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Galvanic corrosion rate and relative surface area

Aviation Mechanic General (AMG) practice question 3 of 31 on Cleaning and Corrosion Control. ACS element AM.I.G.K3 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA small fastener made of a more active metal is installed in a large panel of a less active metal, and an electrolyte is present. What rate of galvanic corrosion should be expected on the fastener?

  1. ARapid and severe
  2. BSlow and superficial
  3. CNone, because the fastener is too small to corrode
Show the answer and explanation

Correct answer: A. Rapid and severe

  • A — correct When the surface area of the corroding (more active) metal is smaller than the surface area of the less active metal, the handbook states that corrosion is rapid and severe.
  • B — incorrect Slow and superficial corrosion is the case when the corroding metal is larger than the less active metal, the reverse of this scenario.
  • C — incorrect Small size does not stop galvanic corrosion; a small anode in contact with a large cathode concentrates the attack on the small part.

BACKGROUND

Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte. The rate depends on the difference in the activities of the two metals: the greater the difference, the faster the corrosion. The rate also depends on the relative size of the parts in contact. If the surface area of the corroding metal is smaller than that of the less active metal, corrosion is rapid and severe; when the corroding metal is larger than the less active metal, corrosion is slow and superficial.

STUDY TIP

Work galvanic questions in two steps: first decide which metal corrodes (the more active one), then compare its area with the other metal's area to judge the rate.

KEY POINTS

  • Galvanic corrosion needs two dissimilar metals in contact plus an electrolyte
  • A larger difference in activity means faster corrosion
  • Small corroding area against a large less-active area gives rapid, severe attack

SOURCES

  • FAA-H-8083-30B ch08 "Galvanic Corrosion"
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MORE ON CLEANING AND CORROSION CONTROL

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.