ME Corrosion and Degradation 1 — Questions and Answers
Question 1: In electrochemical corrosion, the metal that undergoes oxidation (loses electrons) is called the:
- Cathode
- Anode (Correct answer)
- Electrolyte
- Reference electrode
Correct answer: Anode
At the anode in a corrosion cell, metal atoms are oxidized (M to M2+ + 2e-), causing metal dissolution and material loss.
Question 2: Galvanic corrosion occurs when two dissimilar metals are in electrical contact in an electrolyte. The more active (less noble) metal will:
- Be protected and act as the cathode
- Corrode preferentially as the anode (Correct answer)
- Remain unaffected
- Form a protective oxide layer
Correct answer: Corrode preferentially as the anode
In a galvanic couple, the less noble metal (more negative EMF) acts as the anode and corrodes faster, while the more noble metal is cathodically protected.
Question 3: Passivation in metals like stainless steel involves the formation of:
- A thick iron oxide scale
- A thin, adherent, self-healing chromium oxide film (Correct answer)
- A zinc-rich primer coat
- A galvanic protection layer
Correct answer: A thin, adherent, self-healing chromium oxide film
Stainless steels passivate by forming a thin Cr2O3 film (2-3 nm) that acts as a barrier to further corrosion and spontaneously reforms if damaged in oxidizing environments.
Question 4: Pitting corrosion is a particularly dangerous form of localized corrosion because:
- It removes material uniformly across the surface
- It creates deep, narrow pits that are hard to detect and can initiate stress corrosion cracking (Correct answer)
- It only occurs on austenitic alloys
- It requires strongly acidic environments
Correct answer: It creates deep, narrow pits that are hard to detect and can initiate stress corrosion cracking
Pitting creates highly localized deep cavities with a large cathode-to-anode area ratio, making it self-sustaining and autocatalytic; the small pit area is easily overlooked visually.
Question 5: The corrosion protection mechanism of a zinc coating on steel (galvanizing) is primarily:
- Barrier protection only
- Cathodic (sacrificial) protection (Correct answer)
- Anodic protection
- Passivation by a zinc oxide film
Correct answer: Cathodic (sacrificial) protection
Zinc is anodic relative to steel and corrodes sacrificially, providing cathodic protection to the underlying steel even where the coating is scratched or damaged.
Question 6: Intergranular corrosion in sensitized austenitic stainless steel is caused by:
- Uniform dissolution of grain boundaries
- Chromium depletion at grain boundaries due to chromium carbide precipitation (Correct answer)
- Hydrogen embrittlement of grain boundaries
- Pitting at grain boundary intersections
Correct answer: Chromium depletion at grain boundaries due to chromium carbide precipitation
When austenitic stainless steel is heated in the sensitization range (450-850 degrees C), chromium carbides precipitate at grain boundaries, depleting adjacent regions below the 10.5% Cr threshold for passivation.
ME Corrosion and Degradation 1
In electrochemical corrosion, the metal that undergoes oxidation (loses electrons) is called the: