API 571 Corrosion Mechanisms 1 — Questions and Answers
Question 1: What is the most common form of corrosion found in refinery equipment?
- Uniform corrosion (Correct answer)
- Pitting corrosion
- Crevice corrosion
- Galvanic corrosion
Correct answer: Uniform corrosion
Uniform corrosion is the most prevalent form in refineries, causing consistent metal loss across an entire surface.
Question 2: Which type of corrosion occurs at the interface between two dissimilar metals in a conductive electrolyte?
- Erosion corrosion
- Galvanic corrosion (Correct answer)
- Crevice corrosion
- Intergranular corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when two dissimilar metals are electrically connected in a corrosive electrolyte, with the more active metal becoming the anode.
Question 3: What environment best promotes crevice corrosion in process equipment?
- Open, high-flow systems
- High-temperature dry gas streams
- Stagnant or low-flow areas with restricted oxygen access (Correct answer)
- Strongly alkaline solutions
Correct answer: Stagnant or low-flow areas with restricted oxygen access
Crevice corrosion thrives in tight, restricted spaces where stagnant solution creates localized oxygen depletion and anodic conditions.
Question 4: Pitting corrosion is characterized by which type of attack?
- Uniform metal loss across the surface
- Localized attack forming small, deep cavities (Correct answer)
- Grain boundary dissolution
- General surface thinning
Correct answer: Localized attack forming small, deep cavities
Pitting corrosion creates small, deep cavities that concentrate at local anodic sites and is particularly damaging due to its localized, hard-to-detect nature.
Question 5: Which material is most susceptible to intergranular corrosion after sensitization in the 800–1500°F range?
- Carbon steel
- Aluminum alloys
- 304 austenitic stainless steel (Correct answer)
- Copper alloys
Correct answer: 304 austenitic stainless steel
Austenitic stainless steels like 304 precipitate chromium carbides at grain boundaries when heated in the sensitization range, depleting chromium and creating attack susceptibility.
Question 6: Erosion-corrosion is primarily accelerated by which condition?
- Low fluid velocity and laminar flow
- High fluid velocity with entrained particulates (Correct answer)
- Static stagnant conditions
- Cathodic protection currents
Correct answer: High fluid velocity with entrained particulates
High-velocity flow with suspended solids physically removes protective oxide films, exposing fresh metal to continuous corrosive attack.
API 571 Corrosion Mechanisms 1
What is the most common form of corrosion found in refinery equipment?