What two conditions must be present for galvanic corrosion to occur between two pieces of metal?
ACS code: AM.I.G
Correct answer: Dissimilar metals in contact and the presence of an electrolyte
Rationale: Galvanic corrosion is an electrochemical action that requires two dissimilar metals in electrical contact and an electrolyte (such as moisture) to complete the circuit. The greater the difference in electrical potential between the two metals on the galvanic series, the faster the more active (anodic) metal corrodes. Identical metals have no potential difference, and without an electrolyte the corrosion cell cannot form.
When two dissimilar metals form a galvanic couple in the presence of an electrolyte, which metal corrodes?
ACS code: AM.I.G
Correct answer: The more anodic (more active) metal
Rationale: In a galvanic couple the more active metal becomes the anode and corrodes, while the less active metal becomes the cathode and is protected. This is why coupling a small area of anodic metal to a large cathodic area produces rapid, severe attack on the anode. Identifying the anodic member of the pair on the galvanic series predicts which part will deteriorate.
Corrosion that attacks along the grain boundaries of an alloy, often from improper heat treatment, is called
ACS code: AM.I.G
Correct answer: intergranular corrosion
Rationale: Intergranular corrosion is an attack along the grain boundaries of an alloy, frequently the result of a lack of uniformity in the alloy structure caused by improper or interrupted heat treatment. Because it follows the boundaries between grains, it can progress deep into the metal with little visible surface evidence, making it especially dangerous.
The corrosion that occurs between two close-fitting surfaces subject to slight vibration or relative motion is known as
ACS code: AM.I.G
Correct answer: fretting corrosion
Rationale: Fretting corrosion occurs between two mating surfaces normally at rest with respect to each other that are subjected to slight relative motion or vibration. The motion prevents protective films from re-forming and traps abrasive corrosion products, producing pits and a discolored, mottled texture at the contact area.
A thread-like or worm-track corrosion found under painted or coated surfaces in humid conditions is called
ACS code: AM.I.G
Correct answer: filiform corrosion
Rationale: Filiform corrosion is a distinctive thread-like or wormtrack attack that occurs under organic coatings (paint or film) when the relative humidity is high and the surface is slightly acidic. It is recognized by the random thread-like filaments visible beneath the finish and indicates the coating has been undercut.
On a bare magnesium part, the corrosion products that form are typically
ACS code: AM.I.G
Correct answer: a white or gray powder
Rationale: Magnesium is the most chemically active metal used in aircraft, and its corrosion products appear as a white or gray powdery deposit. Reddish-brown scale identifies ferrous (iron and steel) corrosion, while blue-green deposits are characteristic of copper-based alloys, so the color of the corrosion product helps identify the base metal involved.
Which corrosion product is characteristic of corroding iron and steel parts?
ACS code: AM.I.G
Correct answer: A reddish-brown rust or scale
Rationale: Iron and steel form a characteristic reddish-brown oxide commonly known as rust. Unlike the protective oxide on aluminum, this scale is porous and does not stop further attack; it flakes away and exposes fresh metal. White powder indicates aluminum or magnesium, and blue-green film indicates copper alloys.
Alclad aluminum resists corrosion better than bare alloy sheet because it has
ACS code: AM.I.G
Correct answer: a thin coating of pure aluminum rolled onto each surface
Rationale: Alclad sheet consists of a high-strength aluminum alloy core with a thin coating of comparatively pure aluminum metallurgically bonded to each surface. The pure aluminum cladding forms a protective oxide and is anodic to the core, sacrificially protecting it. This is a metallurgical coating, not a primer or a chemically deposited film.
Alodine (a chemical conversion coating) is applied to aluminum primarily to
ACS code: AM.I.G
Correct answer: increase corrosion resistance and improve paint adhesion
Rationale: Alodizing is a simple chemical (conversion) treatment for aluminum alloys that forms a protective film to increase corrosion resistance and provides a good bonding surface for paint. It does not appreciably harden the metal for wear resistance, and it does not add structural thickness or fill pitting—it is a thin conversion coating.
What is the proper action when corrosion is found on a stressed structural member or its depth exceeds allowable limits?
ACS code: AM.I.G
Correct answer: Refer to the manufacturer's data to repair or replace the part
Rationale: When corrosion penetrates a structural member beyond the limits established by the manufacturer, simply cleaning or coating the area is not acceptable because load-carrying capability has been reduced. The mechanic must consult the manufacturer's maintenance or structural repair manual to determine whether the part can be reworked within limits or must be replaced. Painting over corrosion seals in the damage and is never acceptable.
When bare aluminum is exposed to air, the oxide film that forms is
ACS code: AM.I.G
Correct answer: an electrical insulator that protects the metal beneath.
Rationale: Aluminum reacts with air to form a thin, tough, nonconductive oxide film. This layer is an electrical insulator, which is why aluminum bonding and grounding surfaces must be cleaned back to bright metal. The oxide actually protects the metal beneath, so the part does not become weaker.
Besides improving corrosion resistance, anodizing an aluminum surface also
ACS code: AM.I.G
Correct answer: provides a good surface for paint to adhere to.
Rationale: Anodizing thickens the natural oxide into a hard, slightly porous film. Besides improving corrosion protection, that controlled porosity provides an excellent key for paint and primer to adhere to. A fully sealed, nonporous surface would actually be a poorer base for paint.
If caustic soda (sodium hydroxide) leaves a black stain on a metal surface, the metal is most likely
ACS code: AM.I.G
Correct answer: an aluminum alloy.
Rationale: Caustic soda (sodium hydroxide) attacks the alloying elements in aluminum alloy and leaves a dark or black smut on the surface. Pure aluminum and the pure-aluminum clad layer of Alclad react much less and stay light, so a black stain identifies the material as an aluminum alloy.
When diluting a concentrated acid with water, the correct and safe procedure is to
ACS code: AM.I.G
Correct answer: add the acid slowly to the water.
Rationale: Always add the acid slowly to the water while stirring. Dilution releases considerable heat, and the large volume of water absorbs it safely; pouring water onto concentrated acid causes a localized boiling reaction that can spatter corrosive acid out of the container.
A suitable solvent for degreasing aluminum alloy parts is
ACS code: AM.I.G
Correct answer: mineral spirits or naphtha.
Rationale: Aluminum alloys are degreased with petroleum-based solvents such as mineral spirits and naphtha, which dissolve oils and grease without attacking the metal. Dilute sulfuric acid would corrode the alloy and is an etchant rather than a degreaser, so it is not a safe substitute, per FAA-H-8083-30 cleaning practice.
Aircraft transparent plastic (acrylic) windows and windshields should be cleaned with
ACS code: AM.I.G
Correct answer: warm soapy water, then rinsed and dried.
Rationale: Acrylic transparencies scratch and craze easily, so FAA-H-8083-30 directs cleaning with plenty of warm soapy water and a soft cloth, then rinsing and drying, to float off grit without abrasion or chemical attack. Solvents and dry rubbing with leather risk crazing or scratching, so the warm soapy water method is correct.
Abrasive blasting media that were used on an aluminum component and are then re-used on a steel component will
ACS code: AM.I.G
Correct answer: cause corrosion of the steel component.
Rationale: Blasting media used on aluminum pick up aluminum particles; if those same media are then used on a steel part, they embed dissimilar-metal residue that sets up galvanic action and corrosion of the steel. To avoid this contamination, media used on aluminum must not be re-used on steel components.
Intergranular corrosion is generally caused by
ACS code: AM.I.G
Correct answer: improper heat treatment of the metal.
Rationale: Intergranular corrosion is selective attack along the grain boundaries of an alloy. It typically results from improper heat treatment, which leaves the grain-boundary regions chemically different and anodic relative to the grain interiors. Dissimilar-metal contact causes galvanic corrosion instead, and improper assembly is unrelated to this grain-boundary mechanism.
Corrosion caused by electrolytic (galvanic) action is the result of
ACS code: AM.I.G
Correct answer: contact between two unlike metals.
Rationale: Electrolytic (galvanic) corrosion requires an anode, a cathode, and an electrolyte. Joining two unlike metals creates a potential difference that drives current through any moisture present, so the more anodic metal corrodes. Anodizing is a protective film and a wrong quenchant affects heat-treatment properties — neither sets up the galvanic cell described here.
Corrosion may best be described as the destruction of metal by
ACS code: AM.I.G
Correct answer: electrochemical action.
Rationale: Corrosion is fundamentally the electrochemical destruction of a metal, in which the metal gives up electrons (oxidizes) at anodic sites while a reduction reaction occurs at cathodic sites through an electrolyte. The other terms are not recognized corrosion mechanisms; corrosion is an electrochemical, not a mechanical or hydroelectric, process.
Exfoliation corrosion is sometimes also referred to as
ACS code: AM.I.G
Correct answer: layer corrosion.
Rationale: Exfoliation is a form of intergranular corrosion that progresses along planes parallel to the surface, forcing the metal apart into leaf-like layers — hence it is also called layer corrosion. Filiform is a thread-like attack under coatings and subsurface corrosion is a more general term, so neither is the recognized alternative name for exfoliation.
When dissimilar metals are joined in the presence of an electrolyte, one metal forms the anode and the other the cathode. Which one will suffer from corrosion?
ACS code: AM.I.G
Correct answer: The anode.
Rationale: In any corrosion cell the anode is the electrode that loses electrons and dissolves, so it is the part that corrodes. The cathode is protected by the current flow. When dissimilar metals are coupled, the more active (anodic) member sacrifices itself while the cathodic member is preserved.
Which of the following is a common cause of corrosion on aircraft structure?
ACS code: AM.I.G
Correct answer: Spilled battery acid.
Rationale: Spilled battery electrolyte is a classic direct cause of corrosion because the strong acid attacks metal and surrounding structure aggressively and immediately. Untreated metal and water in fuel may eventually contribute to corrosion, but spilled battery acid is the recognized common, active cause requiring prompt neutralizing and cleanup.
Stress corrosion is best described as corrosion occurring in an area that is under
ACS code: AM.I.G
Correct answer: continuous (sustained) loading.
Rationale: Stress corrosion is the combined action of a sustained (continuous, static) tensile stress and a corrosive environment, which together produce cracking that neither would cause alone. Cyclic loading with corrosion is corrosion fatigue, and rubbing between mating surfaces is fretting — so the distinguishing feature here is the continuous, not cyclic, load.
An electrochemical corrosion reaction on an airframe is made possible by the contact of
ACS code: AM.I.G
Correct answer: moisture on the surface of the metal.
Rationale: An electrochemical corrosion reaction needs an electrolyte to carry current between anodic and cathodic sites; on an airframe this is supplied by moisture on the metal surface. Dissimilar metals or a bond merely set up or control the potential difference, but without surface moisture acting as the electrolyte the reaction cannot proceed.
Galvanic corrosion is caused by
ACS code: AM.I.G
Correct answer: joining two dissimilar metals.
Rationale: Galvanic corrosion arises when two dissimilar metals are joined in the presence of an electrolyte, the potential difference driving the more anodic metal to corrode. Incorrect heat treatment causes intergranular attack, and cyclic stress with section loss relates to fatigue, so the defining cause of galvanic corrosion is the dissimilar-metal couple.
In the galvanic series, when the most noble metal is joined to a less noble metal, the noble metal will
ACS code: AM.I.G
Correct answer: allow the less noble metal to corrode first.
Rationale: The most noble metal is cathodic and corrosion-resistant, so when coupled it stays protected and forces the less noble (anodic) metal to corrode preferentially. It does not corrode before the less noble metal; the key effect is that the less noble partner sacrifices itself first.
Stress corrosion cracking is associated with
ACS code: AM.I.G
Correct answer: a corrosion pit in a member under sustained tensile stress.
Rationale: Stress corrosion needs a sustained tensile stress acting together with a corrosive condition; a corrosion pit in a stressed member provides the stress raiser from which cracking initiates and grows. A pit under a purely compressive load will not propagate a stress-corrosion crack, and adding cyclic loading describes corrosion fatigue instead.
Where on a poorly fabricated structural skin repair is stress-corrosion cracking most likely to begin?
ACS code: AM.I.G
Correct answer: Radiating from corners or sharp edges where stress is concentrated.
Rationale: Stress-corrosion cracking initiates where sustained tensile stress is highest. A poorly made repair concentrates stress at corners and sharp edges, so cracks start and radiate outward from those stress raisers. Good repairs use smooth, generous radii precisely to avoid these high-stress points.
A metal that is described as inherently stable (noble) will tend to
ACS code: AM.I.G
Correct answer: Strongly resist corrosion in normal environments.
Rationale: An inherently stable, noble metal is reluctant to react with its environment and therefore strongly resists corrosion. Unstable metals readily revert toward their natural oxide or ore state and are corrosion-prone, which is the opposite condition.
The natural oxide film that forms on the surface of aluminum is best described as
ACS code: AM.I.G
Correct answer: Tightly adherent and non-porous, sealing the surface.
Rationale: Aluminum's corrosion resistance comes from a thin, tightly adherent, non-porous oxide film that forms naturally on its surface. Because the film is non-porous it seals the underlying metal from further attack; a porous film would let the electrolyte through and would not be self-protecting.
Effective corrosion control of an aircraft structure most properly begins at the
ACS code: AM.I.G
Correct answer: Original design stage, before manufacture begins.
Rationale: Corrosion control begins at the design stage, where material selection, dissimilar-metal isolation, drainage, sealing, and accessibility for inspection are built in. Decisions made later can only work within the design already chosen, so preventing corrosion is most effective when addressed in the original design.
In a galvanic corrosion cell formed by two dissimilar metals, which metal corrodes?
ACS code: AM.I.G
Correct answer: The anode, which is the more active (less noble) metal.
Rationale: In a galvanic couple the anode — the more active, less noble metal — gives up electrons and corrodes, while the cathode is protected. The anode is therefore always the part that corrodes in a dissimilar-metal cell.
Galvanic corrosion is the direct result of
ACS code: AM.I.G
Correct answer: Contact between two dissimilar metals in the presence of an electrolyte.
Rationale: Galvanic corrosion is set up by contact between two unlike (dissimilar) metals in the presence of an electrolyte; the resulting potential difference drives the anodic metal to corrode. Surface-treatment faults do not create the dissimilar-metal cell that defines galvanic corrosion.
What are the characteristic signs of fretting corrosion at a tight, slightly moving joint?
ACS code: AM.I.G
Correct answer: Fine black powder on aluminum or reddish-brown staining on steel.
Rationale: Fretting corrosion occurs at tight mating surfaces with slight relative movement and produces fine oxidized debris — a black powder on aluminum or a reddish-brown (cocoa-colored) staining on steel. Intergranular cracking and flaking are signs of other corrosion forms.
Black streaks tracking back from a riveted joint most likely indicate
ACS code: AM.I.G
Correct answer: Fretting corrosion at the working joint.
Rationale: Black streaks tracking back from a rivet hole are oxidized debris worked out from between two surfaces micro-moving against each other under load — the classic signature of fretting corrosion. Galvanic and intergranular corrosion produce different indications.
After a steel part is welded, the weld region may corrode preferentially because
ACS code: AM.I.G
Correct answer: The heat-affected zone becomes anodic to the surrounding metal.
Rationale: Welding heats a band of the parent metal and changes its metallurgical condition, leaving the heat-affected zone anodic relative to the surrounding unaffected metal. That potential difference makes the weld region corrode preferentially. Spatter and paint loss are secondary effects.
Intergranular corrosion is particularly hazardous because it
ACS code: AM.I.G
Correct answer: May progress extensively with little or no visible surface indication.
Rationale: Intergranular corrosion attacks along internal grain boundaries and can progress extensively with little or no sign on the surface, making it dangerous and hard to detect. Visible gray or white powder and flaking are characteristic of surface or exfoliation corrosion, not the hidden intergranular form.
Jointing (sealing) compound applied between mating structural surfaces is used to
ACS code: AM.I.G
Correct answer: Prevent dissimilar-metal (galvanic) corrosion at the interface.
Rationale: Jointing compound is applied between mating surfaces to seal out moisture and electrically separate dissimilar metals, breaking the galvanic cell that would otherwise drive corrosion at the joint. It is not a structural adhesive and is not intended to ease disassembly.
Galvanic corrosion from dissimilar-metal contact is best prevented by
ACS code: AM.I.G
Correct answer: Placing a non-porous dielectric material between the surfaces.
Rationale: Galvanic corrosion between dissimilar metals is best prevented by inserting a non-porous, non-conducting (dielectric) barrier between them, which interrupts the electrical path and keeps electrolyte out so no corrosion cell can form. Bonding connects the metals electrically, and primer alone is not a reliable insulating separation.
When certain stainless steels are welded, the sensitized zone near the weld becomes susceptible to a form of corrosion known as
ACS code: AM.I.G
Correct answer: Weld decay.
Rationale: When some stainless steels are welded, chromium near the weld combines with carbon to form chromium carbides at the grain boundaries, locally depleting the chromium that gives corrosion resistance. The sensitized zone then corrodes intergranularly — a condition known as weld decay.
Corrosion that lifts and flakes the metal surface as corrosion products force grain layers apart parallel to the surface is called
ACS code: AM.I.G
Correct answer: Exfoliation corrosion.
Rationale: Exfoliation corrosion progresses along grain boundaries lying parallel to the surface; the bulky corrosion products force the layers apart, lifting and flaking the metal like the leaves of a book. Electrolysis is the electrochemical process itself, and 'transgranulation' is not a corrosion term.
On aluminum alloy, the typical indication of fretting corrosion is
ACS code: AM.I.G
Correct answer: A fine black powder at the working interface.
Rationale: Fretting corrosion on aluminum alloy produces fine, dark oxidized debris that appears as a black powder at the working interface. On steel the equivalent debris is reddish-brown, so white or brown powder points to other corrosion forms.
What action protects integral fuel tanks from corrosion caused by microbiological growth?
ACS code: AM.I.G
Correct answer: A biocidal additive is used in the fuel.
Rationale: Microbiological growth in integral fuel tanks lives at the fuel/water interface, and its by-products cause corrosion of the tank structure. The accepted protection is to add an approved biocidal additive to the fuel that kills the organisms. Chromate coatings and liners address other concerns and do not control the microbial growth that drives this corrosion.
What type of corrosion attacks the grain boundaries of aluminum alloys that have been improperly or inadequately heat treated?
ACS code: AM.I.G
Correct answer: Intergranular corrosion.
Rationale: Improper or inadequate heat treatment leaves the grain boundaries in an electrochemically more anodic state, so corrosion attacks preferentially along those boundaries — the definition of intergranular corrosion. Stress corrosion additionally requires sustained tensile stress, and surface corrosion is general rather than grain-boundary selective.
The artificial production of an oxide film on the surface of aluminum or its alloys by an electrolytic process is commonly called
ACS code: AM.I.G
Correct answer: anodizing.
Rationale: Deliberately growing a protective oxide film on aluminum or its alloys by an electrolytic process is called anodizing. Alodizing (chromate conversion coating) is a chemical process and parco-lubrizing is a phosphate process for steel, so the artificially produced electrolytic oxide film is anodizing.
Intergranular corrosion in structural aluminum alloy parts
ACS code: AM.I.G
Correct answer: cannot always be detected by surface indications.
Rationale: Because intergranular corrosion runs along internal grain boundaries, it can be well advanced before any sign reaches the surface, so it cannot always be detected by surface indications. It is not confined to non-heat-treated material, and a white powdery deposit is typical of surface corrosion rather than the hidden intergranular form.
Corrosion will generally spread more rapidly when metals are exposed to
ACS code: AM.I.G
Correct answer: high temperatures.
Rationale: Corrosion is an electrochemical reaction whose rate increases with temperature, and warm conditions usually bring the moisture needed to act as an electrolyte. Dry and cold climates both slow the reaction and reduce available moisture, so corrosion spreads most rapidly at high temperatures.
Magnesium alloy components are generally protected against corrosion by
ACS code: AM.I.G
Correct answer: a chemical conversion process.
Rationale: Magnesium alloys are highly anodic and are protected by a chemical conversion treatment such as chromating, which forms a corrosion-resistant film and a good base for paint. An electrolytic deposit or paint alone is not the primary protection — the basic protection is the chemical conversion coating.
When it is necessary to remove corrosion from a steel component in place, a suitable removing solution is based on
ACS code: AM.I.G
Correct answer: phosphoric acid.
Rationale: For removing corrosion from a steel component in situ, a phosphoric-acid-based solution is used because it dissolves the rust and leaves a passivating phosphate film that resists further attack. Chromic and nitric acid treatments are associated with other metals and processes, so phosphoric acid is the suitable base for steel.