API 653 Corrosion & Damage Mechanisms 2 — Questions and Answers
Question 1: Which type of corrosion is characterized by localized attack that produces small pits or holes in the metal surface of a storage tank?
- Uniform corrosion
- Pitting corrosion (Correct answer)
- Galvanic corrosion
- Crevice corrosion
Correct answer: Pitting corrosion
Pitting corrosion produces localized cavities or holes in the metal surface and is particularly dangerous because it can perforate tank walls while overall metal loss remains low.
Question 2: In API 653 context, what is the primary concern with microbiologically influenced corrosion (MIC) in aboveground storage tanks?
- It only affects the tank roof
- Sulfate-reducing bacteria can accelerate corrosion on the tank bottom (Correct answer)
- MIC only occurs above the liquid level
- It produces visible orange rust that is easy to identify
Correct answer: Sulfate-reducing bacteria can accelerate corrosion on the tank bottom
Sulfate-reducing bacteria (SRB) thrive in anaerobic conditions under sludge on tank bottoms and can accelerate corrosion significantly, making MIC a critical concern for floor integrity.
Question 3: What is the term for the preferential corrosion that occurs at the junction of two dissimilar metals in an electrolyte?
- Erosion corrosion
- Galvanic corrosion (Correct answer)
- Stress corrosion cracking
- Intergranular corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, causing the more active (anodic) metal to corrode preferentially.
Question 4: Which damage mechanism causes tank shell cracking when tensile stress combines with a corrosive environment, commonly seen in tanks storing caustic solutions?
- Hydrogen-induced cracking
- Stress corrosion cracking (Correct answer)
- Erosion corrosion
- Uniform corrosion
Correct answer: Stress corrosion cracking
Stress corrosion cracking (SCC) results from the combined action of tensile stress and a specific corrosive environment, such as caustic stress corrosion cracking in tanks storing sodium hydroxide.
Question 5: What does 'annular plate corrosion' specifically refer to in aboveground storage tank inspection?
- Corrosion on the tank roof annular ring
- Corrosion on the bottom plates adjacent to the shell (Correct answer)
- Corrosion on the floating roof pontoon annular space
- Corrosion on the shell-to-roof weld area
Correct answer: Corrosion on the bottom plates adjacent to the shell
Annular plate corrosion refers to corrosion occurring on the tank bottom plates in the area adjacent to and beneath the shell, which is critical because these plates support the entire tank structure.
Question 6: When inspecting a tank for hydrogen blistering, what is the inspector looking for on the steel surface?
- Small circular pits with smooth edges
- Raised bubbles or blisters on the steel surface (Correct answer)
- Thin white crystalline deposits
- Longitudinal cracks along weld seams
Correct answer: Raised bubbles or blisters on the steel surface
Hydrogen blistering appears as raised bubbles or blisters on the steel surface, caused by hydrogen atoms diffusing into the steel and recombining into hydrogen gas molecules at internal voids.
Question 7: Which environmental condition most accelerates external corrosion on the bottom of an aboveground storage tank?
- Dry sandy soil with low chloride content
- Wet, clay-rich soil with high electrical conductivity (Correct answer)
- Gravel pad with good drainage
- Concrete ringwall foundation
Correct answer: Wet, clay-rich soil with high electrical conductivity
Wet, clay-rich soil with high electrical conductivity creates ideal conditions for external bottom corrosion by providing moisture retention and a low-resistance electrolytic path for corrosion currents.
API 653 Corrosion & Damage Mechanisms 2
Which type of corrosion is characterized by localized attack that produces small pits or holes in the metal surface of a storage tank?