API 653 Corrosion & Damage Mechanisms 5

Question 1

What is the primary cause of external corrosion under insulation (CUI) on insulated storage tanks?

Accepted Answer

Water penetrates damaged insulation and becomes trapped against the tank shell surface

Answer

CUI occurs when water infiltrates through damaged, improperly installed, or deteriorated insulation and becomes trapped against the steel surface, creating a persistently wet environment that drives corrosion.

Question 2

Which API 653 inspection finding would most urgently require immediate evaluation for continued safe operation?

Accepted Answer

Bottom plate pitting with measured remaining thickness below the calculated minimum

Answer

Bottom plate thickness below the calculated minimum per API 653 criteria indicates the tank floor has insufficient remaining thickness for safe operation and requires immediate repair or retirement evaluation.

Question 3

In API 653 inspections, what is 'selective seam corrosion' and why is it significant?

Accepted Answer

Corrosion that occurs only at welded seams due to electrochemical differences between weld metal and base metal

Answer

Selective seam corrosion occurs preferentially along weld seams due to microstructural differences, residual stresses, or flux inclusion that make weld areas anodic relative to the base metal.

Question 4

What does 'dealloying' (also called selective leaching) involve when it occurs in tank components?

Accepted Answer

Removal of alloying elements from the steel surface, leaving a porous, weakened structure

Answer

Dealloying involves the selective removal of one element (e.g., zinc from brass, or graphite from gray cast iron) from an alloy, leaving a porous, mechanically weak structure that retains the original shape but lacks structural integrity.

Question 5

A tank inspector finds orange-brown deposits on the internal shell of an atmospheric storage tank containing crude oil. What initial corrosion assessment should be performed?

Accepted Answer

Sample and analyze deposits to distinguish between iron oxide rust, iron sulfide (MIC indicator), and other corrosion products

Answer

Deposit sampling and analysis is essential to differentiate between relatively benign iron oxide rust and more aggressive corrosion products like iron sulfides that indicate MIC or H2S-related attack requiring different remediation approaches.

(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 5