API 653 Corrosion Assessment 5 — Questions and Answers
Question 1: What is the primary advantage of using phased array ultrasonic testing (PAUT) over conventional UT for corrosion mapping in storage tanks?
- Lower equipment cost
- Faster scanning with detailed cross-sectional imaging and better defect characterization (Correct answer)
- No need for couplant
- Ability to test through insulation without removal
Correct answer: Faster scanning with detailed cross-sectional imaging and better defect characterization
PAUT uses multiple elements to electronically steer and focus the beam, enabling faster scanning and superior imaging compared to single-element conventional UT.
Question 2: What is 'cathodic disbondment' and how does it affect corrosion protection of tank exteriors?
- The physical removal of anodes from the tank structure
- The detachment of protective coatings from the metal surface due to hydrogen generation at cathodic areas (Correct answer)
- The dissolution of protective cathodic protection anodes
- The reduction of cathodic protection current output over time
Correct answer: The detachment of protective coatings from the metal surface due to hydrogen generation at cathodic areas
Cathodic disbondment occurs when hydrogen generated at cathodic surfaces under the coating weakens the adhesive bond, causing the coating to detach and lose its protective function.
Question 3: According to API 653, what is the maximum inspection interval for an in-service external inspection of an aboveground storage tank?
- 1 year
- 5 years (Correct answer)
- 10 years
- 15 years
Correct answer: 5 years
API 653 requires external inspections at a maximum interval of 5 years, though risk-based assessment may allow modification of this interval.
Question 4: Which factor is the most reliable indicator of the external corrosion rate for the underside of a tank bottom plate?
- Aboveground atmospheric corrosion rate at the site
- Soil resistivity measurements and cathodic protection survey results (Correct answer)
- The corrosion rate of the tank shell
- The age of the tank since last painting
Correct answer: Soil resistivity measurements and cathodic protection survey results
Soil resistivity and CP survey data (pipe-to-soil potentials, current demand) directly characterize the corrosivity of the environment contacting the tank bottom underside.
Question 5: What does 'general corrosion' (uniform corrosion) mean in the context of API 653 tank assessment?
- Corrosion that affects the entire tank including appurtenances
- Metal loss that occurs at a relatively consistent rate over a broad area of the surface (Correct answer)
- Corrosion that is visible to the naked eye without instruments
- Any form of corrosion detected during a general inspection
Correct answer: Metal loss that occurs at a relatively consistent rate over a broad area of the surface
General or uniform corrosion refers to corrosion distributed relatively evenly across the metal surface, resulting in a consistent reduction in wall thickness.
Question 6: When evaluating a corroded tank bottom annular plate, API 653 specifies what minimum remaining thickness requirement?
- 50% of the nominal thickness
- 0.05 inches regardless of original thickness
- The thickness required to meet the shell-to-bottom joint structural integrity requirements (Correct answer)
- 0.100 inches for all tank sizes
Correct answer: The thickness required to meet the shell-to-bottom joint structural integrity requirements
The annular plate must retain sufficient thickness to maintain the structural integrity of the shell-to-bottom joint, which is the governing requirement per API 653.
Question 7: What is the term for corrosion that occurs on the inner surface of a storage tank roof due to condensation of water vapor from the stored product?
- External atmospheric corrosion
- Vapor space corrosion (Correct answer)
- Underdeposit corrosion
- Erosion corrosion
Correct answer: Vapor space corrosion
Vapor space corrosion occurs when water and corrosive vapors condense on the inner roof surface, causing corrosion that can be accelerated by H2S, CO2, or oxygen in the vapor phase.
API 653 Corrosion Assessment 5
What is the primary advantage of using phased array ultrasonic testing (PAUT) over conventional UT for corrosion mapping in storage tanks?