Corrosion and Degradation Flashcards
6 cards from real ME practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Corrosion and Degradation flashcards as text
Stress corrosion cracking (SCC) requires the simultaneous presence of:
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.
Hydrogen embrittlement in high-strength steels is caused by:
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.
Cavitation erosion occurs in hydraulic components when:
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.
The Evans diagram (mixed potential theory) predicts the corrosion potential and corrosion current by finding:
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.
Crevice corrosion occurs in tight gaps (under fasteners, gaskets) because:
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.
The Pilling-Bedworth (PB) ratio predicts a protective high-temperature oxide scale when:
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.