ME Corrosion and Degradation 2 — Questions and Answers
Question 1: Stress corrosion cracking (SCC) requires the simultaneous presence of:
- High temperature, high pressure, and fatigue loading
- A susceptible material, a specific corrosive environment, and tensile stress (Correct answer)
- Impact loading and an acid environment
- Compressive stress and neutral pH electrolyte
Correct answer: A susceptible material, a specific corrosive environment, and tensile stress
SCC occurs only when all three conditions are simultaneously met: a susceptible alloy, a specific corrosive environment (e.g., chlorides for austenitic stainless steel), and sustained tensile stress.
Question 2: Hydrogen embrittlement in high-strength steels is caused by:
- Hydrogen oxidizing the steel surface
- Atomic hydrogen diffusing into the lattice and reducing ductility and toughness (Correct answer)
- Hydrogen gas bubbles forming at grain boundaries
- Reaction with atmospheric moisture
Correct answer: Atomic hydrogen diffusing into the lattice and reducing ductility and toughness
Atomic hydrogen absorbed from electroplating, welding, or corrosion diffuses interstitially into steel, accumulating at stress concentrations and causing premature brittle fracture.
Question 3: Cavitation erosion occurs in hydraulic components when:
- High velocity particles impact the surface
- Vapor bubbles form and collapse violently near the surface, causing pitting (Correct answer)
- Corrosive fluids slowly dissolve the surface
- Fretting wear occurs at contact surfaces
Correct answer: Vapor bubbles form and collapse violently near the surface, causing pitting
Cavitation creates vapor bubbles in low-pressure zones; when they collapse near a metal surface, micro-jets impact the surface with enormous pressure, causing pitting and material loss.
Question 4: The Evans diagram (mixed potential theory) predicts the corrosion potential and corrosion current by finding:
- The maximum oxidation potential only
- The intersection of the anodic and cathodic polarization curves (Correct answer)
- The pH at which corrosion ceases
- The minimum electrolyte conductivity for corrosion
Correct answer: The intersection of the anodic and cathodic polarization curves
At the mixed potential (corrosion potential), the total anodic oxidation current equals the total cathodic reduction current; this intersection on the Evans diagram gives the corrosion potential and rate.
Question 5: Crevice corrosion occurs in tight gaps (under fasteners, gaskets) because:
- The crevice is exposed to higher oxygen levels
- Oxygen depletion inside the crevice creates an anodic region relative to the outside surface (Correct answer)
- The crevice has lower temperature
- Higher pH inside the crevice dissolves the passive film
Correct answer: Oxygen depletion inside the crevice creates an anodic region relative to the outside surface
Inside a crevice, oxygen is consumed and not replenished, creating a differential aeration cell where the oxygen-depleted interior becomes anodic and actively corrodes.
Question 6: The Pilling-Bedworth (PB) ratio predicts a protective high-temperature oxide scale when:
- PB ratio is less than 1 (porous, non-protective scale)
- PB ratio is between 1 and 2 (compact, adherent, protective scale) (Correct answer)
- PB ratio is greater than 2 (compressive buckling, non-protective)
- PB ratio equals 0
Correct answer: PB ratio is between 1 and 2 (compact, adherent, protective scale)
A PB ratio between 1 and 2 means the oxide volume exceeds the metal volume consumed, producing a compact, adherent, and protective scale; values outside this range produce non-protective scales.
ME Corrosion and Degradation 2
Stress corrosion cracking (SCC) requires the simultaneous presence of: