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Storage humidity that prevents filiform corrosion under paint

Aviation Mechanic General (AMG) practice question 32 of 36 on Cleaning and Corrosion Control. ACS element AM.I.G.K18 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONAn aircraft with a polyurethane finish is stored in a hangar. To prevent filiform corrosion beneath the finish, the hangar air should be kept at what relative humidity?

  1. ABelow 70 percent relative humidity
  2. BBetween 78 and 90 percent relative humidity
  3. CBelow 90 percent relative humidity indoors
Show the answer and explanation

Correct answer: A. Below 70 percent relative humidity

  • A — correct The handbook states that filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent.
  • B — incorrect That span is the humidity range in which filiform corrosion forms on a slightly acidic surface, so holding the air there invites the attack instead of preventing it.
  • C — incorrect Air held anywhere under 90 percent still allows the 78 to 90 percent band in which filiform forms, so this does not prevent it.

BACKGROUND

Filiform corrosion is a special form of oxygen concentration cell that occurs on metal surfaces carrying an organic coating system, and it is recognized by its wormlike trace of corrosion products beneath the paint film. Polyurethane finishes are especially susceptible to it. It occurs when the relative humidity of the air is between 78 and 90 percent and the surface is slightly acidic, and it usually attacks steel and aluminum surfaces. The handbook lists three preventive measures: storing aircraft in an environment with a relative humidity below 70 percent, using coating systems that have a low rate of diffusion for oxygen and water vapor, and washing the aircraft to remove acidic contaminants such as those created by air pollutants. Left in place, filiform can lead to intergranular corrosion, especially around fasteners and at seams.

STUDY TIP

Learn the two humidity figures separately: 78 to 90 percent is where filiform forms, below 70 percent is where it is prevented. Tie each number to the sentence it comes from so a question about storage does not pull up the occurrence range.

KEY POINTS

  • Filiform corrosion grows under an organic coating and shows as wormlike traces beneath the paint film.
  • It forms at 78 to 90 percent relative humidity on a slightly acidic surface, and polyurethane finishes are especially susceptible.
  • Storing the aircraft below 70 percent relative humidity is a stated preventive measure, along with low-diffusion coatings and washing off acidic contaminants.

SOURCES

  • FAA-H-8083-30B ch08 "Filiform Corrosion"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.