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Removing residual flux after soldering

Aviation Mechanic General (AMG) practice question 34 of 36 on Aircraft Materials, Hardware, and Processes. ACS element AM.I.E.K7 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy must residual flux be removed from the metal surface immediately after a soldered joint is completed?

  1. AFlux residues absorb moisture and cause severe pitting.
  2. BFlux residues bond chemically with the solder alloy.
  3. CFlux residues attack the metal only at high temperature.
Show the answer and explanation

Correct answer: A. Flux residues absorb moisture and cause severe pitting.

  • A — correct Flux residues are hygroscopic in nature, absorbing moisture, and unless carefully removed they tend to cause severe pitting of the metal.
  • B — incorrect The concern with leftover flux is corrosion of the base metal, not any bonding between the residue and the solder itself.
  • C — incorrect Flux residues attack the metal by absorbing moisture at ordinary conditions; the damage is not limited to elevated temperatures.

BACKGROUND

Corrosion can be started by foreign material left on a metal surface. Direct chemical attack, also called pure chemical corrosion, results from exposure of a bare surface to caustic liquid or gaseous agents, and one of the most common agents on aircraft is residual flux deposits left by inadequately cleaned welded, brazed, or soldered joints. Many types of flux used in brazing, soldering, and welding are corrosive and chemically attack the metals or alloys they are used with. Because flux residues are hygroscopic, they draw moisture out of the air and keep the attack going long after the joint has cooled. For that reason the residue is removed from the surface immediately after the joining operation.

STUDY TIP

Treat flux cleanup as part of the soldering job itself, not as housekeeping done later. Remember the word hygroscopic and the damage it produces: severe pitting.

KEY POINTS

  • Many brazing, soldering, and welding fluxes are corrosive to the metals they are used on.
  • Residual flux is removed from the surface immediately after the joining operation.
  • Flux residues absorb moisture and, if left in place, tend to cause severe pitting.

SOURCES

  • FAA-H-8083-30B ch08 "Corrosion — Direct Chemical Attack"
  • C:/Users/kuroh/faa-drill/sources/amg-general/ch08.txt (Direct chemical attack / flux residues)
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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.