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.