API 653 Corrosion Assessment 2 — Questions and Answers
Question 1: What type of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?
- Uniform corrosion
- Galvanic corrosion (Correct answer)
- Crevice corrosion
- Pitting corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when dissimilar metals are electrically coupled in an electrolyte, causing the more active (anodic) metal to corrode preferentially.
Question 2: According to API 653, what is the minimum shell thickness (t_min) formula used for corrosion assessment of existing tanks?
- t_min = 2.6D(H-1)G / SE (Correct answer)
- t_min = 4.9D(H-1)G / SE
- t_min = 2.6D(H-1)G / S
- t_min = 4.9D(H-0.3)G / SE
Correct answer: t_min = 2.6D(H-1)G / SE
API 653 uses t_min = 2.6D(H-1)G / SE for existing tanks, which is a modified formula compared to the API 650 design formula.
Question 3: Which corrosion mechanism is characterized by localized attack beneath deposits, gaskets, or at crevices where oxygen is depleted?
- Erosion corrosion
- Stress corrosion cracking
- Crevice corrosion (Correct answer)
- Intergranular corrosion
Correct answer: Crevice corrosion
Crevice corrosion occurs in confined spaces where stagnant solution becomes depleted in oxygen, creating an aggressive local chemistry.
Question 4: When performing an internal inspection of a tank bottom, which measurement technique provides the most complete corrosion mapping?
- Spot ultrasonic thickness readings
- Magnetic flux leakage (MFL) scanning (Correct answer)
- Visual inspection only
- Hammer testing
Correct answer: Magnetic flux leakage (MFL) scanning
Magnetic flux leakage (MFL) scanning provides comprehensive coverage of the tank bottom and can detect both internal and external corrosion on floor plates.
Question 5: What does a high cathodic protection current demand typically indicate when assessing tank bottom corrosion?
- The coating is in excellent condition
- There is significant coating disbondment or bare metal exposure (Correct answer)
- The tank has no external corrosion
- The anode system is oversized
Correct answer: There is significant coating disbondment or bare metal exposure
High cathodic protection current demand indicates that significant coating breakdown has occurred, exposing bare metal that requires protection.
Question 6: Which factor most significantly accelerates microbiologically influenced corrosion (MIC) in aboveground storage tanks?
- High ambient temperature
- Stagnant water accumulation at the tank bottom (Correct answer)
- High-velocity fluid flow
- Alkaline soil conditions
Correct answer: Stagnant water accumulation at the tank bottom
Stagnant water accumulation at the tank bottom creates ideal anaerobic conditions for sulfate-reducing bacteria (SRB) that drive MIC.
Question 7: What is the primary purpose of calculating the remaining corrosion allowance (RCA) during a tank inspection?
- To determine the cost of repairs
- To establish the next inspection interval and fitness for service (Correct answer)
- To select the appropriate coating system
- To determine the tank's original design pressure
Correct answer: To establish the next inspection interval and fitness for service
The remaining corrosion allowance is used to calculate how long the tank can safely operate before reaching minimum acceptable thickness, establishing the inspection interval.
API 653 Corrosion Assessment 2
What type of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?