding structure rapidly and can cause severe corrosion in a short period.
- Wheel wells and landing gear bays — exposed to water, mud, runway de-icing chemicals, and road salt; corrosion here often affects high-stress structural members.
- Engine exhaust areas — exhaust gases contain sulfur and carbon compounds that are highly corrosive, especially on magnesium components.
- Wing lower surfaces and fuel tank areas — water accumulation occurs in low points; fuel tank sumps and inter-spar bays trap water beneath fuel.
- Bilge areas and lower fuselage skins — moisture, cleaning fluids, and spilled hydraulic fluid collect in these zones.
- Control surface hinges and brackets — dissimilar metal contacts and moisture entrapment combine to promote galvanic and crevice corrosion.
- Lap joints, seams, and fastener holes — crevice corrosion develops where moisture is trapped between mating surfaces; galvanic corrosion develops around steel fasteners in aluminum panels.
- Magnesium components — magnesium is the most electrochemically active structural metal used in aircraft and corrodes rapidly when its protective coating is damaged.
Assessing Severity and Determining Acceptability
Once corrosion is found, the technician must assess its depth and extent. The primary reference for acceptability limits is the aircraft manufacturer's Structural Repair Manual (SRM). The SRM specifies maximum allowable depth of corrosion pitting, minimum acceptable material thickness after cleanup, and whether a part must be repaired or replaced. The technician uses a depth gauge, micrometer, or ultrasonic thickness tester to measure remaining material thickness after corrosion products are removed.
FAA-H-8083-30 categorizes corrosion severity as light, moderate, or severe. Light corrosion involves only the surface and can be removed by cleaning without measurable loss of structural material. Moderate corrosion involves measurable pitting or etching and requires cleanup followed by dimensional inspection and possibly a repair or engineering disposition. Severe corrosion involves significant material loss or intergranular/exfoliation damage and typically requires part replacement or a structural repair with engineering approval.
Documentation and Recordkeeping
Every corrosion finding must be documented precisely. Maintenance records should describe the location of corrosion using standard aircraft station and zone references, the type of corrosion observed, the extent and estimated depth, the action taken (cleaned and treated, repaired, or deferred), and the reference authority (SRM chapter and revision). Photographs are strongly encouraged because they provide objective evidence of the condition before and after treatment. Proper documentation supports continuing airworthiness, facilitates trend analysis across a fleet, and protects the technician if a dispute arises about the condition of a part at the time of inspection.
Key Numbers and Rules
- Corrosion cleanup must restore the surface to bare, shiny metal — leaving loose corrosion products in place and simply painting over them is not an acceptable practice.
- After cleanup, the area must be treated with an approved corrosion-inhibiting compound (CIC) or conversion coating (such as alodine on aluminum) before primer and topcoat are reapplied.
- For aluminum alloys, the SRM typically specifies a maximum allowable pit depth (often expressed as a percentage of the skin thickness) and a minimum number of fastener diameters to the edge of any pit that may be left in place.
- Dissimilar metal contacts must be separated by an approved insulating material (sealant, tape, or non-absorbing shim) during reassembly to prevent galvanic corrosion from re-establishing.
- 14 CFR Part 43 requires that all maintenance, including corrosion treatment, be recorded in the aircraft maintenance records with a description of work and the signature and certificate number of the approving technician.
Common Test Traps
- Painting over corrosion — A common distractor answer suggests that applying primer over light corrosion is acceptable to stop its spread. It is not. The SRM and FAA-H-8083-30 are clear that all corrosion products must be removed before any protective coating is applied.
- Confusing exfoliation with paint delamination — Paint peeling from impact or UV degradation can look like exfoliation. The distinguishing feature of exfoliation corrosion is that the metal itself is layering and lifting, not just the paint film.
- Assuming no surface corrosion means no corrosion — Intergranular corrosion may exist beneath an apparently intact surface. NDT methods are required to detect it; visual inspection alone is insufficient in high-risk areas.
- Ignoring fretting debris as contamination — The reddish-brown or gray powder found at joints may be dismissed as dust. Fretting corrosion debris is a sign of relative movement between surfaces, which can indicate loose fasteners or improper fit and must be investigated.
- Galvanic series misapplication — Test questions may ask which metal corrodes preferentially in a galvanic couple. The metal that is more anodic (further from the noble end of the galvanic series) is the one that corrodes; for example, aluminum corrodes preferentially when in contact with stainless steel in the presence of moisture.