11.2 Corrosion Recognition, Control & Damage Limits

11.2 Corrosion Recognition, Control & Damage Limits

[!WARNING] Corrosion is the discrepancy an IA is most likely to under-call. A patch of surface oxide on a stringer looks like housekeeping. Intergranular corrosion in the same location, which can look nearly identical from outside, has removed load-carrying material through the section. The inspection skill is not spotting discoloration — it is classifying what kind of attack produced it, and knowing when the answer stops being "clean and treat" and becomes "this is a major repair."

The guidance is AC 43.13-1B, Chapter 6, Corrosion, Inspection and Protection, supported by the FAA's dedicated corrosion control advisory circular series, AC 43-4.

---

Why Corrosion Happens

Corrosion is an electrochemical process requiring four things: an anode, a cathode, a conductive electrolyte path, and an electrical connection between anode and cathode. Break any one and corrosion stops. Every preventive measure in aviation maintenance is an attack on one of those four:

Environmental drivers: salt air, industrial atmospheres, high humidity, and standing water. Design drivers: faying surfaces, lap joints, dissimilar metal contact, crevices, and any location where water collects and cannot drain.

---

The Types an IA Must Distinguish

Surface (Uniform Etch) Corrosion

General roughening and dulling of the surface, with a powdery white oxide on aluminum or red rust on steel. It is the least serious form provided it has not progressed inward. Removed mechanically, treated, and refinished.

Pitting

Localized attack producing small, sharp-bottomed holes, most common on aluminum and stainless steel. Pitting is more serious than its appearance suggests because each pit is a stress riser and because depth is hard to judge from the surface. Depth must be measured after cleanout and compared against allowable limits.

Galvanic (Dissimilar Metal) Corrosion

Occurs where two metals far apart in the galvanic series are in contact in the presence of an electrolyte. The less noble (more anodic) metal corrodes preferentially. Classic aircraft cases:

• A steel fastener in an aluminum structure — the aluminum corrodes around the fastener.
• Magnesium components in contact with aluminum or steel — magnesium is highly anodic and corrodes aggressively, which is why 5056 rivets (magnesium-compatible) are specified in magnesium structure.
• Carbon fiber against aluminum — carbon is strongly cathodic and drives severe aluminum corrosion.

Prevention is isolation: sealant, a barrier tape, a compatible primer, or an appropriate washer between the surfaces.

Intergranular Corrosion

Attack along the grain boundaries of an alloy, typically the result of improper heat treatment leaving a difference in electrical potential between grain boundary material and the grain center. It proceeds inside the material and can be extensive while the surface still looks acceptable.

Exfoliation

An advanced form of intergranular corrosion in wrought, extruded, or rolled products, where the corrosion products between grain layers force the layers apart. Recognized by lifting, flaking, or leafing of the surface — the metal delaminates like the pages of a book. Exfoliation on an extruded spar cap or stringer is a serious structural finding, because the section has lost material through its depth.

Filiform

Thread-like worm tracks under paint, usually where a coating was applied over a poorly prepared or contaminated surface, and driven by high humidity. Cosmetic at first, but it undermines the coating and opens the way for other forms.

Fretting

Occurs at faying surfaces subject to slight relative motion under load, producing a black or dark powder and eventually pitting and cracking. Found at bolted joints that have loosened and at bearing interfaces.

Stress Corrosion Cracking

Cracking produced by the combination of a sustained tensile stress and a corrosive environment, often with no general corrosion visible at all. Susceptible locations are those with residual assembly stress — interference-fit bushings, tapered pins, over-torqued clamps, and press-fit fittings. Because it produces cracks rather than material loss, it is found by crack detection methods rather than by looking for oxide.

---

Quick Recognition Table

---

Where an IA Looks

[ ] Battery box and the structure downstream of the vent
[ ] Lavatory and galley areas; anywhere fluids are spilled
[ ] Wheel wells, landing gear bays, and the structure behind gear doors
[ ] Lower fuselage skins, belly, and bilge; steel tube lower longerons
[ ] Exhaust trail areas and the structure aft of exhaust outlets
[ ] Wing and empennage attach fittings; hinges and hinge pins
[ ] Faying surfaces, lap joints, and under doublers
[ ] Piano hinges, control surface interiors, and balance weight attachments
[ ] Float and amphibian structure; any seaplane operated in salt water
[ ] Under the floorboards and beneath insulation blankets
[ ] Drain holes: blocked drains are the leading cause of everything above

---

Treatment and the Limits That Govern It

The standard sequence is: clean the area, remove the corrosion products, neutralize any remaining residue, restore the protective finish, and record what was done. For aluminum, removal is by mechanical means appropriate to the alloy, followed by a chemical conversion coating and then primer and topcoat. For steel, removal is followed by an appropriate corrosion-preventive finish.

The decisive question is depth, not area. Every airframe manufacturer publishes allowable damage limits — the maximum material thickness that may be removed from a given part before the part is no longer serviceable. So:

1. Clean out the corrosion completely. Blending around it and painting over the remainder guarantees it returns and hides the extent.
2. Measure the remaining thickness and compare it with the allowable damage limits in the manufacturer's structural repair manual or maintenance manual.
3. If removal stayed within the allowable limits, restore the finish, record the work under § 43.9, and note the location so that the next inspection can compare.
4. If removal exceeded the allowable limits, the part is no longer to type design. It must be repaired using approved data or replaced, and a structural repair to a primary member is a major repair requiring Form 337 (Chapter 5).

There is no fourth option. Cleaning out corrosion past the allowable limits and refinishing without a repair leaves an aircraft that is not airworthy, however clean it looks.

---

Recording Corrosion Findings

An IA's records serve the next inspector. A useful entry states where (station and structure), what type, how deep after cleanout, what limit it was compared against, and what was done. "Cleaned and treated corrosion, belly" tells the next IA nothing; "Removed light pitting corrosion, right side fuselage skin between stations 120 and 132, maximum depth after cleanout 0.004 inch against allowable 0.008 inch per SRM 53-30-01 Rev 12; conversion coated, primed, topcoated" lets the next inspector detect progression.

---

High-Yield Exam Traps

• Galvanic corrosion attacks the less noble metal. In a steel-fastener-in-aluminum joint, the aluminum goes.
• Exfoliation is intergranular corrosion in wrought or extruded product — the leafing appearance is the giveaway, and it is a serious structural finding.
• Stress corrosion cracking needs sustained tensile stress plus a corrosive environment, and it can occur with no general corrosion visible.
• Depth against allowable damage limits decides the disposition, not the surface area.
• Exceeding allowable limits makes it a repair, and on a primary member that means approved data and a Form 337.