API 653 Corrosion & Damage Mechanisms 4 — Questions and Answers
Question 1: What is the mechanism of hydrogen-induced cracking (HIC) in steel storage tanks?
- Hydrogen from water vapor reacts with carbon in steel at high temperatures
- Atomic hydrogen diffuses into steel and collects at internal defects, forming molecular hydrogen gas that causes cracking (Correct answer)
- Hydrogen embrittles the steel surface, causing fatigue cracking
- Hydrogen from cathodic protection systems attacks the weld metal
Correct answer: Atomic hydrogen diffuses into steel and collects at internal defects, forming molecular hydrogen gas that causes cracking
HIC occurs when atomic hydrogen diffuses into the steel matrix and recombines as molecular hydrogen at internal inclusions or laminations, building sufficient pressure to initiate and propagate internal cracks.
Question 2: Which condition in a floating roof tank most commonly leads to corrosion on the internal tank shell above the liquid level?
- Condensation from product vapors contacting the cooler shell surface (Correct answer)
- Cathodic protection current insufficiency above the liquid line
- UV radiation breaking down the primer coat
- Thermal cycling causing paint delamination
Correct answer: Condensation from product vapors contacting the cooler shell surface
Above the liquid level, product vapors condense on the cooler tank shell surface, creating a thin corrosive film that can cause significant localized corrosion in the vapor space zone.
Question 3: In API 653 tank inspections, what does a high corrosion rate in the 'water bottoms' zone of a product storage tank indicate?
- The cathodic protection system is overprotecting the tank
- An aggressive corrosive interface exists between the stored product and accumulated bottom water (Correct answer)
- The tank coating has failed in the lower course only
- The product is causing top-of-line corrosion
Correct answer: An aggressive corrosive interface exists between the stored product and accumulated bottom water
The water bottoms zone, where product meets accumulated water, creates a highly aggressive corrosive environment due to dissolved oxygen, bacteria, chlorides, and other contaminants concentrated in the water layer.
Question 4: What is 'top-of-line corrosion' (TLC) in the context of storage tanks with vapor spaces?
- Corrosion at the highest elevation point on the tank shell exterior
- Corrosion on the upper shell and roof caused by condensation of corrosive vapors (Correct answer)
- Corrosion on the top course from rainwater ingress
- Corrosion from ultraviolet radiation on uncoated roof surfaces
Correct answer: Corrosion on the upper shell and roof caused by condensation of corrosive vapors
Top-of-line corrosion occurs when water and corrosive compounds in vapors condense on cooler upper shell and roof surfaces, creating an aggressive thin-film corrosive environment.
Question 5: During tank inspection, you discover the shell plates have elongated corrosion patterns aligned with the direction of the steel rolling. What does this suggest?
- The corrosion is from external environmental exposure
- Lamellar tearing or lamination defects in the base steel are influencing the corrosion pattern (Correct answer)
- The tank is experiencing flow-induced erosion
- Galvanic corrosion is occurring between the shell and an adjacent structure
Correct answer: Lamellar tearing or lamination defects in the base steel are influencing the corrosion pattern
Elongated corrosion aligned with the steel rolling direction suggests laminations or lamellar defects in the base metal are creating preferential corrosion paths along the plate's rolling direction.
Question 6: What is the corrosion mechanism known as 'amine stress corrosion cracking' relevant to in API 653 inspections?
- It affects tanks storing refined petroleum products at low temperatures
- It affects carbon steel tanks storing amine solutions used in gas treating or water treatment (Correct answer)
- It only affects stainless steel tanks
- It is caused by external exposure to industrial amine compounds
Correct answer: It affects carbon steel tanks storing amine solutions used in gas treating or water treatment
Amine stress corrosion cracking occurs in carbon steel equipment, including storage tanks, that contain amine solutions (such as MEA, DEA) under residual or applied tensile stress.
Question 7: What is the significance of the 'corrosion allowance' specified in API 653 for determining a tank's remaining service life?
- It is the thickness added to the design minimum to account for future corrosion before the next required inspection (Correct answer)
- It represents the total metal thickness lost since the tank was built
- It is the minimum acceptable thickness for structural adequacy
- It defines the maximum acceptable pit depth on the tank shell
Correct answer: It is the thickness added to the design minimum to account for future corrosion before the next required inspection
The corrosion allowance is extra thickness beyond the structural minimum, and remaining life is calculated by dividing the remaining corrosion allowance by the measured corrosion rate.
API 653 Corrosion & Damage Mechanisms 4
What is the mechanism of hydrogen-induced cracking (HIC) in steel storage tanks?