API 653 Corrosion & Damage Mechanisms 4

Question 1

What is the mechanism of hydrogen-induced cracking (HIC) in steel storage tanks?

Accepted Answer

Atomic hydrogen diffuses into steel and collects at internal defects, forming molecular hydrogen gas that causes cracking

Answer

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?

Accepted Answer

Condensation from product vapors contacting the cooler shell surface

Answer

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?

Accepted Answer

An aggressive corrosive interface exists between the stored product and accumulated bottom water

Answer

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?

Accepted Answer

Corrosion on the upper shell and roof caused by condensation of corrosive vapors

Answer

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?

Accepted Answer

Lamellar tearing or lamination defects in the base steel are influencing the corrosion pattern

Answer

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.

(API 653) API 653 Aboveground Storage Tank Inspector Certification Practice Test

(API 653) API 653 Aboveground Storage Tank Inspector Certification Practice Test

API 653 Corrosion & Damage Mechanisms 4