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Insulating a magnesium joint at high service temperature

Aviation Mechanic General (AMG) practice question 12 of 31 on Cleaning and Corrosion Control. ACS element AM.I.G.K12 · 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 dissimilar metal joint involving magnesium alloy will see service temperatures above 250 °F, so vinyl tape cannot be used. What minimum number of primer coats must be applied to each surface?

  1. AThree
  2. BTwo, the same as any other dissimilar joint
  3. COne coat of zinc chromate only
Show the answer and explanation

Correct answer: A. Three

  • A — correct When tape is eliminated because of assembly interference, relative motion, or service temperatures above 250 °F, the handbook requires extra coats of primer, a minimum of three.
  • B — incorrect Two coats is the normal minimum when the 0.003 inch vinyl tape layer is also applied; when tape is omitted, the minimum rises to three coats.
  • C — incorrect A single coat is never sufficient; even the standard treatment calls for at least two coats of zinc chromate or epoxy primer.

BACKGROUND

Contact of different bare metals creates an electrolytic action when moisture is present, and salt water accelerates it. Separating materials such as primer, aluminum tape, washers, grease, or sealant are used as barriers. For joints not involving magnesium, a minimum of two coats of zinc chromate or, preferably, epoxy primer is applied, air dried 6 hours between coats. For joints involving magnesium, each surface gets at least two coats of primer followed by a 0.003 inch pressure-sensitive vinyl tape applied without stretching. When tape thickness interferes with assembly, when relative motion exists, or when service temperatures above 250 °F are anticipated, the tape is eliminated and a minimum of three coats of primer is applied instead.

STUDY TIP

Keep the two magnesium-joint numbers together: 0.003 inch tape at normal temperatures, three coats of primer instead of tape above 250 °F.

KEY POINTS

  • Dissimilar joints need a protective separator (primer, tape, washers, grease, or sealant)
  • Magnesium joints: two primer coats plus 0.003 inch vinyl tape
  • Above 250 °F or with relative motion, replace tape with at least three primer coats

SOURCES

  • FAA-H-8083-30B ch08 "Contacts Involving Magnesium"
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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.